src/Compilers/CSharp/Test/Symbol/Symbols/DefaultInterfaceImplementationTests.cs - main - githubmirror/roslyn

src/Compilers/CSharp/Test/Symbol/Symbols/DefaultInterfaceImplementationTests.cs
70 244 строки3 MB
Fred Silberberg
Add C# 15 language version (#84799)
11 авг 2026, 23:51
11 авг 2026, 23:511284a4a
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

#nullable disable

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using Basic.Reference.Assemblies;
using Microsoft.CodeAnalysis.CSharp.Symbols;
using Microsoft.CodeAnalysis.CSharp.Symbols.Metadata.PE;
using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
using Microsoft.CodeAnalysis.Test.Utilities;
using Roslyn.Test.Utilities;
using Xunit;

namespace Microsoft.CodeAnalysis.CSharp.UnitTests.Symbols
{
    [CompilerTrait(CompilerFeature.DefaultInterfaceImplementation)]
    public class DefaultInterfaceImplementationTests : CSharpTestBase
    {
        private sealed class CSharp13_CSharp14_Preview()
            : CombinatorialValuesAttribute(LanguageVersion.CSharp13, LanguageVersion.CSharp14, LanguageVersion.Preview);

        [Theory]
        [CombinatorialData]
        [WorkItem(33083, "https://github.com/dotnet/roslyn/issues/33083")]
        public void MethodImplementation_011(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            ValidateMethodImplementation_011(source1, isStatic);
        }

        private static Verification VerifyOnMonoOrCoreClr
        {
            get
            {
                return ExecutionConditionUtil.IsMonoOrCoreClr ? Verification.Passes : Verification.Skipped;
            }
        }

        private static Verification Verify(bool isStatic)
        {
            // IL Verify complains about static constrained calls: "Missing callvirt following constrained prefix."
            return isStatic ? Verification.Skipped : VerifyOnMonoOrCoreClr;
        }

        private static bool Execute(bool isStatic, bool haveImplementationInDerivedInterface = false, bool hasImplementationOfVirtualInDerivedType = false)
        {
            return ExecutionConditionUtil.IsMonoOrCoreClr;
        }

        private static Verification VerifyOnMonoOrCoreClr_FailsIlVerify
        {
            get
            {
                return ExecutionConditionUtil.IsMonoOrCoreClr ? Verification.FailsILVerify : Verification.Skipped;
            }
        }

        private static Verification VerifyFailsIlVerify(bool isStatic)
        {
            return !isStatic && ExecutionConditionUtil.IsMonoOrCoreClr ? Verification.FailsILVerify : Verification.Skipped;
        }

        private void ValidateMethodImplementation_011(string source, bool isStatic = false)
        {
            foreach (string access in new[] { "x.M1();", "new System.Action(x.M1)();" })
            {
                foreach (string typeKind in new[] { "class", "struct" })
                {
                    string source1;

                    if (!isStatic)
                    {
                        source1 =
typeKind + " Test1 " + @": I1
{
    static void Main()
    {
        I1 x = new Test1();
        " + access + @"
    }
}
";
                    }
                    else
                    {
                        source1 =
typeKind + " Test1 " + @": I1
{
    static void Main()
    {
        Test<Test1>();
    }
#pragma warning disable CS8981 // The type name 'x' only contains lower-cased ascii characters. Such names may become reserved for the language.
    static void Test<x>() where x : I1
    {
        " + access + @"
    }
}
";
                    }

                    source1 = source + source1;

                    var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe, targetFramework: TargetFramework.Net60,
                                                         parseOptions: TestOptions.RegularPreview);
                    Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                    compilation1.VerifyDiagnostics();

                    void Validate1(ModuleSymbol m)
                    {
                        ValidateMethodImplementationTest1_011(m, "void I1.M1()", isStatic);
                    }

                    Validate1(compilation1.SourceModule);

                    CompileAndVerify(compilation1,
                                     expectedOutput: Execute(isStatic) ? "M1" : null,
                                     verify: Verify(isStatic), symbolValidator: Validate1);

                    string source2;

                    if (!isStatic)
                    {
                        source2 =
typeKind + " Test2 " + @": I1
{
    static void Main()
    {
        I1 x = new Test2();
        " + access + @"
    }
}
";
                    }
                    else
                    {
                        source2 =
typeKind + " Test2 " + @": I1
{
    static void Main()
    {
        Test<Test2>();
    }
#pragma warning disable CS8981 // The type name 'x' only contains lower-cased ascii characters. Such names may become reserved for the language.
    static void Test<x>() where x : I1
    {
        " + access + @"
    }
}
";
                    }

                    foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
                    {
                        var compilation2 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                             parseOptions: TestOptions.RegularPreview,
                                                             targetFramework: TargetFramework.Net60);
                        Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                        void Validate2(ModuleSymbol m)
                        {
                            ValidateMethodImplementationTest2_011(m, "void I1.M1()");
                        }

                        Validate2(compilation2.SourceModule);

                        compilation2.VerifyDiagnostics();
                        CompileAndVerify(compilation2,
                                         expectedOutput: Execute(isStatic) ? "M1" : null,
                                         verify: Verify(isStatic), symbolValidator: Validate2);
                    }
                }
            }
        }

        private static void ValidateMethodImplementationTest1_011(ModuleSymbol m, string expectedImplementation, bool isStatic, string methodName = null)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");

            var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.Name);

            var m1 = i1.GetMembers().OfType<MethodSymbol>().Where(m => methodName is null ? true : m.Name == methodName).Single();

            Assert.True(i1.IsAbstract);
            Assert.True(i1.IsMetadataAbstract);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsAbstract);
            Assert.True(m1.IsVirtual);
            Assert.False(m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);

            if (m is PEModuleSymbol peModule)
            {
                int rva;
                peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)m1.OriginalDefinition).Handle, out _, out _, out _, out rva);
                Assert.NotEqual(0, rva);
            }

            Assert.Equal(expectedImplementation, test1.FindImplementationForInterfaceMember(m1).ToTestDisplayString());
            Assert.Same(m1, i1.FindImplementationForInterfaceMember(m1));
        }

        private static void ValidateMethodImplementationTest2_011(ModuleSymbol m, string expectedImplementation, string methodName = null)
        {
            var test2 = m.GlobalNamespace.GetTypeMember("Test2");
            var i1 = test2.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.Name);
            var m1 = i1.GetMember<MethodSymbol>(methodName ?? "M1");

            Assert.Equal(expectedImplementation, test2.FindImplementationForInterfaceMember(m1).ToTestDisplayString());
            Assert.Same(m1, i1.FindImplementationForInterfaceMember(m1));
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_012(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{
    " + implModifiers + @"public void M1() 
    {
        System.Console.WriteLine(""Test1 M1"");
    }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                ValidateMethodImplementationTest1_011(m, "void Test1.M1()", isStatic);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                             expectedOutput: Execute(isStatic) ? "Test1 M1" : null,
                             verify: Verify(isStatic), symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    " + implModifiers + @"public void M1() 
    {
        System.Console.WriteLine(""Test2 M1"");
    }
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 x = new Test2();
        x.M1();
    }
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test2>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source2 +=
@"
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate2(ModuleSymbol m)
            {
                ValidateMethodImplementationTest2_011(m, "void Test2.M1()");
            }

            Validate2(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                             expectedOutput: Execute(isStatic) ? "Test2 M1" : null,
                             verify: Verify(isStatic), symbolValidator: Validate2);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate2(compilation3.SourceModule);
            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                             expectedOutput: Execute(isStatic) ? "Test2 M1" : null,
                             verify: Verify(isStatic), symbolValidator: Validate2);
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_013(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""Test1 M1"");
    }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                ValidateMethodImplementationTest1_011(m, "void Test1.I1.M1()", isStatic);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                             expectedOutput: Execute(isStatic) ? "Test1 M1" : null,
                             verify: Verify(isStatic), symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""Test2 M1"");
    }
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 x = new Test2();
        x.M1();
    }
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test2>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source2 +=
@"
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate2(ModuleSymbol m)
            {
                ValidateMethodImplementationTest2_011(m, "void Test2.I1.M1()");
            }

            Validate2(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                             expectedOutput: Execute(isStatic) ? "Test2 M1" : null,
                             verify: Verify(isStatic), symbolValidator: Validate2);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate2(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                             expectedOutput: Execute(isStatic) ? "Test2 M1" : null,
                             verify: Verify(isStatic), symbolValidator: Validate2);
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_021(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
    " + declModifiers + @"void M2() 
    {
        System.Console.WriteLine(""M2"");
    }
}

class Base
{
    " + implModifiers + @"void M1() { }
}

class Derived : Base, I1
{
    " + implModifiers + @"void M2() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Derived();
        x.M1();
        x.M2();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        T.M2();
    }
";
            }

            source1 +=
@"
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var m1 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M1");
                var m2 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M2");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Same(m1, derived.FindImplementationForInterfaceMember(m1));
                Assert.Same(m2, derived.FindImplementationForInterfaceMember(m2));
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"M1
M2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_022(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
    " + declModifiers + @"void M2() 
    {
        System.Console.WriteLine(""M2"");
    }
}

class Base : Test
{
    " + implModifiers + @"void M1() { }
}

class Derived : Base, I1
{
    " + implModifiers + @"void M2() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Derived();
        x.M1();
        x.M2();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        T.M2();
    }
";
            }

            source1 +=
@"
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var m1 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M1");
                var m2 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M2");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Same(m1, derived.FindImplementationForInterfaceMember(m1));
                Assert.Same(m2, derived.FindImplementationForInterfaceMember(m2));
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"M1
M2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_023(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() {}
    " + declModifiers + @"void M2() {}
}

class Base : Test
{
    " + implModifiers + @"void M1() { }
}

class Derived : Base, I1
{
    " + implModifiers + @"void M2() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Derived();
        x.M1();
        x.M2();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        T.M2();
    }
";
            }

            source1 +=
@"
}

class Test : I1 
{
    " + implModifiers + @"void I1.M1()
    {
        System.Console.WriteLine(""Test.M1"");
    }
    " + implModifiers + @"void I1.M2()
    {
        System.Console.WriteLine(""Test.M2"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var m1 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M1");
                var m2 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M2");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("void Test.I1.M1()", derived.FindImplementationForInterfaceMember(m1).ToTestDisplayString());
                Assert.Equal("void Test.I1.M2()", derived.FindImplementationForInterfaceMember(m2).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"Test.M1
Test.M2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_024(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() {}
    " + declModifiers + @"void M2() {}
}

class Base : Test
{
    new " + implModifiers + @"void M1() { }
}

class Derived : Base, I1
{
    new " + implModifiers + @"void M2() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Derived();
        x.M1();
        x.M2();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        T.M2();
    }
";
            }

            source1 +=
@"
}

class Test : I1 
{
    " + implModifiers + @"public void M1()
    {
        System.Console.WriteLine(""Test.M1"");
    }
    " + implModifiers + @"public void M2()
    {
        System.Console.WriteLine(""Test.M2"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var m1 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M1");
                var m2 = m.GlobalNamespace.GetMember<MethodSymbol>("I1.M2");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("void Test.M1()", derived.FindImplementationForInterfaceMember(m1).ToTestDisplayString());
                Assert.Equal("void Test.M2()", derived.FindImplementationForInterfaceMember(m2).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"Test.M1
Test.M2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_031(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = !isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{
    public " + implModifiers + @" void M1() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));

            CompileAndVerify(compilation1,
                expectedOutput: Execute(isStatic) ? "M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_032(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = !isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : Test2, I1
{
    public " + implModifiers + @" void M1() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));

            CompileAndVerify(compilation1,
                expectedOutput: Execute(isStatic) ? "M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_033(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";
            string notImplModifiers = !isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() { System.Console.WriteLine(""I1.M1""); }
}

class Test1 : Test2, I1
{
    public " + notImplModifiers + @" void M1() { }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 
{
    " + implModifiers + @"void I1.M1()
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Equal("void Test2.I1.M1()", test1.FindImplementationForInterfaceMember(m1).ToTestDisplayString());

            CompileAndVerify(compilation1,
                expectedOutput: Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? "Test2.M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_034(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";
            string notImplModifiers = !isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() { System.Console.WriteLine(""I1.M1""); }
}

class Test1 : Test2, I1
{
    new public " + notImplModifiers + @" void M1() { System.Console.WriteLine(""Test1.M1""); }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 
{
    " + implModifiers + @"public void M1()
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Equal("void Test2.M1()", test1.FindImplementationForInterfaceMember(m1).ToTestDisplayString());

            CompileAndVerify(compilation1,
                expectedOutput: Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? "Test2.M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_041(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1()
    {
        System.Console.WriteLine(""M1"");
    }
    " + declModifiers + @"int M2() => 2; 
}

class Test1 : I1
{
    " + implModifiers + @"public int M1() { return 0; }
    " + implModifiers + @"public ref int M2() { throw null; }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
        System.Console.WriteLine(x.M2());
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        System.Console.WriteLine(T.M2());
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var m2 = compilation1.GetMember<MethodSymbol>("I1.M2");

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
            Assert.Same(m2, test1.FindImplementationForInterfaceMember(m2));

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"M1
2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_042(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1()
    {
        System.Console.WriteLine(""M1"");
    }
    " + declModifiers + @"int M2() => 2; 
}

class Test1 : Test2, I1
{
    " + implModifiers + @"public int M1() { return 0; }
    " + implModifiers + @"public ref int M2() { throw null; }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
        System.Console.WriteLine(x.M2());
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        System.Console.WriteLine(T.M2());
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var m2 = compilation1.GetMember<MethodSymbol>("I1.M2");

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
            Assert.Same(m2, test1.FindImplementationForInterfaceMember(m2));

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"M1
2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_043(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() {}
    " + declModifiers + @"int M2() => 1; 
}

class Test1 : Test2, I1
{
    " + implModifiers + @"public int M1() { return 0; }
    " + implModifiers + @"public ref int M2() { throw null; }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
        System.Console.WriteLine(x.M2());
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        System.Console.WriteLine(T.M2());
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 
{
    " + implModifiers + @"void I1.M1()
    {
        System.Console.WriteLine(""Test2.M1"");
    }
    " + implModifiers + @"int I1.M2() => 2; 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var m2 = compilation1.GetMember<MethodSymbol>("I1.M2");

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Equal("void Test2.I1.M1()", test1.FindImplementationForInterfaceMember(m1).ToTestDisplayString());
            Assert.Equal("System.Int32 Test2.I1.M2()", test1.FindImplementationForInterfaceMember(m2).ToTestDisplayString());

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"Test2.M1
2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_044(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"void M1() {}
    " + declModifiers + @"int M2() => 1; 
}

class Test1 : Test2, I1
{
    " + implModifiers + @"new public int M1() { return 0; }
    " + implModifiers + @"new public ref int M2() { throw null; }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
        System.Console.WriteLine(x.M2());
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
        System.Console.WriteLine(T.M2());
    }
";
            }

            source1 +=
@"
}

class Test2 : I1 
{
    " + implModifiers + @"public void M1()
    {
        System.Console.WriteLine(""Test2.M1"");
    }
    " + implModifiers + @"public int M2() => 2; 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");
            var m2 = compilation1.GetMember<MethodSymbol>("I1.M2");

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Equal("void Test2.M1()", test1.FindImplementationForInterfaceMember(m1).ToTestDisplayString());
            Assert.Equal("System.Int32 Test2.M2()", test1.FindImplementationForInterfaceMember(m2).ToTestDisplayString());

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"Test2.M1
2",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test1Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test1");
                    Assert.Equal("I1", test1Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_051(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");

            Assert.False(m1.IsAbstract);
            Assert.True(m1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (4,10): error CS8501: Target runtime doesn't support default interface implementation.
                    //     void M1() 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(4, 10)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,25): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual void M1() 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(4, 25)
                    );
            }

            Assert.True(m1.IsMetadataVirtual());

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: isStatic ? TestOptions.Regular11 : TestOptions.Regular8);

            m1 = compilation3.GetMember<MethodSymbol>("I1.M1");
            var test2 = compilation3.GetTypeByMetadataName("Test2");

            Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8502: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8929: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2").WithLocation(2, 15)
                    );
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_052(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Net60,
                                                 parseOptions: TestOptions.RegularPreview);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
class Test2 : I1
{}
";
            foreach (var reference in new[] { compilation1.EmitToImageReference(), compilation1.ToMetadataReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference },
                                                     options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: isStatic ? TestOptions.Regular11 : TestOptions.Regular8);

                var m1 = compilation3.GetMember<MethodSymbol>("I1.M1");
                var test2 = compilation3.GetTypeByMetadataName("Test2");

                Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

                if (!isStatic)
                {
                    compilation3.VerifyDiagnostics(
                        // (2,15): error CS8502: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because the target runtime doesn't support default interface implementation.
                        // class Test2 : I1
                        Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2").WithLocation(2, 15)
                        );
                }
                else
                {
                    compilation3.VerifyDiagnostics(
                        // (2,15): error CS8929: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                        // class Test2 : I1
                        Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2").WithLocation(2, 15)
                        );
                }
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_053(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular11,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
public interface I2
{
    void M2();
}

class Test2 : I2
{
    public void M2() {}
}
";

            foreach (var reference in new[] { compilation1.EmitToImageReference(), compilation1.ToMetadataReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugDll,
                                                     targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: TestOptions.Regular7_3);
                Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                var m1 = compilation3.GetMember<MethodSymbol>("I1.M1");
                var test2 = compilation3.GetTypeByMetadataName("Test2");

                Assert.Null(test2.FindImplementationForInterfaceMember(m1));

                compilation3.VerifyDiagnostics();
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_061(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3, skipUsesIsNullable: true);

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");

            Assert.False(m1.IsAbstract);
            Assert.True(m1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (4,10): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     void M1() 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "M1").WithArguments("default interface implementation", "8.0").WithLocation(4, 10),
                    // (4,10): error CS8701: Target runtime doesn't support default interface implementation.
                    //     void M1() 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(4, 10)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,25): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual void M1() 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("virtual", "10.0", "11.0").WithLocation(4, 25),
                    // (4,25): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual void M1() 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(4, 25)
                    );
            }

            Assert.True(m1.IsMetadataVirtual());

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3);

            m1 = compilation3.GetMember<MethodSymbol>("I1.M1");
            var test2 = compilation3.GetTypeByMetadataName("Test2");

            Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8506: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8706: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2").WithLocation(2, 15)
                    );
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_071(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: isStatic ? TargetFramework.Net60 : TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");

            Assert.False(m1.IsAbstract);
            Assert.True(m1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (4,10): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     void M1() 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "M1").WithArguments("default interface implementation", "8.0").WithLocation(4, 10)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,25): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual void M1() 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("virtual", "10.0", "11.0").WithLocation(4, 25)
                    );
            }

            Assert.True(m1.IsMetadataVirtual());

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular11,
                                                 targetFramework: isStatic ? TargetFramework.Net60 : TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            m1 = compilation2.GetMember<MethodSymbol>("I1.M1");
            var test2 = compilation2.GetTypeByMetadataName("Test2");

            Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

            compilation2.VerifyDiagnostics();

            CompileAndVerify(compilation2, verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test2Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test2");
                    Assert.Equal("I1", test2Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                });

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: isStatic ? TargetFramework.Net60 : TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            m1 = compilation3.GetMember<MethodSymbol>("I1.M1");
            test2 = compilation3.GetTypeByMetadataName("Test2");

            Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8506: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8706: 'I1.M1()' cannot implement interface member 'I1.M1()' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.M1()", "I1.M1()", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15)
                    );
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_081(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"I1 M1() 
    {
        throw null;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular11);

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");

            Assert.False(m1.IsAbstract);
            Assert.True(m1.IsVirtual);

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (4,8): error CS8501: Target runtime doesn't support default interface implementation.
                    //     I1 M1() 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(4, 8)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,23): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual I1 M1() 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(4, 23)
                    );
            }

            Assert.True(m1.IsMetadataVirtual());
        }

        [Fact]
        public void MethodImplementation_091()
        {
            var source1 =
@"
public interface I1
{
    static void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");

            Assert.False(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsStatic);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Null(test1.FindImplementationForInterfaceMember(m1));

            compilation1.VerifyDiagnostics(
                // (4,17): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static void M1() 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "M1").WithArguments("default interface implementation", "8.0").WithLocation(4, 17)
                );

            Assert.False(m1.IsMetadataVirtual());

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular8,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular14,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (4,17): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static void M1() 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "M1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 17)
                );

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics();

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics();
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementation_101(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

public interface I2 : I1
{}

class Test1 : I2
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular11,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            var m1 = compilation1.GetMember<MethodSymbol>("I1.M1");

            Assert.False(m1.IsAbstract);
            Assert.True(m1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));

            compilation1.VerifyDiagnostics();
            Assert.True(m1.IsMetadataVirtual());

            CompileAndVerify(compilation1,
                expectedOutput: Execute(isStatic) ? "M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var i1 = m.GlobalNamespace.GetTypeMember("I1");
                    var result = (PEMethodSymbol)i1.GetMember("M1");

                    Assert.True(result.IsMetadataVirtual());
                    Assert.False(result.IsAbstract);
                    Assert.True(result.IsVirtual);
                    Assert.True(i1.IsAbstract);
                    Assert.True(i1.IsMetadataAbstract);

                    int rva;
                    ((PEModuleSymbol)m).Module.GetMethodDefPropsOrThrow(result.Handle, out _, out _, out _, out rva);
                    Assert.NotEqual(0, rva);

                    var test1Result = m.GlobalNamespace.GetTypeMember("Test1");
                    var interfaces = test1Result.InterfacesNoUseSiteDiagnostics().ToArray();
                    Assert.Equal(2, interfaces.Length);
                    Assert.Equal("I2", interfaces[0].ToTestDisplayString());
                    Assert.Equal("I1", interfaces[1].ToTestDisplayString());
                });

            var source2 =
@"
class Test2 : I2
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 x = new Test2();
        x.M1();
    }
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test2>();
    }

    static void Test<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source2 +=
@"
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular11,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            m1 = compilation2.GetMember<MethodSymbol>("I1.M1");
            var test2 = compilation2.GetTypeByMetadataName("Test2");

            Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: Execute(isStatic) ? "M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test2Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test2");
                    var interfaces = test2Result.InterfacesNoUseSiteDiagnostics().ToArray();
                    Assert.Equal(2, interfaces.Length);
                    Assert.Equal("I2", interfaces[0].ToTestDisplayString());
                    Assert.Equal("I1", interfaces[1].ToTestDisplayString());
                });

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular11,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            m1 = compilation3.GetMember<MethodSymbol>("I1.M1");
            test2 = compilation3.GetTypeByMetadataName("Test2");

            Assert.Same(m1, test2.FindImplementationForInterfaceMember(m1));

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: Execute(isStatic) ? "M1" : null,
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test2Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test2");
                    var interfaces = test2Result.InterfacesNoUseSiteDiagnostics().ToArray();
                    Assert.Equal(2, interfaces.Length);
                    Assert.Equal("I2", interfaces[0].ToTestDisplayString());
                    Assert.Equal("I1", interfaces[1].ToTestDisplayString());
                });
        }

        [Fact]
        public void MethodImplementation_111()
        {
            var source1 =
@"
#nullable enable

public interface I1<T>
{
    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2 : I1<string>
{
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2, I1<string?>
{
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.Regular,
                                                        targetFramework: TargetFramework.NetCoreApp);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I2", "I1<System.String>", "I1<System.String?>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2, I1<string?>
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"
I1.M1
I1.M1
",
                    verify: VerifyOnMonoOrCoreClr);
            }
        }

        [Fact]
        public void MethodImplementation_112()
        {
            var source1 =
@"
#nullable enable

public interface I1<T>
{
    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2 : I1<string>
{
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I1<string?>, I2
{
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.Regular,
                                                        targetFramework: TargetFramework.NetCoreApp);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I1<System.String?>", "I2", "I1<System.String>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I1<System.String>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[0].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I1<System.String>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I1<string?>, I2
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string>", "Test1").WithLocation(4, 7)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"
I1.M1
I1.M1
",
                    verify: VerifyOnMonoOrCoreClr);
            }
        }

        [Fact]
        public void MethodImplementation_113()
        {
            var source1 =
@"
#nullable enable

public interface I1<T>
{
    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2 : I1<string>
{
}

public interface I3 : I1<string?>
{
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2, I3
{
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.Regular,
                                                        targetFramework: TargetFramework.NetCoreApp);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2, I3
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"
I1.M1
I1.M1
",
                    verify: VerifyOnMonoOrCoreClr);
            }
        }

        [Fact]
        public void MethodImplementation_114()
        {
            var source1 =
@"
#nullable enable

public interface I1<T>
{
    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2 : I1<string>
{
}

public interface I3 : I1<string?>
{
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I3, I2
{
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.Regular,
                                                        targetFramework: TargetFramework.NetCoreApp);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I3", "I1<System.String?>", "I2", "I1<System.String>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I1<System.String>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I1<System.String>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I3, I2
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string>", "Test1").WithLocation(4, 7)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"
I1.M1
I1.M1
",
                    verify: VerifyOnMonoOrCoreClr);
            }
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_101(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
    }
}

class Test1 : I1
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1.P1;
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        _ = T.P1;
    }
";
            }

            source1 +=
@"
}
";
            ValidatePropertyImplementation_101(source1, isStatic);
        }

        private void ValidatePropertyImplementation_101(string source1, bool isStatic = false)
        {
            ValidatePropertyImplementation_101(source1, "P1", haveGet: true, haveSet: false,
                accessCode: @"
        _ = i1.P1;
",
                expectedOutput: "get P1",
                isStatic: isStatic);
        }

        private void ValidatePropertyImplementation_101(string source1, string propertyName, bool haveGet, bool haveSet, string accessCode, string expectedOutput, bool isStatic = false)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                ValidatePropertyImplementationTest1_101(m, propertyName, haveGet, haveSet, isStatic);
            }

            Validate1(compilation1.SourceModule);
            CompileAndVerify(compilation1,
                expectedOutput: Execute(isStatic) ? expectedOutput : null,
                verify: Verify(isStatic),
                symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 i1 = new Test2();
" + accessCode + @"
    }
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test2>();
    }

    static void Test<i1>() where i1 : I1
    {
" + accessCode + @"
    }
";
            }

            source2 +=
@"
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate2(ModuleSymbol m)
            {
                ValidatePropertyImplementationTest2_101(m, propertyName, haveGet, haveSet);
            }

            Validate2(compilation2.SourceModule);
            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: Execute(isStatic) ? expectedOutput : null,
                verify: Verify(isStatic),
                symbolValidator: Validate2);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate2(compilation3.SourceModule);
            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: Execute(isStatic) ? expectedOutput : null,
                verify: Verify(isStatic),
                symbolValidator: Validate2);
        }

        private static void ValidatePropertyImplementationTest1_101(ModuleSymbol m, string propertyName, bool haveGet, bool haveSet, bool isStatic)
        {
            var i1 = m.GlobalNamespace.GetTypeMember("I1");
            var p1 = i1.GetMember<PropertySymbol>(propertyName);

            Assert.Equal(!haveSet, p1.IsReadOnly);
            Assert.Equal(!haveGet, p1.IsWriteOnly);

            if (haveGet)
            {
                ValidateAccessor(p1.GetMethod);
            }
            else
            {
                Assert.Null(p1.GetMethod);
            }

            if (haveSet)
            {
                ValidateAccessor(p1.SetMethod);
            }
            else
            {
                Assert.Null(p1.SetMethod);
            }

            void ValidateAccessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.Equal(isStatic, accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            Assert.False(p1.IsAbstract);
            Assert.True(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.Equal(isStatic, p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.True(i1.IsAbstract);
            Assert.True(i1.IsMetadataAbstract);

            if (m is PEModuleSymbol peModule)
            {
                int rva;

                if (haveGet)
                {
                    peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)p1.GetMethod).Handle, out _, out _, out _, out rva);
                    Assert.NotEqual(0, rva);
                }

                if (haveSet)
                {
                    peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)p1.SetMethod).Handle, out _, out _, out _, out rva);
                    Assert.NotEqual(0, rva);
                }
            }

            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            Assert.Equal("I1", test1.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
            Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));

            if (haveGet)
            {
                Assert.Same(p1.GetMethod, test1.FindImplementationForInterfaceMember(p1.GetMethod));
            }

            if (haveSet)
            {
                Assert.Same(p1.SetMethod, test1.FindImplementationForInterfaceMember(p1.SetMethod));
            }
        }

        private static void ValidatePropertyImplementationTest2_101(ModuleSymbol m, string propertyName, bool haveGet, bool haveSet)
        {
            var test2 = m.GlobalNamespace.GetTypeMember("Test2");
            Assert.Equal("I1", test2.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

            var p1 = test2.InterfacesNoUseSiteDiagnostics().Single().GetMember<PropertySymbol>(propertyName);
            Assert.Same(p1, test2.FindImplementationForInterfaceMember(p1));

            if (haveGet)
            {
                var getP1 = p1.GetMethod;
                Assert.Same(getP1, test2.FindImplementationForInterfaceMember(getP1));
            }

            if (haveSet)
            {
                var setP1 = p1.SetMethod;
                Assert.Same(setP1, test2.FindImplementationForInterfaceMember(setP1));
            }
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_102(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 = i1.P1;
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.P1 = T.P1;
    }
";
            }

            source1 +=
@"
}
";
            ValidatePropertyImplementation_102(source1, isStatic);
        }

        private void ValidatePropertyImplementation_102(string source1, bool isStatic = false)
        {
            ValidatePropertyImplementation_101(source1, "P1", haveGet: true, haveSet: true,
                accessCode: @"
        i1.P1 = i1.P1;
",
                expectedOutput:
@"get P1
set P1",
                isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_103(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 
    {
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 = 1;
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        T.P1 = 1;
    }
";
            }

            source1 +=
@"
}
";
            ValidatePropertyImplementation_103(source1, isStatic);
        }

        private void ValidatePropertyImplementation_103(string source1, bool isStatic = false)
        {
            ValidatePropertyImplementation_101(source1, "P1", haveGet: false, haveSet: true,
                accessCode: @"
        i1.P1 = 1;
",
                expectedOutput: "set P1",
                isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_104(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 => Test1.GetInt();
}

class Test1 : I1
{
    public static int GetInt()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1.P1;
    }
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        _ = T.P1;
    }
";
            }

            source1 +=
@"
}
";
            ValidatePropertyImplementation_101(source1, isStatic);
        }

        [Fact]
        public void PropertyImplementation_105()
        {
            {
                var source1 =
    @"
public interface I1
{
    int P1 {add; remove;} => 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics(
                    // (4,13): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 13),
                    // (4,18): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 18),
                    // (4,9): error CS0548: 'I1.P1': property or indexer must have at least one accessor
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 9),
                    // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "int P1 {add; remove;} => 0;").WithLocation(4, 5)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.False(p1.IsVirtual);
                Assert.False(p1.IsStatic);
                Assert.Null(p1.GetMethod);
                Assert.Null(p1.SetMethod);
                Assert.True(p1.IsReadOnly);
                Assert.True(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static abstract int P1 {add; remove;} => 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics(
                    // (4,13): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 29),
                    // (4,18): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 34),
                    // (4,9): error CS0548: 'I1.P1': property or indexer must have at least one accessor
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 25),
                    // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "static abstract int P1 {add; remove;} => 0;").WithLocation(4, 5)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.False(p1.IsVirtual);
                Assert.True(p1.IsStatic);
                Assert.Null(p1.GetMethod);
                Assert.Null(p1.SetMethod);
                Assert.True(p1.IsReadOnly);
                Assert.True(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static virtual int P1 {add; remove;} => 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular11,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics(
                    // (4,13): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 28),
                    // (4,18): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 33),
                    // (4,9): error CS0548: 'I1.P1': property or indexer must have at least one accessor
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 24),
                    // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                    //     int P1 {add; remove;} => 0;
                    Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "static virtual int P1 {add; remove;} => 0;").WithLocation(4, 5)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.False(p1.IsAbstract);
                Assert.True(p1.IsVirtual);
                Assert.True(p1.IsStatic);
                Assert.Null(p1.GetMethod);
                Assert.Null(p1.SetMethod);
                Assert.True(p1.IsReadOnly);
                Assert.True(p1.IsWriteOnly);
            }
        }

        [Fact]
        public void PropertyImplementation_106()
        {
            {
                var source1 =
    @"
public interface I1
{
    int P1 {get; set;} => 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics(
                    // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                    //     int P1 {get; set;} => 0;
                    Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "int P1 {get; set;} => 0;").WithLocation(4, 5)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.True(p1.GetMethod.IsAbstract);
                Assert.True(p1.SetMethod.IsAbstract);
                Assert.False(p1.IsReadOnly);
                Assert.False(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static abstract int P1 {get; set;} => 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics(
                    // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                    //     static abstract int P1 {get; set;} => 0;
                    Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "static abstract int P1 {get; set;} => 0;").WithLocation(4, 5)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.True(p1.GetMethod.IsAbstract);
                Assert.True(p1.SetMethod.IsAbstract);
                Assert.False(p1.IsReadOnly);
                Assert.False(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static virtual int P1 {get; set;} => 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics(
                    // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                    //     int P1 {get; set;} => 0;
                    Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "static virtual int P1 {get; set;} => 0;").WithLocation(4, 5)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsVirtual);
                Assert.True(p1.GetMethod.IsVirtual);
                Assert.True(p1.SetMethod.IsVirtual);
                Assert.False(p1.IsReadOnly);
                Assert.False(p1.IsWriteOnly);
            }
        }

        [Fact]
        public void PropertyImplementation_107()
        {
            {
                var source1 =
    @"
public interface I1
{
    int P1 {add; remove;} = 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyEmitDiagnostics(
                    // (4,13): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 13),
                    // (4,18): error CS1014: A get, set or init accessor expected
                    //     int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 18),
                    // (4,9): error CS8053: Instance properties in interfaces cannot have initializers.
                    //     int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P1").WithLocation(4, 9),
                    // (4,9): error CS0548: 'I1.P1': property or indexer must have at least one accessor
                    //     int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 9)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.Null(p1.GetMethod);
                Assert.Null(p1.SetMethod);
                Assert.True(p1.IsReadOnly);
                Assert.True(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static abstract int P1 {add; remove;} = 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyEmitDiagnostics(
                    // (4,29): error CS1014: A get or set accessor expected
                    //     static abstract int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 29),
                    // (4,34): error CS1014: A get or set accessor expected
                    //     static abstract int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 34),
                    // (4,25): error CS8050: Only auto-implemented properties, or properties that use the 'field' keyword, can have initializers.
                    //     static abstract int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P1").WithLocation(4, 25),
                    // (4,25): error CS0548: 'I1.P1': property or indexer must have at least one accessor
                    //     static abstract int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 25)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.Null(p1.GetMethod);
                Assert.Null(p1.SetMethod);
                Assert.True(p1.IsReadOnly);
                Assert.True(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static virtual int P1 {add; remove;} = 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyEmitDiagnostics(
                    // (4,28): error CS1014: A get or set accessor expected
                    //     static virtual int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 28),
                    // (4,33): error CS1014: A get or set accessor expected
                    //     static virtual int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 33),
                    // (4,24): error CS8050: Only auto-implemented properties, or properties that use the 'field' keyword, can have initializers.
                    //     static virtual int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P1").WithLocation(4, 24),
                    // (4,24): error CS0548: 'I1.P1': property or indexer must have at least one accessor
                    //     static virtual int P1 {add; remove;} = 0;
                    Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 24)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsVirtual);
                Assert.Null(p1.GetMethod);
                Assert.Null(p1.SetMethod);
                Assert.True(p1.IsReadOnly);
                Assert.True(p1.IsWriteOnly);
            }
        }

        [Fact]
        public void PropertyImplementation_108()
        {
            {
                var source1 =
    @"
public interface I1
{
    int P1 {get; set;} = 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyEmitDiagnostics(
                    // (4,9): error CS8053: Instance properties in interfaces cannot have initializers..
                    //     int P1 {get; set;} = 0;
                    Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P1").WithLocation(4, 9)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.True(p1.GetMethod.IsAbstract);
                Assert.True(p1.SetMethod.IsAbstract);
                Assert.False(p1.IsReadOnly);
                Assert.False(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static abstract int P1 {get; set;} = 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyEmitDiagnostics(
                    // (4,25): error CS8050: Only auto-implemented properties, or properties that use the 'field' keyword, can have initializers.
                    //     static abstract int P1 {get; set;} = 0;
                    Diagnostic(ErrorCode.ERR_InitializerOnNonAutoProperty, "P1").WithLocation(4, 25)
                    );

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsAbstract);
                Assert.True(p1.GetMethod.IsAbstract);
                Assert.True(p1.SetMethod.IsAbstract);
                Assert.False(p1.IsReadOnly);
                Assert.False(p1.IsWriteOnly);
            }
            {
                var source1 =
    @"
public interface I1
{
    static virtual int P1 {get; set;} = 0;
}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyEmitDiagnostics();

                var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
                Assert.True(p1.IsVirtual);
                Assert.True(p1.GetMethod.IsVirtual);
                Assert.True(p1.SetMethod.IsVirtual);
                Assert.False(p1.IsReadOnly);
                Assert.False(p1.IsWriteOnly);
            }
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_109A(bool isStatic, [CSharp13_CSharp14_Preview] LanguageVersion langVer)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set;
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular.WithLanguageVersion(langVer),
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            bool useCSharp13 = langVer == LanguageVersion.CSharp13;

            switch (isStatic, useCSharp13)
            {
                case (true, true):
                    compilation1.VerifyDiagnostics(
                        // (4,24): error CS9260: Feature 'field keyword' is not available in C# 13.0. Please use language version 14.0 or greater.
                        //     static virtual int P1 
                        Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion13, "P1").WithArguments("field keyword", "14.0").WithLocation(4, 24));
                    break;
                case (true, false):
                    compilation1.VerifyDiagnostics(
                        // (6,9): warning CS9266: The 'get' accessor of property 'I1.P1' should use 'field' because the other accessor is using it.
                        //         get
                        Diagnostic(ErrorCode.WRN_AccessorDoesNotUseBackingField, "get").WithArguments("get", "I1.P1").WithLocation(6, 9));
                    break;
                case (false, true):
                    compilation1.VerifyDiagnostics(
                        // (4,9): error CS9260: Feature 'field keyword' is not available in C# 13.0. Please use language version 14.0 or greater.
                        //     int P1 
                        Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion13, "P1").WithArguments("field keyword", "14.0").WithLocation(4, 9),
                        // (4,9): error CS0525: Interfaces cannot contain instance fields
                        //     int P1 
                        Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "P1").WithLocation(4, 9));
                    break;
                case (false, false):
                    // See also earlier LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-04-18.md,
                    // we don't want to allow only one accessor to have an implementation.
                    compilation1.VerifyDiagnostics(
                        // (4,9): error CS0525: Interfaces cannot contain instance fields
                        //     int P1 
                        Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "P1").WithLocation(4, 9),
                        // (6,9): warning CS9266: The 'get' accessor of property 'I1.P1' should use 'field' because the other accessor is using it.
                        //         get
                        Diagnostic(ErrorCode.WRN_AccessorDoesNotUseBackingField, "get").WithArguments("get", "I1.P1").WithLocation(6, 9));
                    break;
            }

            var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
            var getP1 = p1.GetMethod;
            var setP1 = p1.SetMethod;
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);

            var field1 = ((SourcePropertySymbolBase)p1).BackingField;
            Assert.Equal("System.Int32 I1.<P1>k__BackingField", field1?.ToTestDisplayString());

            Assert.False(p1.IsAbstract);
            Assert.True(p1.IsVirtual);
            Assert.False(getP1.IsAbstract);
            Assert.True(getP1.IsVirtual);
            Assert.False(setP1.IsAbstract);
            Assert.True(setP1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(getP1, test1.FindImplementationForInterfaceMember(getP1));
            Assert.Same(setP1, test1.FindImplementationForInterfaceMember(setP1));

            Assert.True(getP1.IsMetadataVirtual());
            Assert.True(setP1.IsMetadataVirtual());
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_109B([CSharp13_CSharp14_Preview] LanguageVersion langVer)
        {
            var source1 =
@"
public interface I1
{
    static int P1 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set;
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular.WithLanguageVersion(langVer),
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            bool useCSharp13 = langVer == LanguageVersion.CSharp13;

            if (useCSharp13)
            {
                compilation1.VerifyDiagnostics(
                    // (4,16): error CS9260: Feature 'field keyword' is not available in C# 13.0. Please use language version 14.0 or greater.
                    //     static int P1 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion13, "P1").WithArguments("field keyword", "14.0").WithLocation(4, 16));
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (6,9): warning CS9266: The 'get' accessor of property 'I1.P1' should use 'field' because the other accessor is using it.
                    //         get
                    Diagnostic(ErrorCode.WRN_AccessorDoesNotUseBackingField, "get").WithArguments("get", "I1.P1").WithLocation(6, 9));
            }

            var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
            var getP1 = p1.GetMethod;
            var setP1 = p1.SetMethod;
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);

            var field1 = ((SourcePropertySymbolBase)p1).BackingField;
            Assert.Equal("System.Int32 I1.<P1>k__BackingField", field1?.ToTestDisplayString());

            Assert.False(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(getP1.IsAbstract);
            Assert.False(getP1.IsVirtual);
            Assert.False(setP1.IsAbstract);
            Assert.False(setP1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test1.FindImplementationForInterfaceMember(getP1));
            Assert.Null(test1.FindImplementationForInterfaceMember(setP1));

            Assert.False(getP1.IsMetadataVirtual());
            Assert.False(setP1.IsMetadataVirtual());
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_110A(bool isStatic, [CSharp13_CSharp14_Preview] LanguageVersion langVer)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 
    {
        get;
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular.WithLanguageVersion(langVer),
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            bool useCSharp13 = langVer == LanguageVersion.CSharp13;

            switch (isStatic, useCSharp13)
            {
                case (true, true):
                    compilation1.VerifyDiagnostics(
                        // (4,24): error CS9260: Feature 'field keyword' is not available in C# 13.0. Please use language version 14.0 or greater.
                        //     static virtual int P1 
                        Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion13, "P1").WithArguments("field keyword", "14.0").WithLocation(4, 24));
                    break;
                case (true, false):
                    compilation1.VerifyDiagnostics(
                        // (7,9): warning CS9266: The 'set' accessor of property 'I1.P1' should use 'field' because the other accessor is using it.
                        //         set => System.Console.WriteLine("set P1");
                        Diagnostic(ErrorCode.WRN_AccessorDoesNotUseBackingField, "set").WithArguments("set", "I1.P1").WithLocation(7, 9));
                    break;
                case (false, true):
                    compilation1.VerifyDiagnostics(
                        // (4,9): error CS9260: Feature 'field keyword' is not available in C# 13.0. Please use language version 14.0 or greater.
                        //     int P1 
                        Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion13, "P1").WithArguments("field keyword", "14.0").WithLocation(4, 9),
                        // (4,9): error CS0525: Interfaces cannot contain instance fields
                        //     int P1 
                        Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "P1").WithLocation(4, 9));
                    break;
                case (false, false):
                    // See also earlier LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-04-18.md,
                    // we don't want to allow only one accessor to have an implementation.
                    compilation1.VerifyDiagnostics(
                        // (4,9): error CS0525: Interfaces cannot contain instance fields
                        //     int P1 
                        Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "P1").WithLocation(4, 9),
                        // (7,9): warning CS9266: The 'set' accessor of property 'I1.P1' should use 'field' because the other accessor is using it.
                        //         set => System.Console.WriteLine("set P1");
                        Diagnostic(ErrorCode.WRN_AccessorDoesNotUseBackingField, "set").WithArguments("set", "I1.P1").WithLocation(7, 9));
                    break;
            }

            var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
            var getP1 = p1.GetMethod;
            var setP1 = p1.SetMethod;
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);

            var field1 = ((SourcePropertySymbolBase)p1).BackingField;
            Assert.Equal("System.Int32 I1.<P1>k__BackingField", field1?.ToTestDisplayString());

            Assert.False(p1.IsAbstract);
            Assert.True(p1.IsVirtual);
            Assert.False(getP1.IsAbstract);
            Assert.True(getP1.IsVirtual);
            Assert.False(setP1.IsAbstract);
            Assert.True(setP1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(getP1, test1.FindImplementationForInterfaceMember(getP1));
            Assert.Same(setP1, test1.FindImplementationForInterfaceMember(setP1));

            Assert.True(getP1.IsMetadataVirtual());
            Assert.True(setP1.IsMetadataVirtual());
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_110B([CSharp13_CSharp14_Preview] LanguageVersion langVer)
        {
            var source1 =
@"
public interface I1
{
    static int P1 
    {
        get;
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular.WithLanguageVersion(langVer),
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            bool useCSharp13 = langVer == LanguageVersion.CSharp13;

            if (useCSharp13)
            {
                compilation1.VerifyDiagnostics(
                    // (4,16): error CS9260: Feature 'field keyword' is not available in C# 13.0. Please use language version 14.0 or greater.
                    //     static int P1 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion13, "P1").WithArguments("field keyword", "14.0").WithLocation(4, 16));
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (7,9): warning CS9266: The 'set' accessor of property 'I1.P1' should use 'field' because the other accessor is using it.
                    //         set => System.Console.WriteLine("set P1");
                    Diagnostic(ErrorCode.WRN_AccessorDoesNotUseBackingField, "set").WithArguments("set", "I1.P1").WithLocation(7, 9));
            }

            var p1 = compilation1.GetMember<PropertySymbol>("I1.P1");
            var getP1 = p1.GetMethod;
            var setP1 = p1.SetMethod;
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);

            var field1 = ((SourcePropertySymbolBase)p1).BackingField;
            Assert.Equal("System.Int32 I1.<P1>k__BackingField", field1?.ToTestDisplayString());

            Assert.False(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(getP1.IsAbstract);
            Assert.False(getP1.IsVirtual);
            Assert.False(setP1.IsAbstract);
            Assert.False(setP1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test1.FindImplementationForInterfaceMember(getP1));
            Assert.Null(test1.FindImplementationForInterfaceMember(setP1));

            Assert.False(getP1.IsMetadataVirtual());
            Assert.False(setP1.IsMetadataVirtual());
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_201(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P2 => 2;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P4 { get => 4; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P6 { set => System.Console.WriteLine(6); }
    " + declModifiers + @"int P7 { get { return 7;} set {System.Console.WriteLine(71);} }
    " + declModifiers + @"int P8 { get { return 8;} set {System.Console.WriteLine(81);} }
}

class Base
{
    " + implModifiers + @"int P1 => 10;
    " + implModifiers + @"int P3 { get => 30; }
    " + implModifiers + @"int P5 { set => System.Console.WriteLine(50); }
    " + implModifiers + @"int P7 { get { return 70;} set {} }
}

class Derived : Base, I1
{
    " + implModifiers + @"int P2 => 20;
    " + implModifiers + @"int P4 { get => 40; }
    " + implModifiers + @"int P6 { set => System.Console.WriteLine(60); }
    " + implModifiers + @"int P8 { get { return 80;} set {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        System.Console.WriteLine(i1.P1);
        System.Console.WriteLine(i1.P2);
        System.Console.WriteLine(i1.P3);
        System.Console.WriteLine(i1.P4);
        i1.P5 = 0;
        i1.P6 = 0;
        System.Console.WriteLine(i1.P7);
        i1.P7 = 0;
        System.Console.WriteLine(i1.P8);
        i1.P8 = 0;
    }
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            ValidatePropertyImplementation_201(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"1
2
3
4
5
6
7
71
8
81
",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    ValidatePropertyImplementation_201(m);
                });
        }

        private static void ValidatePropertyImplementation_201(ModuleSymbol m)
        {
            var p1 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P1");
            var p2 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P2");
            var p3 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P3");
            var p4 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P4");
            var p5 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P5");
            var p6 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P6");
            var p7 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P7");
            var p8 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P8");

            var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

            Assert.Same(p1, derived.FindImplementationForInterfaceMember(p1));
            Assert.Same(p2, derived.FindImplementationForInterfaceMember(p2));
            Assert.Same(p3, derived.FindImplementationForInterfaceMember(p3));
            Assert.Same(p4, derived.FindImplementationForInterfaceMember(p4));
            Assert.Same(p5, derived.FindImplementationForInterfaceMember(p5));
            Assert.Same(p6, derived.FindImplementationForInterfaceMember(p6));
            Assert.Same(p7, derived.FindImplementationForInterfaceMember(p7));
            Assert.Same(p8, derived.FindImplementationForInterfaceMember(p8));

            Assert.Same(p1.GetMethod, derived.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Same(p2.GetMethod, derived.FindImplementationForInterfaceMember(p2.GetMethod));
            Assert.Same(p3.GetMethod, derived.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Same(p4.GetMethod, derived.FindImplementationForInterfaceMember(p4.GetMethod));
            Assert.Same(p5.SetMethod, derived.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Same(p6.SetMethod, derived.FindImplementationForInterfaceMember(p6.SetMethod));
            Assert.Same(p7.GetMethod, derived.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Same(p8.GetMethod, derived.FindImplementationForInterfaceMember(p8.GetMethod));
            Assert.Same(p7.SetMethod, derived.FindImplementationForInterfaceMember(p7.SetMethod));
            Assert.Same(p8.SetMethod, derived.FindImplementationForInterfaceMember(p8.SetMethod));
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_202(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P2 => 2;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P4 { get => 4; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P6 { set => System.Console.WriteLine(6); }
    " + declModifiers + @"int P7 { get { return 7;} set {System.Console.WriteLine(71);} }
    " + declModifiers + @"int P8 { get { return 8;} set {System.Console.WriteLine(81);} }
}

class Base : Test
{
    " + implModifiers + @"int P1 => 10;
    " + implModifiers + @"int P3 { get => 30; }
    " + implModifiers + @"int P5 { set => System.Console.WriteLine(50); }
    " + implModifiers + @"int P7 { get { return 70;} set {} }
}

class Derived : Base, I1
{
    " + implModifiers + @"int P2 => 20;
    " + implModifiers + @"int P4 { get => 40; }
    " + implModifiers + @"int P6 { set => System.Console.WriteLine(60); }
    " + implModifiers + @"int P8 { get { return 80;} set {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        System.Console.WriteLine(i1.P1);
        System.Console.WriteLine(i1.P2);
        System.Console.WriteLine(i1.P3);
        System.Console.WriteLine(i1.P4);
        i1.P5 = 0;
        i1.P6 = 0;
        System.Console.WriteLine(i1.P7);
        i1.P7 = 0;
        System.Console.WriteLine(i1.P8);
        i1.P8 = 0;
    }
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            ValidatePropertyImplementation_201(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"1
2
3
4
5
6
7
71
8
81
",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    ValidatePropertyImplementation_201(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_203(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P2 => 2;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P4 { get => 4; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P6 { set => System.Console.WriteLine(6); }
    " + declModifiers + @"int P7 { get { return 7;} set {} }
    " + declModifiers + @"int P8 { get { return 8;} set {} }
}

class Base : Test
{
    " + implModifiers + @"int P1 => 10;
    " + implModifiers + @"int P3 { get => 30; }
    " + implModifiers + @"int P5 { set => System.Console.WriteLine(50); }
    " + implModifiers + @"int P7 { get { return 70;} set {} }
}

class Derived : Base, I1
{
    " + implModifiers + @"int P2 => 20;
    " + implModifiers + @"int P4 { get => 40; }
    " + implModifiers + @"int P6 { set => System.Console.WriteLine(60); }
    " + implModifiers + @"int P8 { get { return 80;} set {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        System.Console.WriteLine(i1.P1);
        System.Console.WriteLine(i1.P2);
        System.Console.WriteLine(i1.P3);
        System.Console.WriteLine(i1.P4);
        i1.P5 = 0;
        i1.P6 = 0;
        System.Console.WriteLine(i1.P7);
        i1.P7 = 0;
        System.Console.WriteLine(i1.P8);
        i1.P8 = 0;
    }
}

class Test : I1 
{
    " + implModifiers + @"int I1.P1 => 100;
    " + implModifiers + @"int I1.P2 => 200;
    " + implModifiers + @"int I1.P3 { get => 300; }
    " + implModifiers + @"int I1.P4 { get => 400; }
    " + implModifiers + @"int I1.P5 { set => System.Console.WriteLine(500); }
    " + implModifiers + @"int I1.P6 { set => System.Console.WriteLine(600); }
    " + implModifiers + @"int I1.P7 { get { return 700;} set {System.Console.WriteLine(701);} }
    " + implModifiers + @"int I1.P8 { get { return 800;} set {System.Console.WriteLine(801);} }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var p1 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P1");
                var p2 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P2");
                var p3 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P3");
                var p4 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P4");
                var p5 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P5");
                var p6 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P6");
                var p7 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P7");
                var p8 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P8");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("System.Int32 Test.I1.P1 { get; }", derived.FindImplementationForInterfaceMember(p1).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P2 { get; }", derived.FindImplementationForInterfaceMember(p2).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P3 { get; }", derived.FindImplementationForInterfaceMember(p3).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P4 { get; }", derived.FindImplementationForInterfaceMember(p4).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P5 { set; }", derived.FindImplementationForInterfaceMember(p5).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P6 { set; }", derived.FindImplementationForInterfaceMember(p6).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P7 { get; set; }", derived.FindImplementationForInterfaceMember(p7).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P8 { get; set; }", derived.FindImplementationForInterfaceMember(p8).ToTestDisplayString());

                Assert.Equal("System.Int32 Test.I1.P1.get", derived.FindImplementationForInterfaceMember(p1.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P2.get", derived.FindImplementationForInterfaceMember(p2.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P3.get", derived.FindImplementationForInterfaceMember(p3.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P4.get", derived.FindImplementationForInterfaceMember(p4.GetMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.P5.set", derived.FindImplementationForInterfaceMember(p5.SetMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.P6.set", derived.FindImplementationForInterfaceMember(p6.SetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P7.get", derived.FindImplementationForInterfaceMember(p7.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1.P8.get", derived.FindImplementationForInterfaceMember(p8.GetMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.P7.set", derived.FindImplementationForInterfaceMember(p7.SetMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.P8.set", derived.FindImplementationForInterfaceMember(p8.SetMethod).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"100
200
300
400
500
600
700
701
800
801
",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_204(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P2 => 2;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P4 { get => 4; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P6 { set => System.Console.WriteLine(6); }
    " + declModifiers + @"int P7 { get { return 7;} set {} }
    " + declModifiers + @"int P8 { get { return 8;} set {} }
}

class Base : Test
{
    " + implModifiers + @"new int P1 => 10;
    " + implModifiers + @"new int P3 { get => 30; }
    " + implModifiers + @"new int P5 { set => System.Console.WriteLine(50); }
    " + implModifiers + @"new int P7 { get { return 70;} set {} }
}

class Derived : Base, I1
{
    " + implModifiers + @"new int P2 => 20;
    " + implModifiers + @"new int P4 { get => 40; }
    " + implModifiers + @"new int P6 { set => System.Console.WriteLine(60); }
    " + implModifiers + @"new int P8 { get { return 80;} set {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        System.Console.WriteLine(i1.P1);
        System.Console.WriteLine(i1.P2);
        System.Console.WriteLine(i1.P3);
        System.Console.WriteLine(i1.P4);
        i1.P5 = 0;
        i1.P6 = 0;
        System.Console.WriteLine(i1.P7);
        i1.P7 = 0;
        System.Console.WriteLine(i1.P8);
        i1.P8 = 0;
    }
}

class Test : I1 
{
    " + implModifiers + @"public int P1 => 100;
    " + implModifiers + @"public int P2 => 200;
    " + implModifiers + @"public int P3 { get => 300; }
    " + implModifiers + @"public int P4 { get => 400; }
    " + implModifiers + @"public int P5 { set => System.Console.WriteLine(500); }
    " + implModifiers + @"public int P6 { set => System.Console.WriteLine(600); }
    " + implModifiers + @"public int P7 { get { return 700;} set {System.Console.WriteLine(701);} }
    " + implModifiers + @"public int P8 { get { return 800;} set {System.Console.WriteLine(801);} }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var p1 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P1");
                var p2 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P2");
                var p3 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P3");
                var p4 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P4");
                var p5 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P5");
                var p6 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P6");
                var p7 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P7");
                var p8 = m.GlobalNamespace.GetMember<PropertySymbol>("I1.P8");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("System.Int32 Test.P1 { get; }", derived.FindImplementationForInterfaceMember(p1).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P2 { get; }", derived.FindImplementationForInterfaceMember(p2).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P3 { get; }", derived.FindImplementationForInterfaceMember(p3).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P4 { get; }", derived.FindImplementationForInterfaceMember(p4).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P5 { set; }", derived.FindImplementationForInterfaceMember(p5).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P6 { set; }", derived.FindImplementationForInterfaceMember(p6).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P7 { get; set; }", derived.FindImplementationForInterfaceMember(p7).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P8 { get; set; }", derived.FindImplementationForInterfaceMember(p8).ToTestDisplayString());

                Assert.Equal("System.Int32 Test.P1.get", derived.FindImplementationForInterfaceMember(p1.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P2.get", derived.FindImplementationForInterfaceMember(p2.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P3.get", derived.FindImplementationForInterfaceMember(p3.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P4.get", derived.FindImplementationForInterfaceMember(p4.GetMethod).ToTestDisplayString());
                Assert.Equal("void Test.P5.set", derived.FindImplementationForInterfaceMember(p5.SetMethod).ToTestDisplayString());
                Assert.Equal("void Test.P6.set", derived.FindImplementationForInterfaceMember(p6.SetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P7.get", derived.FindImplementationForInterfaceMember(p7.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.P8.get", derived.FindImplementationForInterfaceMember(p8.GetMethod).ToTestDisplayString());
                Assert.Equal("void Test.P7.set", derived.FindImplementationForInterfaceMember(p7.SetMethod).ToTestDisplayString());
                Assert.Equal("void Test.P8.set", derived.FindImplementationForInterfaceMember(p8.SetMethod).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"100
200
300
400
500
600
700
701
800
801
",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_501(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P3 
    { get => 3; }
    " + declModifiers + @"int P5 
    { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P7 
    { 
        get { return 7;} 
        set {} 
    }
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (4,15): error CS8501: Target runtime doesn't support default interface implementation.
                    //     int P1 => 1;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "1").WithLocation(4, 15),
                    // (6,7): error CS8501: Target runtime doesn't support default interface implementation.
                    //     { get => 3; }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(6, 7),
                    // (8,7): error CS8501: Target runtime doesn't support default interface implementation.
                    //     { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(8, 7),
                    // (11,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         get { return 7;} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(11, 9),
                    // (12,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         set {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(12, 9)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,30): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual int P1 => 1;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "1").WithLocation(4, 30),
                    // (6,7): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     { get => 3; }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "get").WithLocation(6, 7),
                    // (8,7): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "set").WithLocation(8, 7),
                    // (11,9): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //         get { return 7;} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "get").WithLocation(11, 9),
                    // (12,9): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //         set {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "set").WithLocation(12, 9)
                    );
            }

            ValidatePropertyImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8502: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8929: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2").WithLocation(2, 15)
                    );
            }

            ValidatePropertyImplementation_501(compilation3.SourceModule, "Test2");
        }

        private static void ValidatePropertyImplementation_501(ModuleSymbol m, string typeName)
        {
            var derived = m.GlobalNamespace.GetTypeMember(typeName);
            var i1 = derived.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.ToTestDisplayString());

            var p1 = i1.GetMember<PropertySymbol>("P1");
            var p3 = i1.GetMember<PropertySymbol>("P3");
            var p5 = i1.GetMember<PropertySymbol>("P5");
            var p7 = i1.GetMember<PropertySymbol>("P7");

            Assert.True(p1.IsVirtual);
            Assert.True(p3.IsVirtual);
            Assert.True(p5.IsVirtual);
            Assert.True(p7.IsVirtual);

            Assert.False(p1.IsAbstract);
            Assert.False(p3.IsAbstract);
            Assert.False(p5.IsAbstract);
            Assert.False(p7.IsAbstract);

            Assert.Same(p1, derived.FindImplementationForInterfaceMember(p1));
            Assert.Same(p3, derived.FindImplementationForInterfaceMember(p3));
            Assert.Same(p5, derived.FindImplementationForInterfaceMember(p5));
            Assert.Same(p7, derived.FindImplementationForInterfaceMember(p7));

            Assert.True(p1.GetMethod.IsVirtual);
            Assert.True(p3.GetMethod.IsVirtual);
            Assert.True(p5.SetMethod.IsVirtual);
            Assert.True(p7.GetMethod.IsVirtual);
            Assert.True(p7.SetMethod.IsVirtual);

            Assert.True(p1.GetMethod.IsMetadataVirtual());
            Assert.True(p3.GetMethod.IsMetadataVirtual());
            Assert.True(p5.SetMethod.IsMetadataVirtual());
            Assert.True(p7.GetMethod.IsMetadataVirtual());
            Assert.True(p7.SetMethod.IsMetadataVirtual());

            Assert.False(p1.GetMethod.IsAbstract);
            Assert.False(p3.GetMethod.IsAbstract);
            Assert.False(p5.SetMethod.IsAbstract);
            Assert.False(p7.GetMethod.IsAbstract);
            Assert.False(p7.SetMethod.IsAbstract);

            Assert.Same(p1.GetMethod, derived.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Same(p3.GetMethod, derived.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Same(p5.SetMethod, derived.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Same(p7.GetMethod, derived.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Same(p7.SetMethod, derived.FindImplementationForInterfaceMember(p7.SetMethod));
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_502(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P7 { get { return 7;} set {} }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() },
                                                 options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8502: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8929: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2").WithLocation(2, 15)
                    );
            }

            ValidatePropertyImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_503(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P7 { get { return 7;} set {} }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
public interface I2
{
    void M2();
}

class Test2 : I2
{
    public void M2() {}
}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() },
                                                 targetFramework: TargetFramework.Mscorlib461Extended, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            var test2 = compilation3.GetTypeByMetadataName("Test2");
            var i1 = compilation3.GetTypeByMetadataName("I1");
            Assert.Equal("I1", i1.ToTestDisplayString());

            var p1 = i1.GetMember<PropertySymbol>("P1");
            var p3 = i1.GetMember<PropertySymbol>("P3");
            var p5 = i1.GetMember<PropertySymbol>("P5");
            var p7 = i1.GetMember<PropertySymbol>("P7");

            Assert.Null(test2.FindImplementationForInterfaceMember(p1));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5));
            Assert.Null(test2.FindImplementationForInterfaceMember(p7));

            Assert.Null(test2.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p7.SetMethod));

            compilation3.VerifyDiagnostics();
        }

        [Fact]
        public void PropertyImplementation_601()
        {
            {
                var source1 =
    @"
public interface I1
{
    int P1 => 1;
    int P3 { get => 3; }
    int P5 { set => System.Console.WriteLine(5); }
    int P7 { get { return 7;} set {} }
}

class Test1 : I1
{}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: TestOptions.Regular7_3, skipUsesIsNullable: true);
                Assert.False(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation1.VerifyDiagnostics(
                    // (4,15): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P1 => 1;
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "1").WithArguments("default interface implementation", "8.0").WithLocation(4, 15),
                    // (4,15): error CS8701: Target runtime doesn't support default interface implementation.
                    //     int P1 => 1;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "1").WithLocation(4, 15),
                    // (5,14): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P3 { get => 3; }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(5, 14),
                    // (5,14): error CS8701: Target runtime doesn't support default interface implementation.
                    //     int P3 { get => 3; }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(5, 14),
                    // (6,14): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P5 { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(6, 14),
                    // (6,14): error CS8701: Target runtime doesn't support default interface implementation.
                    //     int P5 { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(6, 14),
                    // (7,14): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(7, 14),
                    // (7,14): error CS8701: Target runtime doesn't support default interface implementation.
                    //     int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(7, 14),
                    // (7,31): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(7, 31),
                    // (7,31): error CS8701: Target runtime doesn't support default interface implementation.
                    //     int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(7, 31)
                    );

                ValidatePropertyImplementation_501(compilation1.SourceModule, "Test1");

                var source2 =
    @"
class Test2 : I1
{}
";

                var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                     options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: TestOptions.Regular7_3);
                Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8506: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2").WithLocation(2, 15)
                    );

                ValidatePropertyImplementation_501(compilation3.SourceModule, "Test2");
            }

            {
                var source1 =
    @"
public interface I1
{
    static virtual int P1 => 1;
    static virtual int P3 { get => 3; }
    static virtual int P5 { set => System.Console.WriteLine(5); }
    static virtual int P7 { get { return 7;} set {} }
}

class Test1 : I1
{}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                     parseOptions: TestOptions.Regular10, skipUsesIsNullable: true);

                compilation1.VerifyDiagnostics(
                    // (4,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P1 => 1;
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("virtual", "10.0", "11.0").WithLocation(4, 24),
                    // (4,30): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual int P1 => 1;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "1").WithLocation(4, 30),
                    // (5,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P3 { get => 3; }
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("virtual", "10.0", "11.0").WithLocation(5, 24),
                    // (5,29): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual int P3 { get => 3; }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "get").WithLocation(5, 29),
                    // (6,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P5 { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P5").WithArguments("virtual", "10.0", "11.0").WithLocation(6, 24),
                    // (6,29): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual int P5 { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "set").WithLocation(6, 29),
                    // (7,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P7").WithArguments("virtual", "10.0", "11.0").WithLocation(7, 24),
                    // (7,29): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "get").WithLocation(7, 29),
                    // (7,46): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static virtual int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "set").WithLocation(7, 46)
                    );

                ValidatePropertyImplementation_501(compilation1.SourceModule, "Test1");

                var source2 =
    @"
class Test2 : I1
{}
";

                var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                     options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                     parseOptions: TestOptions.Regular10);

                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8706: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2").WithLocation(2, 15)
                    );

                ValidatePropertyImplementation_501(compilation3.SourceModule, "Test2");
            }
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementation_701(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"int P1 => 1;
    " + declModifiers + @"int P3 { get => 3; }
    " + declModifiers + @"int P5 { set => System.Console.WriteLine(5); }
    " + declModifiers + @"int P7 { get { return 7;} set {} }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (4,15): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P1 => 1;
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "1").WithArguments("default interface implementation", "8.0").WithLocation(4, 15),
                    // (5,14): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P3 { get => 3; }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(5, 14),
                    // (6,14): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P5 { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(6, 14),
                    // (7,14): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(7, 14),
                    // (7,31): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(7, 31)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P1 => 1;
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("virtual", "10.0", "11.0").WithLocation(4, 24),
                    // (5,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P3 { get => 3; }
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("virtual", "10.0", "11.0").WithLocation(5, 24),
                    // (6,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P5 { set => System.Console.WriteLine(5); }
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P5").WithArguments("virtual", "10.0", "11.0").WithLocation(6, 24),
                    // (7,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual int P7 { get { return 7;} set {} }
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P7").WithArguments("virtual", "10.0", "11.0").WithLocation(7, 24)
                    );
            }

            ValidatePropertyImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidatePropertyImplementation_501(compilation2.SourceModule, "Test2");

            CompileAndVerify(compilation2, verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test2Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test2");
                    Assert.Equal("I1", test2Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                    ValidatePropertyImplementation_501(m, "Test2");
                });

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8506: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8706: 'I1.P1.get' cannot implement interface member 'I1.P1.get' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P1.get", "I1.P1.get", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P3.get' cannot implement interface member 'I1.P3.get' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P3.get", "I1.P3.get", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P5.set' cannot implement interface member 'I1.P5.set' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P5.set", "I1.P5.set", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P7.get' cannot implement interface member 'I1.P7.get' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.get", "I1.P7.get", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.P7.set' cannot implement interface member 'I1.P7.set' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.P7.set", "I1.P7.set", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15)
                    );
            }

            ValidatePropertyImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Fact]
        public void PropertyImplementation_901()
        {
            var source1 =
@"
public interface I1
{
    static int P1 => 1;
    static int P3 { get => 3; }
    static int P5 { set => System.Console.WriteLine(5); }
    static int P7 { get { return 7;} set {} }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,16): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static int P1 => 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "P1").WithArguments("default interface implementation", "8.0").WithLocation(4, 16),
                // (5,16): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static int P3 { get => 3; }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "P3").WithArguments("default interface implementation", "8.0").WithLocation(5, 16),
                // (6,16): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static int P5 { set => System.Console.WriteLine(5); }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "P5").WithArguments("default interface implementation", "8.0").WithLocation(6, 16),
                // (7,16): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static int P7 { get { return 7;} set {} }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "P7").WithArguments("default interface implementation", "8.0").WithLocation(7, 16)
                );

            var derived = compilation1.SourceModule.GlobalNamespace.GetTypeMember("Test1");
            var i1 = derived.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.ToTestDisplayString());

            var p1 = i1.GetMember<PropertySymbol>("P1");
            var p3 = i1.GetMember<PropertySymbol>("P3");
            var p5 = i1.GetMember<PropertySymbol>("P5");
            var p7 = i1.GetMember<PropertySymbol>("P7");

            Assert.True(p1.IsStatic);
            Assert.True(p3.IsStatic);
            Assert.True(p5.IsStatic);
            Assert.True(p7.IsStatic);

            Assert.False(p1.IsVirtual);
            Assert.False(p3.IsVirtual);
            Assert.False(p5.IsVirtual);
            Assert.False(p7.IsVirtual);

            Assert.False(p1.IsAbstract);
            Assert.False(p3.IsAbstract);
            Assert.False(p5.IsAbstract);
            Assert.False(p7.IsAbstract);

            Assert.Null(derived.FindImplementationForInterfaceMember(p1));
            Assert.Null(derived.FindImplementationForInterfaceMember(p3));
            Assert.Null(derived.FindImplementationForInterfaceMember(p5));
            Assert.Null(derived.FindImplementationForInterfaceMember(p7));

            Assert.True(p1.GetMethod.IsStatic);
            Assert.True(p3.GetMethod.IsStatic);
            Assert.True(p5.SetMethod.IsStatic);
            Assert.True(p7.GetMethod.IsStatic);
            Assert.True(p7.SetMethod.IsStatic);

            Assert.False(p1.GetMethod.IsVirtual);
            Assert.False(p3.GetMethod.IsVirtual);
            Assert.False(p5.SetMethod.IsVirtual);
            Assert.False(p7.GetMethod.IsVirtual);
            Assert.False(p7.SetMethod.IsVirtual);

            Assert.False(p1.GetMethod.IsMetadataVirtual());
            Assert.False(p3.GetMethod.IsMetadataVirtual());
            Assert.False(p5.SetMethod.IsMetadataVirtual());
            Assert.False(p7.GetMethod.IsMetadataVirtual());
            Assert.False(p7.SetMethod.IsMetadataVirtual());

            Assert.False(p1.GetMethod.IsAbstract);
            Assert.False(p3.GetMethod.IsAbstract);
            Assert.False(p5.SetMethod.IsAbstract);
            Assert.False(p7.GetMethod.IsAbstract);
            Assert.False(p7.SetMethod.IsAbstract);

            Assert.Null(derived.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Null(derived.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Null(derived.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Null(derived.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Null(derived.FindImplementationForInterfaceMember(p7.SetMethod));
        }

        [Fact]
        public void IndexerImplementation_101()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1[0];
    }
}
";
            ValidateIndexerImplementation_101(source1);
        }

        private void ValidateIndexerImplementation_101(string source1)
        {
            ValidatePropertyImplementation_101(source1, "this[]", haveGet: true, haveSet: false,
                accessCode: @"
        _ = i1[0];
",
                expectedOutput: "get P1");
        }

        [Fact]
        public void IndexerImplementation_102()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1[0] = i1[0];
    }
}
";
            ValidateIndexerImplementation_102(source1);
        }

        private void ValidateIndexerImplementation_102(string source1)
        {
            ValidatePropertyImplementation_101(source1, "this[]", haveGet: true, haveSet: true,
                accessCode: @"
        i1[0] = i1[0];
",
                expectedOutput:
@"get P1
set P1");
        }

        [Fact]
        public void IndexerImplementation_103()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] 
    {
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1[0] = 1;
    }
}
";
            ValidateIndexerImplementation_103(source1);
        }

        private void ValidateIndexerImplementation_103(string source1)
        {
            ValidatePropertyImplementation_101(source1, "this[]", haveGet: false, haveSet: true,
                accessCode: @"
        i1[0] = 1;
",
                expectedOutput: "set P1");
        }

        [Fact]
        public void IndexerImplementation_104()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] => Test1.GetInt();
}

class Test1 : I1
{
    public static int GetInt()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1[0];
    }
}
";
            ValidateIndexerImplementation_101(source1);
        }

        [Fact]
        public void IndexerImplementation_105()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] {add; remove;} => 0;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,22): error CS1014: A get, set or init accessor expected
                //     int this[int i] {add; remove;} => 0;
                Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 22),
                // (4,27): error CS1014: A get, set or init accessor expected
                //     int this[int i] {add; remove;} => 0;
                Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 27),
                // (4,9): error CS0548: 'I1.this[int]': property or indexer must have at least one accessor
                //     int this[int i] {add; remove;} => 0;
                Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "this").WithArguments("I1.this[int]").WithLocation(4, 9),
                // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                //     int this[int i] {add; remove;} => 0;
                Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "int this[int i] {add; remove;} => 0;").WithLocation(4, 5)
                );

            var p1 = compilation1.GetMember<PropertySymbol>("I1.this[]");
            Assert.True(p1.IsAbstract);
            Assert.Null(p1.GetMethod);
            Assert.Null(p1.SetMethod);
            Assert.True(p1.IsReadOnly);
            Assert.True(p1.IsWriteOnly);
        }

        [Fact]
        public void IndexerImplementation_106()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] {get; set;} => 0;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                //     int this[int i] {get; set;} => 0;
                Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "int this[int i] {get; set;} => 0;").WithLocation(4, 5)
                );

            var p1 = compilation1.GetMember<PropertySymbol>("I1.this[]");
            Assert.True(p1.IsAbstract);
            Assert.True(p1.GetMethod.IsAbstract);
            Assert.True(p1.SetMethod.IsAbstract);
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);
        }

        [Fact]
        public void IndexerImplementation_107()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] {add; remove;} = 0;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,36): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     int this[int i] {add; remove;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(4, 36),
                // (4,22): error CS1014: A get, set or init accessor expected
                //     int this[int i] {add; remove;} = 0;
                Diagnostic(ErrorCode.ERR_GetOrSetExpected, "add").WithLocation(4, 22),
                // (4,27): error CS1014: A get, set or init accessor expected
                //     int this[int i] {add; remove;} = 0;
                Diagnostic(ErrorCode.ERR_GetOrSetExpected, "remove").WithLocation(4, 27),
                // (4,9): error CS0548: 'I1.this[int]': property or indexer must have at least one accessor
                //     int this[int i] {add; remove;} = 0;
                Diagnostic(ErrorCode.ERR_PropertyWithNoAccessors, "this").WithArguments("I1.this[int]").WithLocation(4, 9)
                );

            var p1 = compilation1.GetMember<PropertySymbol>("I1.this[]");
            Assert.True(p1.IsAbstract);
            Assert.Null(p1.GetMethod);
            Assert.Null(p1.SetMethod);
            Assert.True(p1.IsReadOnly);
            Assert.True(p1.IsWriteOnly);
        }

        [Fact]
        public void IndexerImplementation_108()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] {get; set;} = 0;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,33): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     int this[int i] {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(4, 33)
                );

            var p1 = compilation1.GetMember<PropertySymbol>("I1.this[]");
            Assert.True(p1.IsAbstract);
            Assert.True(p1.GetMethod.IsAbstract);
            Assert.True(p1.SetMethod.IsAbstract);
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);
        }

        [Fact]
        public void IndexerImplementation_109()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set;
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            // According to LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-04-18.md,
            // we don't want to allow only one accessor to have an implementation.
            compilation1.VerifyDiagnostics(
                // (11,9): error CS0501: 'I1.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                //         set;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.this[int].set")
                );

            var p1 = compilation1.GetMember<PropertySymbol>("I1.this[]");
            var getP1 = p1.GetMethod;
            var setP1 = p1.SetMethod;
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);

            Assert.False(p1.IsAbstract);
            Assert.True(p1.IsVirtual);
            Assert.False(getP1.IsAbstract);
            Assert.True(getP1.IsVirtual);
            Assert.False(setP1.IsAbstract);
            Assert.True(setP1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(getP1, test1.FindImplementationForInterfaceMember(getP1));
            Assert.Same(setP1, test1.FindImplementationForInterfaceMember(setP1));

            Assert.True(getP1.IsMetadataVirtual());
            Assert.True(setP1.IsMetadataVirtual());
        }

        [Fact]
        public void IndexerImplementation_110()
        {
            var source1 =
@"
public interface I1
{
    int this[int i] 
    {
        get;
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            // According to LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-04-18.md,
            // we don't want to allow only one accessor to have an implementation.
            compilation1.VerifyDiagnostics(
                // (6,9): error CS0501: 'I1.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                //         get;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.this[int].get")
                );

            var p1 = compilation1.GetMember<PropertySymbol>("I1.this[]");
            var getP1 = p1.GetMethod;
            var setP1 = p1.SetMethod;
            Assert.False(p1.IsReadOnly);
            Assert.False(p1.IsWriteOnly);

            Assert.False(p1.IsAbstract);
            Assert.True(p1.IsVirtual);
            Assert.False(getP1.IsAbstract);
            Assert.True(getP1.IsVirtual);
            Assert.False(setP1.IsAbstract);
            Assert.True(setP1.IsVirtual);

            var test1 = compilation1.GetTypeByMetadataName("Test1");

            Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(getP1, test1.FindImplementationForInterfaceMember(getP1));
            Assert.Same(setP1, test1.FindImplementationForInterfaceMember(setP1));

            Assert.True(getP1.IsMetadataVirtual());
            Assert.True(setP1.IsMetadataVirtual());
        }

        [Fact]
        public void IndexerImplementation_201()
        {
            var source1 =
@"
interface I1
{
    int this[sbyte i] => 1;
    int this[byte i] => 2;
    int this[short i] { get => 3; }
    int this[ushort i] { get => 4; }
    int this[int i] { set => System.Console.WriteLine(5); }
    int this[uint i] { set => System.Console.WriteLine(6); }
    int this[long i] { get { return 7;} set {System.Console.WriteLine(71);} }
    int this[ulong i] { get { return 8;} set {System.Console.WriteLine(81);} }
}

class Base
{
    int this[sbyte i] => 10;
    int this[short i] { get => 30; }
    int this[int i] { set => System.Console.WriteLine(50); }
    int this[long i] { get { return 70;} set {} }
}

class Derived : Base, I1
{
    int this[byte i] => 20;
    int this[ushort i] { get => 40; }
    int this[uint i] { set => System.Console.WriteLine(60); }
    int this[ulong i] { get { return 80;} set {} }

    static void Main()
    {
        I1 i1 = new Derived();
        System.Console.WriteLine(i1[(sbyte)0]);
        System.Console.WriteLine(i1[(byte)0]);
        System.Console.WriteLine(i1[(short)0]);
        System.Console.WriteLine(i1[(ushort)0]);
        i1[(int)0] = 0;
        i1[(uint)0] = 0;
        System.Console.WriteLine(i1[(long)0]);
        i1[(long)0] = 0;
        System.Console.WriteLine(i1[(ulong)0]);
        i1[(ulong)0] = 0;
    }
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            ValidateIndexerImplementation_201(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"1
2
3
4
5
6
7
71
8
81
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    ValidateIndexerImplementation_201(m);
                });
        }

        private static void ValidateIndexerImplementation_201(ModuleSymbol m)
        {
            var i1 = m.GlobalNamespace.GetTypeMember("I1");
            var indexers = i1.GetMembers("this[]");
            var p1 = (PropertySymbol)indexers[0];
            var p2 = (PropertySymbol)indexers[1];
            var p3 = (PropertySymbol)indexers[2];
            var p4 = (PropertySymbol)indexers[3];
            var p5 = (PropertySymbol)indexers[4];
            var p6 = (PropertySymbol)indexers[5];
            var p7 = (PropertySymbol)indexers[6];
            var p8 = (PropertySymbol)indexers[7];

            var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

            Assert.Same(p1, derived.FindImplementationForInterfaceMember(p1));
            Assert.Same(p2, derived.FindImplementationForInterfaceMember(p2));
            Assert.Same(p3, derived.FindImplementationForInterfaceMember(p3));
            Assert.Same(p4, derived.FindImplementationForInterfaceMember(p4));
            Assert.Same(p5, derived.FindImplementationForInterfaceMember(p5));
            Assert.Same(p6, derived.FindImplementationForInterfaceMember(p6));
            Assert.Same(p7, derived.FindImplementationForInterfaceMember(p7));
            Assert.Same(p8, derived.FindImplementationForInterfaceMember(p8));

            Assert.Same(p1.GetMethod, derived.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Same(p2.GetMethod, derived.FindImplementationForInterfaceMember(p2.GetMethod));
            Assert.Same(p3.GetMethod, derived.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Same(p4.GetMethod, derived.FindImplementationForInterfaceMember(p4.GetMethod));
            Assert.Same(p5.SetMethod, derived.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Same(p6.SetMethod, derived.FindImplementationForInterfaceMember(p6.SetMethod));
            Assert.Same(p7.GetMethod, derived.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Same(p8.GetMethod, derived.FindImplementationForInterfaceMember(p8.GetMethod));
            Assert.Same(p7.SetMethod, derived.FindImplementationForInterfaceMember(p7.SetMethod));
            Assert.Same(p8.SetMethod, derived.FindImplementationForInterfaceMember(p8.SetMethod));
        }

        [Fact]
        public void IndexerImplementation_202()
        {
            var source1 =
@"
interface I1
{
    int this[sbyte i] => 1;
    int this[byte i] => 2;
    int this[short i] { get => 3; }
    int this[ushort i] { get => 4; }
    int this[int i] { set => System.Console.WriteLine(5); }
    int this[uint i] { set => System.Console.WriteLine(6); }
    int this[long i] { get { return 7;} set {System.Console.WriteLine(71);} }
    int this[ulong i] { get { return 8;} set {System.Console.WriteLine(81);} }
}

class Base : Test
{
    int this[sbyte i] => 10;
    int this[short i] { get => 30; }
    int this[int i] { set => System.Console.WriteLine(50); }
    int this[long i] { get { return 70;} set {} }
}

class Derived : Base, I1
{
    int this[byte i] => 20;
    int this[ushort i] { get => 40; }
    int this[uint i] { set => System.Console.WriteLine(60); }
    int this[ulong i] { get { return 80;} set {} }

    static void Main()
    {
        I1 i1 = new Derived();
        System.Console.WriteLine(i1[(sbyte)0]);
        System.Console.WriteLine(i1[(byte)0]);
        System.Console.WriteLine(i1[(short)0]);
        System.Console.WriteLine(i1[(ushort)0]);
        i1[(int)0] = 0;
        i1[(uint)0] = 0;
        System.Console.WriteLine(i1[(long)0]);
        i1[(long)0] = 0;
        System.Console.WriteLine(i1[(ulong)0]);
        i1[(ulong)0] = 0;
    }
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            ValidateIndexerImplementation_201(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"1
2
3
4
5
6
7
71
8
81
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    ValidateIndexerImplementation_201(m);
                });
        }

        [Fact]
        public void IndexerImplementation_203()
        {
            var source1 =
@"
interface I1
{
    int this[sbyte i] => 1;
    int this[byte i] => 2;
    int this[short i] { get => 3; }
    int this[ushort i] { get => 4; }
    int this[int i] { set => System.Console.WriteLine(5); }
    int this[uint i] { set => System.Console.WriteLine(6); }
    int this[long i] { get { return 7;} set {} }
    int this[ulong i] { get { return 8;} set {} }
}

class Base : Test
{
    int this[sbyte i] => 10;
    int this[short i] { get => 30; }
    int this[int i] { set => System.Console.WriteLine(50); }
    int this[long i] { get { return 70;} set {} }
}

class Derived : Base, I1
{
    int this[byte i] => 20;
    int this[ushort i] { get => 40; }
    int this[uint i] { set => System.Console.WriteLine(60); }
    int this[ulong i] { get { return 80;} set {} }

    static void Main()
    {
        I1 i1 = new Derived();
        System.Console.WriteLine(i1[(sbyte)0]);
        System.Console.WriteLine(i1[(byte)0]);
        System.Console.WriteLine(i1[(short)0]);
        System.Console.WriteLine(i1[(ushort)0]);
        i1[(int)0] = 0;
        i1[(uint)0] = 0;
        System.Console.WriteLine(i1[(long)0]);
        i1[(long)0] = 0;
        System.Console.WriteLine(i1[(ulong)0]);
        i1[(ulong)0] = 0;
    }
}

class Test : I1 
{
    int I1.this[sbyte i] => 100;
    int I1.this[byte i] => 200;
    int I1.this[short i] { get => 300; }
    int I1.this[ushort i] { get => 400; }
    int I1.this[int i] { set => System.Console.WriteLine(500); }
    int I1.this[uint i] { set => System.Console.WriteLine(600); }
    int I1.this[long i] { get { return 700;} set {System.Console.WriteLine(701);} }
    int I1.this[ulong i] { get { return 800;} set {System.Console.WriteLine(801);} }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var indexers = i1.GetMembers("this[]");
                var p1 = (PropertySymbol)indexers[0];
                var p2 = (PropertySymbol)indexers[1];
                var p3 = (PropertySymbol)indexers[2];
                var p4 = (PropertySymbol)indexers[3];
                var p5 = (PropertySymbol)indexers[4];
                var p6 = (PropertySymbol)indexers[5];
                var p7 = (PropertySymbol)indexers[6];
                var p8 = (PropertySymbol)indexers[7];

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                string name = m is PEModuleSymbol ? "Item" : "this";

                Assert.Equal("System.Int32 Test.I1." + name + "[System.SByte i] { get; }", derived.FindImplementationForInterfaceMember(p1).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.Byte i] { get; }", derived.FindImplementationForInterfaceMember(p2).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.Int16 i] { get; }", derived.FindImplementationForInterfaceMember(p3).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.UInt16 i] { get; }", derived.FindImplementationForInterfaceMember(p4).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.Int32 i] { set; }", derived.FindImplementationForInterfaceMember(p5).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.UInt32 i] { set; }", derived.FindImplementationForInterfaceMember(p6).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.Int64 i] { get; set; }", derived.FindImplementationForInterfaceMember(p7).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.I1." + name + "[System.UInt64 i] { get; set; }", derived.FindImplementationForInterfaceMember(p8).ToTestDisplayString());

                if (m is PEModuleSymbol)
                {
                    Assert.Equal("System.Int32 Test.I1.get_Item(System.SByte i)", derived.FindImplementationForInterfaceMember(p1.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.get_Item(System.Byte i)", derived.FindImplementationForInterfaceMember(p2.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.get_Item(System.Int16 i)", derived.FindImplementationForInterfaceMember(p3.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.get_Item(System.UInt16 i)", derived.FindImplementationForInterfaceMember(p4.GetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.set_Item(System.Int32 i, System.Int32 value)", derived.FindImplementationForInterfaceMember(p5.SetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.set_Item(System.UInt32 i, System.Int32 value)", derived.FindImplementationForInterfaceMember(p6.SetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.get_Item(System.Int64 i)", derived.FindImplementationForInterfaceMember(p7.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.get_Item(System.UInt64 i)", derived.FindImplementationForInterfaceMember(p8.GetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.set_Item(System.Int64 i, System.Int32 value)", derived.FindImplementationForInterfaceMember(p7.SetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.set_Item(System.UInt64 i, System.Int32 value)", derived.FindImplementationForInterfaceMember(p8.SetMethod).ToTestDisplayString());
                }
                else
                {
                    Assert.Equal("System.Int32 Test.I1.this[System.SByte i].get", derived.FindImplementationForInterfaceMember(p1.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.this[System.Byte i].get", derived.FindImplementationForInterfaceMember(p2.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.this[System.Int16 i].get", derived.FindImplementationForInterfaceMember(p3.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.this[System.UInt16 i].get", derived.FindImplementationForInterfaceMember(p4.GetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.this[System.Int32 i].set", derived.FindImplementationForInterfaceMember(p5.SetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.this[System.UInt32 i].set", derived.FindImplementationForInterfaceMember(p6.SetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.this[System.Int64 i].get", derived.FindImplementationForInterfaceMember(p7.GetMethod).ToTestDisplayString());
                    Assert.Equal("System.Int32 Test.I1.this[System.UInt64 i].get", derived.FindImplementationForInterfaceMember(p8.GetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.this[System.Int64 i].set", derived.FindImplementationForInterfaceMember(p7.SetMethod).ToTestDisplayString());
                    Assert.Equal("void Test.I1.this[System.UInt64 i].set", derived.FindImplementationForInterfaceMember(p8.SetMethod).ToTestDisplayString());
                }
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"100
200
300
400
500
600
700
701
800
801
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Fact]
        public void IndexerImplementation_204()
        {
            var source1 =
@"
interface I1
{
    int this[sbyte i] => 1;
    int this[byte i] => 2;
    int this[short i] { get => 3; }
    int this[ushort i] { get => 4; }
    int this[int i] { set => System.Console.WriteLine(5); }
    int this[uint i] { set => System.Console.WriteLine(6); }
    int this[long i] { get { return 7;} set {} }
    int this[ulong i] { get { return 8;} set {} }
}

class Base : Test
{
    new int this[sbyte i] => 10;
    new int this[short i] { get => 30; }
    new int this[int i] { set => System.Console.WriteLine(50); }
    new int this[long i] { get { return 70;} set {} }
}

class Derived : Base, I1
{
    new int this[byte i] => 20;
    new int this[ushort i] { get => 40; }
    new int this[uint i] { set => System.Console.WriteLine(60); }
    new int this[ulong i] { get { return 80;} set {} }

    static void Main()
    {
        I1 i1 = new Derived();
        System.Console.WriteLine(i1[(sbyte)0]);
        System.Console.WriteLine(i1[(byte)0]);
        System.Console.WriteLine(i1[(short)0]);
        System.Console.WriteLine(i1[(ushort)0]);
        i1[(int)0] = 0;
        i1[(uint)0] = 0;
        System.Console.WriteLine(i1[(long)0]);
        i1[(long)0] = 0;
        System.Console.WriteLine(i1[(ulong)0]);
        i1[(ulong)0] = 0;
    }
}

class Test : I1 
{
    public int this[sbyte i] => 100;
    public int this[byte i] => 200;
    public int this[short i] { get => 300; }
    public int this[ushort i] { get => 400; }
    public int this[int i] { set => System.Console.WriteLine(500); }
    public int this[uint i] { set => System.Console.WriteLine(600); }
    public int this[long i] { get { return 700;} set {System.Console.WriteLine(701);} }
    public int this[ulong i] { get { return 800;} set {System.Console.WriteLine(801);} }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var indexers = i1.GetMembers("this[]");
                var p1 = (PropertySymbol)indexers[0];
                var p2 = (PropertySymbol)indexers[1];
                var p3 = (PropertySymbol)indexers[2];
                var p4 = (PropertySymbol)indexers[3];
                var p5 = (PropertySymbol)indexers[4];
                var p6 = (PropertySymbol)indexers[5];
                var p7 = (PropertySymbol)indexers[6];
                var p8 = (PropertySymbol)indexers[7];

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("System.Int32 Test.this[System.SByte i] { get; }", derived.FindImplementationForInterfaceMember(p1).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Byte i] { get; }", derived.FindImplementationForInterfaceMember(p2).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Int16 i] { get; }", derived.FindImplementationForInterfaceMember(p3).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.UInt16 i] { get; }", derived.FindImplementationForInterfaceMember(p4).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Int32 i] { set; }", derived.FindImplementationForInterfaceMember(p5).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.UInt32 i] { set; }", derived.FindImplementationForInterfaceMember(p6).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Int64 i] { get; set; }", derived.FindImplementationForInterfaceMember(p7).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.UInt64 i] { get; set; }", derived.FindImplementationForInterfaceMember(p8).ToTestDisplayString());

                Assert.Equal("System.Int32 Test.this[System.SByte i].get", derived.FindImplementationForInterfaceMember(p1.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Byte i].get", derived.FindImplementationForInterfaceMember(p2.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Int16 i].get", derived.FindImplementationForInterfaceMember(p3.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.UInt16 i].get", derived.FindImplementationForInterfaceMember(p4.GetMethod).ToTestDisplayString());
                Assert.Equal("void Test.this[System.Int32 i].set", derived.FindImplementationForInterfaceMember(p5.SetMethod).ToTestDisplayString());
                Assert.Equal("void Test.this[System.UInt32 i].set", derived.FindImplementationForInterfaceMember(p6.SetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.Int64 i].get", derived.FindImplementationForInterfaceMember(p7.GetMethod).ToTestDisplayString());
                Assert.Equal("System.Int32 Test.this[System.UInt64 i].get", derived.FindImplementationForInterfaceMember(p8.GetMethod).ToTestDisplayString());
                Assert.Equal("void Test.this[System.Int64 i].set", derived.FindImplementationForInterfaceMember(p7.SetMethod).ToTestDisplayString());
                Assert.Equal("void Test.this[System.UInt64 i].set", derived.FindImplementationForInterfaceMember(p8.SetMethod).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"100
200
300
400
500
600
700
701
800
801
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Fact]
        public void IndexerImplementation_501()
        {
            var source1 =
@"
public interface I1
{
    int this[sbyte i] => 1;
    int this[short i] 
    { get => 3; }
    int this[int i] 
    { set => System.Console.WriteLine(5); }
    int this[long i] 
    { 
        get { return 7;} 
        set {} 
    }
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular, skipUsesIsNullable: true);
            Assert.False(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,26): error CS8501: Target runtime doesn't support default interface implementation.
                //     int this[sbyte i] => 1;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "1").WithLocation(4, 26),
                // (6,7): error CS8501: Target runtime doesn't support default interface implementation.
                //     { get => 3; }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(6, 7),
                // (8,7): error CS8501: Target runtime doesn't support default interface implementation.
                //     { set => System.Console.WriteLine(5); }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(8, 7),
                // (11,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         get { return 7;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(11, 9),
                // (12,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         set {} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(12, 9)
                );

            ValidateIndexerImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (2,15): error CS8502: 'I1.this[long].set' cannot implement interface member 'I1.this[long].set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].set", "I1.this[long].set", "Test2"),
                // (2,15): error CS8502: 'I1.this[sbyte].get' cannot implement interface member 'I1.this[sbyte].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[sbyte].get", "I1.this[sbyte].get", "Test2"),
                // (2,15): error CS8502: 'I1.this[short].get' cannot implement interface member 'I1.this[short].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[short].get", "I1.this[short].get", "Test2"),
                // (2,15): error CS8502: 'I1.this[int].set' cannot implement interface member 'I1.this[int].set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[int].set", "I1.this[int].set", "Test2"),
                // (2,15): error CS8502: 'I1.this[long].get' cannot implement interface member 'I1.this[long].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].get", "I1.this[long].get", "Test2")
                );

            ValidateIndexerImplementation_501(compilation3.SourceModule, "Test2");
        }

        private static void ValidateIndexerImplementation_501(ModuleSymbol m, string typeName)
        {
            var derived = m.GlobalNamespace.GetTypeMember(typeName);
            var i1 = derived.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.ToTestDisplayString());

            var indexers = i1.GetMembers("this[]");
            var p1 = (PropertySymbol)indexers[0];
            var p3 = (PropertySymbol)indexers[1];
            var p5 = (PropertySymbol)indexers[2];
            var p7 = (PropertySymbol)indexers[3];

            Assert.True(p1.IsVirtual);
            Assert.True(p3.IsVirtual);
            Assert.True(p5.IsVirtual);
            Assert.True(p7.IsVirtual);

            Assert.False(p1.IsAbstract);
            Assert.False(p3.IsAbstract);
            Assert.False(p5.IsAbstract);
            Assert.False(p7.IsAbstract);

            Assert.Same(p1, derived.FindImplementationForInterfaceMember(p1));
            Assert.Same(p3, derived.FindImplementationForInterfaceMember(p3));
            Assert.Same(p5, derived.FindImplementationForInterfaceMember(p5));
            Assert.Same(p7, derived.FindImplementationForInterfaceMember(p7));

            Assert.True(p1.GetMethod.IsVirtual);
            Assert.True(p3.GetMethod.IsVirtual);
            Assert.True(p5.SetMethod.IsVirtual);
            Assert.True(p7.GetMethod.IsVirtual);
            Assert.True(p7.SetMethod.IsVirtual);

            Assert.True(p1.GetMethod.IsMetadataVirtual());
            Assert.True(p3.GetMethod.IsMetadataVirtual());
            Assert.True(p5.SetMethod.IsMetadataVirtual());
            Assert.True(p7.GetMethod.IsMetadataVirtual());
            Assert.True(p7.SetMethod.IsMetadataVirtual());

            Assert.False(p1.GetMethod.IsAbstract);
            Assert.False(p3.GetMethod.IsAbstract);
            Assert.False(p5.SetMethod.IsAbstract);
            Assert.False(p7.GetMethod.IsAbstract);
            Assert.False(p7.SetMethod.IsAbstract);

            Assert.Same(p1.GetMethod, derived.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Same(p3.GetMethod, derived.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Same(p5.SetMethod, derived.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Same(p7.GetMethod, derived.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Same(p7.SetMethod, derived.FindImplementationForInterfaceMember(p7.SetMethod));
        }

        [Fact]
        public void IndexerImplementation_502()
        {
            var source1 =
@"
public interface I1
{
    int this[sbyte i] => 1;
    int this[short i] 
    { get => 3; }
    int this[int i] 
    { set => System.Console.WriteLine(5); }
    int this[long i] 
    { 
        get { return 7;} 
        set {} 
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (2,15): error CS8502: 'I1.this[short].get' cannot implement interface member 'I1.this[short].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[short].get", "I1.this[short].get", "Test2"),
                // (2,15): error CS8502: 'I1.this[int].set' cannot implement interface member 'I1.this[int].set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[int].set", "I1.this[int].set", "Test2"),
                // (2,15): error CS8502: 'I1.this[long].get' cannot implement interface member 'I1.this[long].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].get", "I1.this[long].get", "Test2"),
                // (2,15): error CS8502: 'I1.this[long].set' cannot implement interface member 'I1.this[long].set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].set", "I1.this[long].set", "Test2"),
                // (2,15): error CS8502: 'I1.this[sbyte].get' cannot implement interface member 'I1.this[sbyte].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[sbyte].get", "I1.this[sbyte].get", "Test2")
                );

            ValidateIndexerImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Fact]
        public void IndexerImplementation_503()
        {
            var source1 =
@"
public interface I1
{
    int this[sbyte i] => 1;
    int this[short i] 
    { get => 3; }
    int this[int i] 
    { set => System.Console.WriteLine(5); }
    int this[long i] 
    { 
        get { return 7;} 
        set {} 
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
public interface I2
{
    void M2();
}

class Test2 : I2
{
    public void M2() {}
}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            var test2 = compilation3.GetTypeByMetadataName("Test2");
            var i1 = compilation3.GetTypeByMetadataName("I1");
            Assert.Equal("I1", i1.ToTestDisplayString());

            var indexers = i1.GetMembers("this[]");
            var p1 = (PropertySymbol)indexers[0];
            var p3 = (PropertySymbol)indexers[1];
            var p5 = (PropertySymbol)indexers[2];
            var p7 = (PropertySymbol)indexers[3];

            Assert.Null(test2.FindImplementationForInterfaceMember(p1));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5));
            Assert.Null(test2.FindImplementationForInterfaceMember(p7));

            Assert.Null(test2.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5.SetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p7.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p7.SetMethod));

            compilation3.VerifyDiagnostics();
        }

        [Fact]
        public void IndexerImplementation_601()
        {
            var source1 =
@"
public interface I1
{
    int this[sbyte i] => 1;
    int this[short i] 
    { get => 3; }
    int this[int i] 
    { set => System.Console.WriteLine(5); }
    int this[long i] 
    { 
        get { return 7;} 
        set {} 
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular7_3, skipUsesIsNullable: true);
            Assert.False(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,26): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     int this[sbyte i] => 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "1").WithArguments("default interface implementation", "8.0").WithLocation(4, 26),
                // (4,26): error CS8701: Target runtime doesn't support default interface implementation.
                //     int this[sbyte i] => 1;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "1").WithLocation(4, 26),
                // (6,7): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     { get => 3; }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(6, 7),
                // (6,7): error CS8701: Target runtime doesn't support default interface implementation.
                //     { get => 3; }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(6, 7),
                // (8,7): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     { set => System.Console.WriteLine(5); }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(8, 7),
                // (8,7): error CS8701: Target runtime doesn't support default interface implementation.
                //     { set => System.Console.WriteLine(5); }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(8, 7),
                // (11,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         get { return 7;} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(11, 9),
                // (11,9): error CS8701: Target runtime doesn't support default interface implementation.
                //         get { return 7;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(11, 9),
                // (12,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         set {} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(12, 9),
                // (12,9): error CS8701: Target runtime doesn't support default interface implementation.
                //         set {} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(12, 9)
                );

            ValidateIndexerImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular7_3);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (2,15): error CS8506: 'I1.this[long].set' cannot implement interface member 'I1.this[long].set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].set", "I1.this[long].set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8502: 'I1.this[long].set' cannot implement interface member 'I1.this[long].set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].set", "I1.this[long].set", "Test2").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[sbyte].get' cannot implement interface member 'I1.this[sbyte].get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[sbyte].get", "I1.this[sbyte].get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8502: 'I1.this[sbyte].get' cannot implement interface member 'I1.this[sbyte].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[sbyte].get", "I1.this[sbyte].get", "Test2").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[short].get' cannot implement interface member 'I1.this[short].get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[short].get", "I1.this[short].get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8502: 'I1.this[short].get' cannot implement interface member 'I1.this[short].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[short].get", "I1.this[short].get", "Test2").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[int].set' cannot implement interface member 'I1.this[int].set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[int].set", "I1.this[int].set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8502: 'I1.this[int].set' cannot implement interface member 'I1.this[int].set' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[int].set", "I1.this[int].set", "Test2").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[long].get' cannot implement interface member 'I1.this[long].get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].get", "I1.this[long].get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8502: 'I1.this[long].get' cannot implement interface member 'I1.this[long].get' in type 'Test2' because the target runtime doesn't support default interface implementation.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].get", "I1.this[long].get", "Test2").WithLocation(2, 15)
                );

            ValidateIndexerImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Fact]
        public void IndexerImplementation_701()
        {
            var source1 =
@"
public interface I1
{
    int this[sbyte i] => 1;
    int this[short i] 
    { get => 3; }
    int this[int i] 
    { set => System.Console.WriteLine(5); }
    int this[long i] 
    { 
        get { return 7;} 
        set {} 
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,26): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     int this[sbyte i] => 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "1").WithArguments("default interface implementation", "8.0").WithLocation(4, 26),
                // (6,7): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     { get => 3; }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(6, 7),
                // (8,7): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     { set => System.Console.WriteLine(5); }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(8, 7),
                // (11,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         get { return 7;} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(11, 9),
                // (12,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         set {} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(12, 9)
                );

            ValidateIndexerImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateIndexerImplementation_501(compilation2.SourceModule, "Test2");

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr,
                symbolValidator: (m) =>
                {
                    var test2Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test2");
                    Assert.Equal("I1", test2Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                    ValidateIndexerImplementation_501(m, "Test2");
                });

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation3.VerifyDiagnostics(
                // (2,15): error CS8506: 'I1.this[long].set' cannot implement interface member 'I1.this[long].set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].set", "I1.this[long].set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[sbyte].get' cannot implement interface member 'I1.this[sbyte].get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[sbyte].get", "I1.this[sbyte].get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[short].get' cannot implement interface member 'I1.this[short].get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[short].get", "I1.this[short].get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[int].set' cannot implement interface member 'I1.this[int].set' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[int].set", "I1.this[int].set", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                // (2,15): error CS8506: 'I1.this[long].get' cannot implement interface member 'I1.this[long].get' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.this[long].get", "I1.this[long].get", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15)
                );

            ValidateIndexerImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Fact]
        public void IndexerImplementation_901()
        {
            var source1 =
@"
public interface I1
{
    static int this[sbyte i] => 1;
    static int this[short i] 
    { get => 3; }
    static int this[int i] 
    { set => System.Console.WriteLine(5); }
    static int this[long i] 
    { 
        get { return 7;} 
        set {} 
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,16): error CS0106: The modifier 'static' is not valid for this item
                //     static int this[sbyte i] => 1;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(4, 16),
                // (4,33): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static int this[sbyte i] => 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "1").WithArguments("default interface implementation", "8.0").WithLocation(4, 33),
                // (5,16): error CS0106: The modifier 'static' is not valid for this item
                //     static int this[short i] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(5, 16),
                // (6,7): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     { get => 3; }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(6, 7),
                // (7,16): error CS0106: The modifier 'static' is not valid for this item
                //     static int this[int i] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(7, 16),
                // (8,7): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     { set => System.Console.WriteLine(5); }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(8, 7),
                // (9,16): error CS0106: The modifier 'static' is not valid for this item
                //     static int this[long i] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(9, 16),
                // (11,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         get { return 7;} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(11, 9),
                // (12,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         set {} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(12, 9)
                );

            ValidateIndexerImplementation_501(compilation1.SourceModule, "Test1");
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_101(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        add
        {
            System.Console.WriteLine(""add E1"");
        }
    }
}

class Test1 : I1
{}
";
            ValidateEventImplementation_101(source1,
                new[] {
                // (4,25): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 25 + declModifiers.Length)
                },
                haveAdd: true, haveRemove: false, isStatic: isStatic);
        }

        private void ValidateEventImplementation_101(string source1, DiagnosticDescription[] expected, bool haveAdd, bool haveRemove, bool isStatic = false)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyEmitDiagnostics(expected);

            ValidateEventImplementationTest1_101(compilation1.SourceModule, haveAdd, haveRemove, isStatic);

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate2(ModuleSymbol m)
            {
                ValidateEventImplementationTest2_101(m, haveAdd, haveRemove);
            }

            Validate2(compilation2.SourceModule);
            compilation2.VerifyDiagnostics();
            Assert.NotEmpty(expected);
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate2);
        }

        private static void ValidateEventImplementationTest1_101(ModuleSymbol m, bool haveAdd, bool haveRemove, bool isStatic = false)
        {
            var i1 = m.GlobalNamespace.GetTypeMember("I1");
            var e1 = i1.GetMember<EventSymbol>("E1");
            var addE1 = e1.AddMethod;
            var rmvE1 = e1.RemoveMethod;

            if (haveAdd)
            {
                ValidateAccessor(addE1);
            }
            else
            {
                Assert.Null(addE1);
            }

            if (haveRemove)
            {
                ValidateAccessor(rmvE1);
            }
            else
            {
                Assert.Null(rmvE1);
            }

            void ValidateAccessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.Equal(isStatic, accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            Assert.False(e1.IsAbstract);
            Assert.True(e1.IsVirtual);
            Assert.False(e1.IsSealed);
            Assert.Equal(isStatic, e1.IsStatic);
            Assert.False(e1.IsExtern);
            Assert.False(e1.IsOverride);
            Assert.Equal(Accessibility.Public, e1.DeclaredAccessibility);

            Assert.True(i1.IsAbstract);
            Assert.True(i1.IsMetadataAbstract);

            if (m is PEModuleSymbol peModule)
            {
                int rva;

                if (haveAdd)
                {
                    peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)addE1).Handle, out _, out _, out _, out rva);
                    Assert.NotEqual(0, rva);
                }

                if (haveRemove)
                {
                    peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)rmvE1).Handle, out _, out _, out _, out rva);
                    Assert.NotEqual(0, rva);
                }
            }

            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            Assert.Equal("I1", test1.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
            Assert.Same(e1, test1.FindImplementationForInterfaceMember(e1));

            if (haveAdd)
            {
                Assert.Same(addE1, test1.FindImplementationForInterfaceMember(addE1));
            }

            if (haveRemove)
            {
                Assert.Same(rmvE1, test1.FindImplementationForInterfaceMember(rmvE1));
            }
        }

        private static void ValidateEventImplementationTest2_101(ModuleSymbol m, bool haveAdd, bool haveRemove)
        {
            var test2 = m.GlobalNamespace.GetTypeMember("Test2");
            Assert.Equal("I1", test2.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

            var e1 = test2.InterfacesNoUseSiteDiagnostics().Single().GetMember<EventSymbol>("E1");
            Assert.Same(e1, test2.FindImplementationForInterfaceMember(e1));

            if (haveAdd)
            {
                var addP1 = e1.AddMethod;
                Assert.Same(addP1, test2.FindImplementationForInterfaceMember(addP1));
            }

            if (haveRemove)
            {
                var rmvP1 = e1.RemoveMethod;
                Assert.Same(rmvP1, test2.FindImplementationForInterfaceMember(rmvP1));
            }
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_102(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        add => System.Console.WriteLine(""add E1"");
        remove => System.Console.WriteLine(""remove E1"");
    }
}

class Test1 : I1
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Test1();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        i1.E1 += null;
        i1.E1 -= null;
    }
}
";
            ValidateEventImplementation_102(source1, isStatic);
        }

        private void ValidateEventImplementation_102(string source1, bool isStatic = false)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                ValidateEventImplementationTest1_101(m, haveAdd: true, haveRemove: true, isStatic: isStatic);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"add E1
remove E1
",
                verify: Verify(isStatic),
                symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 i1 = new Test2();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test2>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source2 +=
@"
        i1.E1 += null;
        i1.E1 -= null;
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate2(ModuleSymbol m)
            {
                ValidateEventImplementationTest2_101(m, haveAdd: true, haveRemove: true);
            }

            Validate2(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic) ? null :
@"add E1
remove E1
",
                verify: Verify(isStatic),
                symbolValidator: Validate2);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate2(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic) ? null :
@"add E1
remove E1
",
                verify: Verify(isStatic),
                symbolValidator: Validate2);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_103(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        remove
        {
            System.Console.WriteLine(""remove E1"");
        }
    }
}

class Test1 : I1
{}
";

            ValidateEventImplementation_101(source1,
                new[] {
                // (4,25): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 25 + declModifiers.Length)
                },
                haveAdd: false, haveRemove: true, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_104(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1
    {
        add;
    }
}

class Test1 : I1
{}
";

            ValidateEventImplementation_101(source1,
                new[] {
                // (6,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 12),
                // (4,25): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 25 + declModifiers.Length)
                },
                haveAdd: true, haveRemove: false, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_105(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        remove;
    }
}

class Test1 : I1
{}
";

            ValidateEventImplementation_101(source1,
                new[] {
                // (6,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 15),
                // (4,25): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 25 + declModifiers.Length)
                },
                haveAdd: false, haveRemove: true, isStatic: isStatic);
        }

        [Fact]
        public void EventImplementation_106()
        {
            var source1 =
@"
public interface I1
{
    event System.Action E1 
    {
    }
}

class Test1 : I1
{}
";
            ValidateEventImplementation_101(source1,
                new[] {
                // (4,25): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 25)
                },
                haveAdd: false, haveRemove: false);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_107(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        add;
        remove;
    }
}

class Test1 : I1
{}
";
            ValidateEventImplementation_101(source1,
                new[] {
                // (6,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 12),
                // (7,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(7, 15)
                },
                haveAdd: true, haveRemove: true, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_108(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        get;
        set;
    } => 0;
}

class Test1 : I1
{}
";
            ValidateEventImplementation_101(source1,
                new[] {
                // (8,7): error CS1519: Invalid token '=>' in class, record, struct, or interface member declaration
                //     } => 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=>").WithArguments("=>").WithLocation(8, 7),
                // (6,9): error CS1055: An add or remove accessor expected
                //         get;
                Diagnostic(ErrorCode.ERR_AddOrRemoveExpected, "get").WithLocation(6, 9),
                // (7,9): error CS1055: An add or remove accessor expected
                //         set;
                Diagnostic(ErrorCode.ERR_AddOrRemoveExpected, "set").WithLocation(7, 9),
                // (4,25): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1")
                },
                haveAdd: false, haveRemove: false, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_109(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        add => throw null;
        remove;
    }
}

class Test1 : I1
{}
";
            ValidateEventImplementation_101(source1,
                new[] {
                // (7,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(7, 15)
                },
                haveAdd: true, haveRemove: true, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_110(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E1 
    {
        add;
        remove => throw null;
    }
}

class Test1 : I1
{}
";
            ValidateEventImplementation_101(source1,
                new[] {
                // (6,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 12)
                },
                haveAdd: true, haveRemove: true, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_201(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"event System.Action E7 { add {System.Console.WriteLine(""add E7"");} remove {System.Console.WriteLine(""remove E7"");} }
    " + declModifiers + @"event System.Action E8 { add {System.Console.WriteLine(""add E8"");} remove {System.Console.WriteLine(""remove E8"");} }
}

class Base
{
    " + implModifiers + @"event System.Action E7;
}

class Derived : Base, I1
{
    " + implModifiers + @"event System.Action E8 { add {} remove {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        i1.E7 += null;
        i1.E7 -= null;
        i1.E8 += null;
        i1.E8 -= null;
    }
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (10,25): warning CS0067: The event 'Base.E7' is never used
                //     event System.Action E7;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "E7").WithArguments("Base.E7").WithLocation(10, 25 + implModifiers.Length)
                );

            ValidateEventImplementation_201(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic) ? null :
@"add E7
remove E7
add E8
remove E8",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    ValidateEventImplementation_201(m);
                });
        }

        private static void ValidateEventImplementation_201(ModuleSymbol m)
        {
            var e7 = m.GlobalNamespace.GetMember<EventSymbol>("I1.E7");
            var e8 = m.GlobalNamespace.GetMember<EventSymbol>("I1.E8");

            var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

            Assert.Same(e7, derived.FindImplementationForInterfaceMember(e7));
            Assert.Same(e8, derived.FindImplementationForInterfaceMember(e8));

            Assert.Same(e7.AddMethod, derived.FindImplementationForInterfaceMember(e7.AddMethod));
            Assert.Same(e8.AddMethod, derived.FindImplementationForInterfaceMember(e8.AddMethod));
            Assert.Same(e7.RemoveMethod, derived.FindImplementationForInterfaceMember(e7.RemoveMethod));
            Assert.Same(e8.RemoveMethod, derived.FindImplementationForInterfaceMember(e8.RemoveMethod));
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_202(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"event System.Action E7 { add {System.Console.WriteLine(""add E7"");} remove {System.Console.WriteLine(""remove E7"");} }
    " + declModifiers + @"event System.Action E8 { add {System.Console.WriteLine(""add E8"");} remove {System.Console.WriteLine(""remove E8"");} }
}

class Base : Test
{
    " + implModifiers + @"event System.Action E7;
}

class Derived : Base, I1
{
    " + implModifiers + @"event System.Action E8 { add {} remove {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        i1.E7 += null;
        i1.E7 -= null;
        i1.E8 += null;
        i1.E8 -= null;
    }
}

class Test : I1 {}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (10,25): warning CS0067: The event 'Base.E7' is never used
                //     event System.Action E7;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "E7").WithArguments("Base.E7").WithLocation(10, 25 + implModifiers.Length)
                );

            ValidateEventImplementation_201(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"add E7
remove E7
add E8
remove E8",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    ValidateEventImplementation_201(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_203(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"event System.Action E7 { add {} remove {} }
    " + declModifiers + @"event System.Action E8 { add {} remove {} }
}

class Base : Test
{
    " + implModifiers + @"event System.Action E7;
}

class Derived : Base, I1
{
    " + implModifiers + @"event System.Action E8 { add {} remove {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        i1.E7 += null;
        i1.E7 -= null;
        i1.E8 += null;
        i1.E8 -= null;
    }
}

class Test : I1 
{
    " + implModifiers + @"event System.Action I1.E7 { add {System.Console.WriteLine(""add E7"");} remove {System.Console.WriteLine(""remove E7"");} }
    " + implModifiers + @"event System.Action I1.E8 { add {System.Console.WriteLine(""add E8"");} remove {System.Console.WriteLine(""remove E8"");} }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (10,25): warning CS0067: The event 'Base.E7' is never used
                //     event System.Action E7;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "E7").WithArguments("Base.E7").WithLocation(10, 25 + implModifiers.Length)
                );

            void Validate(ModuleSymbol m)
            {
                var e7 = m.GlobalNamespace.GetMember<EventSymbol>("I1.E7");
                var e8 = m.GlobalNamespace.GetMember<EventSymbol>("I1.E8");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("event System.Action Test.I1.E7", derived.FindImplementationForInterfaceMember(e7).ToTestDisplayString());
                Assert.Equal("event System.Action Test.I1.E8", derived.FindImplementationForInterfaceMember(e8).ToTestDisplayString());

                Assert.Equal("void Test.I1.E7.add", derived.FindImplementationForInterfaceMember(e7.AddMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.E8.add", derived.FindImplementationForInterfaceMember(e8.AddMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.E7.remove", derived.FindImplementationForInterfaceMember(e7.RemoveMethod).ToTestDisplayString());
                Assert.Equal("void Test.I1.E8.remove", derived.FindImplementationForInterfaceMember(e8.RemoveMethod).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"add E7
remove E7
add E8
remove E8",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_204(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
interface I1
{
    " + declModifiers + @"event System.Action E7 { add {} remove {} }
    " + declModifiers + @"event System.Action E8 { add {} remove {} }
}

class Base : Test
{
    " + implModifiers + @"new event System.Action E7;
}

class Derived : Base, I1
{
    " + implModifiers + @"new event System.Action E8 { add {} remove {} }

";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I1 i1 = new Derived();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Derived>();
    }

    static void Test<i1>() where i1 : I1
    {
";
            }

            source1 +=
@"
        i1.E7 += null;
        i1.E7 -= null;
        i1.E8 += null;
        i1.E8 -= null;
    }
}

class Test : I1 
{
    " + implModifiers + @"public event System.Action E7 { add {System.Console.WriteLine(""add E7"");} remove {System.Console.WriteLine(""remove E7"");} }
    " + implModifiers + @"public event System.Action E8 { add {System.Console.WriteLine(""add E8"");} remove {System.Console.WriteLine(""remove E8"");} }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (10,29): warning CS0067: The event 'Base.E7' is never used
                //     new event System.Action E7;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "E7").WithArguments("Base.E7").WithLocation(10, 29 + implModifiers.Length)
                );

            void Validate(ModuleSymbol m)
            {
                var e7 = m.GlobalNamespace.GetMember<EventSymbol>("I1.E7");
                var e8 = m.GlobalNamespace.GetMember<EventSymbol>("I1.E8");

                var derived = m.ContainingAssembly.GetTypeByMetadataName("Derived");

                Assert.Equal("event System.Action Test.E7", derived.FindImplementationForInterfaceMember(e7).ToTestDisplayString());
                Assert.Equal("event System.Action Test.E8", derived.FindImplementationForInterfaceMember(e8).ToTestDisplayString());

                Assert.Equal("void Test.E7.add", derived.FindImplementationForInterfaceMember(e7.AddMethod).ToTestDisplayString());
                Assert.Equal("void Test.E8.add", derived.FindImplementationForInterfaceMember(e8.AddMethod).ToTestDisplayString());
                Assert.Equal("void Test.E7.remove", derived.FindImplementationForInterfaceMember(e7.RemoveMethod).ToTestDisplayString());
                Assert.Equal("void Test.E8.remove", derived.FindImplementationForInterfaceMember(e8.RemoveMethod).ToTestDisplayString());
            }

            Validate(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, hasImplementationOfVirtualInDerivedType: true) ? null :
@"add E7
remove E7
add E8
remove E8",
                verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var derivedResult = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Derived");
                    Assert.Equal("I1", derivedResult.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());

                    Validate(m);
                });
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_501(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (6,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         add {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(6, 9),
                    // (7,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         remove {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(7, 9)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (6,9): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //         add {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "add").WithLocation(6, 9),
                    // (7,9): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //         remove {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "remove").WithLocation(7, 9)
                    );
            }

            ValidateEventImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8502: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8929: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2").WithLocation(2, 15)
                    );
            }

            ValidateEventImplementation_501(compilation3.SourceModule, "Test2");
        }

        private static void ValidateEventImplementation_501(ModuleSymbol m, string typeName)
        {
            var derived = m.GlobalNamespace.GetTypeMember(typeName);
            var i1 = derived.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.ToTestDisplayString());

            var e7 = i1.GetMember<EventSymbol>("E7");

            Assert.True(e7.IsVirtual);
            Assert.False(e7.IsAbstract);

            Assert.Same(e7, derived.FindImplementationForInterfaceMember(e7));

            Assert.True(e7.AddMethod.IsVirtual);
            Assert.True(e7.RemoveMethod.IsVirtual);

            Assert.True(e7.AddMethod.IsMetadataVirtual());
            Assert.True(e7.RemoveMethod.IsMetadataVirtual());

            Assert.False(e7.AddMethod.IsAbstract);
            Assert.False(e7.RemoveMethod.IsAbstract);

            Assert.Same(e7.AddMethod, derived.FindImplementationForInterfaceMember(e7.AddMethod));
            Assert.Same(e7.RemoveMethod, derived.FindImplementationForInterfaceMember(e7.RemoveMethod));
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_502(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() },
                                                 targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8502: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8929: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2").WithLocation(2, 15)
                    );
            }

            ValidateEventImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_503(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
public interface I2
{
    void M2();
}

class Test2 : I2
{
    public void M2() {}
}
";

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            var test2 = compilation3.GetTypeByMetadataName("Test2");
            var i1 = compilation3.GetTypeByMetadataName("I1");
            Assert.Equal("I1", i1.ToTestDisplayString());

            var e7 = i1.GetMember<EventSymbol>("E7");

            Assert.Null(test2.FindImplementationForInterfaceMember(e7));

            Assert.Null(test2.FindImplementationForInterfaceMember(e7.AddMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(e7.RemoveMethod));

            compilation3.VerifyDiagnostics();
        }

        [Fact]
        public void EventImplementation_601()
        {
            {
                var source1 =
    @"
public interface I1
{
    event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}

class Test1 : I1
{}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: TestOptions.Regular7_3, skipUsesIsNullable: true);
                Assert.False(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation1.VerifyDiagnostics(
                    // (6,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //         add {} 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(6, 9),
                    // (6,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         add {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(6, 9),
                    // (7,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //         remove {} 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(7, 9),
                    // (7,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         remove {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(7, 9)
                    );

                ValidateEventImplementation_501(compilation1.SourceModule, "Test1");

                var source2 =
    @"
class Test2 : I1
{}
";

                var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                     options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: TestOptions.Regular7_3);
                Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8506: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8502: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because the target runtime doesn't support default interface implementation.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2").WithLocation(2, 15)
                    );

                ValidateEventImplementation_501(compilation3.SourceModule, "Test2");
            }

            {
                var source1 =
    @"
public interface I1
{
    static virtual event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}

class Test1 : I1
{}
";
                var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                     parseOptions: TestOptions.Regular10, skipUsesIsNullable: true);

                compilation1.VerifyDiagnostics(
                    // (4,40): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual event System.Action E7 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "E7").WithArguments("virtual", "10.0", "11.0").WithLocation(4, 40),
                    // (6,9): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //         add {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "add").WithLocation(6, 9),
                    // (7,9): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //         remove {} 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "remove").WithLocation(7, 9)
                    );

                ValidateEventImplementation_501(compilation1.SourceModule, "Test1");

                var source2 =
    @"
class Test2 : I1
{}
";

                var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                     options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                     parseOptions: TestOptions.Regular10);

                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8706: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8929: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because the target runtime doesn't support static abstract members in interfaces.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2").WithLocation(2, 15)
                    );

                ValidateEventImplementation_501(compilation3.SourceModule, "Test2");
            }
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementation_701(bool isStatic)
        {
            string declModifiers = isStatic ? "static virtual " : "";

            var source1 =
@"
public interface I1
{
    " + declModifiers + @"event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (6,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //         add {} 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(6, 9),
                    // (7,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //         remove {} 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(7, 9)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,40): error CS8703: The modifier 'virtual' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static virtual event System.Action E7 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "E7").WithArguments("virtual", "10.0", "11.0").WithLocation(4, 40)
                    );
            }

            ValidateEventImplementation_501(compilation1.SourceModule, "Test1");

            var source2 =
@"
class Test2 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateEventImplementation_501(compilation2.SourceModule, "Test2");

            CompileAndVerify(compilation2, verify: Verify(isStatic),
                symbolValidator: (m) =>
                {
                    var test2Result = (PENamedTypeSymbol)m.GlobalNamespace.GetTypeMember("Test2");
                    Assert.Equal("I1", test2Result.InterfacesNoUseSiteDiagnostics().Single().ToTestDisplayString());
                    ValidateEventImplementation_501(m, "Test2");
                });

            var compilation3 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8506: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15),
                    // (2,15): error CS8506: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2", "default interface implementation", "7.3", "8.0").WithLocation(2, 15)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (2,15): error CS8706: 'I1.E7.add' cannot implement interface member 'I1.E7.add' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.add", "I1.E7.add", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15),
                    // (2,15): error CS8706: 'I1.E7.remove' cannot implement interface member 'I1.E7.remove' in type 'Test2' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.E7.remove", "I1.E7.remove", "Test2", "static abstract members in interfaces", "10.0", "11.0").WithLocation(2, 15)
                    );
            }

            ValidateEventImplementation_501(compilation3.SourceModule, "Test2");
        }

        [Fact]
        public void EventImplementation_901()
        {
            var source1 =
@"
public interface I1
{
    static event System.Action E7 
    { 
        add {} 
        remove {} 
    }
}

class Test1 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,32): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static event System.Action E7 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "E7").WithArguments("default interface implementation", "8.0").WithLocation(4, 32)
                );

            var derived = compilation1.GlobalNamespace.GetTypeMember("Test1");
            var i1 = derived.InterfacesNoUseSiteDiagnostics().Single();
            Assert.Equal("I1", i1.ToTestDisplayString());

            var e7 = i1.GetMember<EventSymbol>("E7");

            Assert.False(e7.IsVirtual);
            Assert.False(e7.IsAbstract);
            Assert.True(e7.IsStatic);

            Assert.Null(derived.FindImplementationForInterfaceMember(e7));

            Assert.False(e7.AddMethod.IsVirtual);
            Assert.False(e7.RemoveMethod.IsVirtual);

            Assert.False(e7.AddMethod.IsMetadataVirtual());
            Assert.False(e7.RemoveMethod.IsMetadataVirtual());

            Assert.False(e7.AddMethod.IsAbstract);
            Assert.False(e7.RemoveMethod.IsAbstract);

            Assert.True(e7.AddMethod.IsStatic);
            Assert.True(e7.RemoveMethod.IsStatic);

            Assert.Null(derived.FindImplementationForInterfaceMember(e7.AddMethod));
            Assert.Null(derived.FindImplementationForInterfaceMember(e7.RemoveMethod));
        }

        [Fact]
        public void BaseIsNotAllowed_01()
        {
            var source1 =
@"
public interface I1
{
    void M1() 
    {
        base.GetHashCode();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (6,9): error CS0174: A base class is required for a 'base' reference
                //         base.GetHashCode();
                Diagnostic(ErrorCode.ERR_NoBaseClass, "base").WithLocation(6, 9)
                );
        }

        [Fact]
        public void ThisIsAllowed_01()
        {
            var source1 =
@"
public interface I1
{
    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    }

    int P1
    {
        get
        {
            System.Console.WriteLine(""I1.get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""I1.set_P1"");
    }

    event System.Action E1
    {
        add => System.Console.WriteLine(""I1.add_E1"");
        remove => System.Console.WriteLine(""I1.remove_E1"");
    }
}

public interface I2 : I1
{
    void M2() 
    {
        System.Console.WriteLine(""I2.M2"");
        System.Console.WriteLine(this.GetHashCode());
        this.M1();
        this.P1 = this.P1;
        this.E1 += null;
        this.E1 -= null;
        this.M3();
        this.P3 = this.P3;
        this.E3 += null;
        this.E3 -= null;
    }

    int P2
    {
        get
        {
            System.Console.WriteLine(""I2.get_P2"");
            System.Console.WriteLine(this.GetHashCode());
            this.M1();
            this.P1 = this.P1;
            this.E1 += null;
            this.E1 -= null;
            this.M3();
            this.P3 = this.P3;
            this.E3 += null;
            this.E3 -= null;
            return 0;
        }
        set
        {
            System.Console.WriteLine(""I2.set_P2"");
            System.Console.WriteLine(this.GetHashCode());
            this.M1();
            this.P1 = this.P1;
            this.E1 += null;
            this.E1 -= null;
            this.M3();
            this.P3 = this.P3;
            this.E3 += null;
            this.E3 -= null;
        }
    }

    event System.Action E2
    {
        add
        {
            System.Console.WriteLine(""I2.add_E2"");
            System.Console.WriteLine(this.GetHashCode());
            this.M1();
            this.P1 = this.P1;
            this.E1 += null;
            this.E1 -= null;
            this.M3();
            this.P3 = this.P3;
            this.E3 += null;
            this.E3 -= null;
        }
        remove
        {
            System.Console.WriteLine(""I2.remove_E2"");
            System.Console.WriteLine(this.GetHashCode());
            this.M1();
            this.P1 = this.P1;
            this.E1 += null;
            this.E1 -= null;
            this.M3();
            this.P3 = this.P3;
            this.E3 += null;
            this.E3 -= null;
        }
    }

    void M3() 
    {
        System.Console.WriteLine(""I2.M3"");
    }

    int P3
    {
        get
        {
            System.Console.WriteLine(""I2.get_P3"");
            return 0;
        }
        set => System.Console.WriteLine(""I2.set_P3"");
    }

    event System.Action E3
    {
        add => System.Console.WriteLine(""I2.add_E3"");
        remove => System.Console.WriteLine(""I2.remove_E3"");
    }
}


class Test1 : I2
{
    static void Main()
    {
        I2 x = new Test1();
        x.M2();
        x.P2 = x.P2;
        x.E2 += null;
        x.E2 -= null;
    }

    public override int GetHashCode()
    {
        return 123;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"I2.M2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.get_P2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.set_P2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.add_E2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.remove_E2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
",
                verify: VerifyOnMonoOrCoreClr);
        }

        [Fact]
        public void ThisIsAllowed_02()
        {
            var source1 =
@"
public interface I1
{
    public int F1;
}

public interface I2 : I1
{
    void M2() 
    {
        this.F1 = this.F2;
    }

    int P2
    {
        get
        {
            this.F1 = this.F2;
            return 0;
        }
        set
        {
            this.F1 = this.F2;
        }
    }

    event System.Action E2
    {
        add
        {
            this.F1 = this.F2;
        }
        remove
        {
            this.F1 = this.F2;
        }
    }

    public int F2;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,16): error CS0525: Interfaces cannot contain fields
                //     public int F1;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F1").WithLocation(4, 16),
                // (39,16): error CS0525: Interfaces cannot contain fields
                //     public int F2;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F2").WithLocation(39, 16)
                );
        }

        [Fact]
        public void ImplicitThisIsAllowed_01()
        {
            var source1 =
@"
public interface I1
{
    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    }

    int P1
    {
        get
        {
            System.Console.WriteLine(""I1.get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""I1.set_P1"");
    }

    event System.Action E1
    {
        add => System.Console.WriteLine(""I1.add_E1"");
        remove => System.Console.WriteLine(""I1.remove_E1"");
    }
}

public interface I2 : I1
{
    void M2() 
    {
        System.Console.WriteLine(""I2.M2"");
        System.Console.WriteLine(GetHashCode());
        M1();
        P1 = P1;
        E1 += null;
        E1 -= null;
        M3();
        P3 = P3;
        E3 += null;
        E3 -= null;
    }

    int P2
    {
        get
        {
            System.Console.WriteLine(""I2.get_P2"");
            System.Console.WriteLine(GetHashCode());
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            M3();
            P3 = P3;
            E3 += null;
            E3 -= null;
            return 0;
        }
        set
        {
            System.Console.WriteLine(""I2.set_P2"");
            System.Console.WriteLine(GetHashCode());
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            M3();
            P3 = P3;
            E3 += null;
            E3 -= null;
        }
    }

    event System.Action E2
    {
        add
        {
            System.Console.WriteLine(""I2.add_E2"");
            System.Console.WriteLine(GetHashCode());
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            M3();
            P3 = P3;
            E3 += null;
            E3 -= null;
        }
        remove
        {
            System.Console.WriteLine(""I2.remove_E2"");
            System.Console.WriteLine(GetHashCode());
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            M3();
            P3 = P3;
            E3 += null;
            E3 -= null;
        }
    }

    void M3() 
    {
        System.Console.WriteLine(""I2.M3"");
    }

    int P3
    {
        get
        {
            System.Console.WriteLine(""I2.get_P3"");
            return 0;
        }
        set => System.Console.WriteLine(""I2.set_P3"");
    }

    event System.Action E3
    {
        add => System.Console.WriteLine(""I2.add_E3"");
        remove => System.Console.WriteLine(""I2.remove_E3"");
    }
}


class Test1 : I2
{
    static void Main()
    {
        I2 x = new Test1();
        x.M2();
        x.P2 = x.P2;
        x.E2 += null;
        x.E2 -= null;
    }

    public override int GetHashCode()
    {
        return 123;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"I2.M2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.get_P2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.set_P2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.add_E2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
I2.remove_E2
123
I1.M1
I1.get_P1
I1.set_P1
I1.add_E1
I1.remove_E1
I2.M3
I2.get_P3
I2.set_P3
I2.add_E3
I2.remove_E3
",
                verify: VerifyOnMonoOrCoreClr);
        }

        [Fact]
        public void ImplicitThisIsAllowed_02()
        {
            var source1 =
@"
public interface I1
{
    public int F1;
}

public interface I2 : I1
{
    void M2() 
    {
        F1 = F2;
    }

    int P2
    {
        get
        {
            F1 = F2;
            return 0;
        }
        set
        {
            F1 = F2;
        }
    }

    event System.Action E2
    {
        add
        {
            F1 = F2;
        }
        remove
        {
            F1 = F2;
        }
    }

    public int F2;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,16): error CS0525: Interfaces cannot contain fields
                //     public int F1;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F1").WithLocation(4, 16),
                // (39,16): error CS0525: Interfaces cannot contain fields
                //     public int F2;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F2").WithLocation(39, 16)
                );
        }

        [Fact]
        public void MethodModifiers_01()
        {
            var source1 =
@"
public interface I1
{
    public void M01();
    protected void M02();
    protected internal void M03();
    internal void M04();
    private void M05();
    static void M06();
    virtual void M07();
    sealed void M08();
    override void M09();
    abstract void M10();
    extern void M11();
    async void M12();
    private protected void M13();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (12,19): error CS0106: The modifier 'override' is not valid for this item
                //     override void M09();
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M09").WithArguments("override").WithLocation(12, 19),
                // (15,16): error CS1994: The 'async' modifier can only be used in methods that have a body.
                //     async void M12();
                Diagnostic(ErrorCode.ERR_BadAsyncLacksBody, "M12").WithLocation(15, 16),
                // (8,18): error CS0501: 'I1.M05()' must declare a body because it is not marked abstract, extern, or partial
                //     private void M05();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M05").WithArguments("I1.M05()").WithLocation(8, 18),
                // (9,17): error CS0501: 'I1.M06()' must declare a body because it is not marked abstract, extern, or partial
                //     static void M06();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M06").WithArguments("I1.M06()").WithLocation(9, 17),
                // (10,18): error CS0501: 'I1.M07()' must declare a body because it is not marked abstract, extern, or partial
                //     virtual void M07();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M07").WithArguments("I1.M07()").WithLocation(10, 18),
                // (11,17): error CS0501: 'I1.M08()' must declare a body because it is not marked abstract, extern, or partial
                //     sealed void M08();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M08").WithArguments("I1.M08()").WithLocation(11, 17),
                // (14,17): warning CS0626: Method, operator, or accessor 'I1.M11()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void M11();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M11").WithArguments("I1.M11()").WithLocation(14, 17)
                );

            ValidateSymbolsMethodModifiers_01(compilation1);
        }

        private static void ValidateSymbolsMethodModifiers_01(CSharpCompilation compilation1)
        {
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m01 = i1.GetMember<MethodSymbol>("M01");

            Assert.True(m01.IsAbstract);
            Assert.False(m01.IsVirtual);
            Assert.True(m01.IsMetadataVirtual());
            Assert.False(m01.IsSealed);
            Assert.False(m01.IsStatic);
            Assert.False(m01.IsExtern);
            Assert.False(m01.IsAsync);
            Assert.False(m01.IsOverride);
            Assert.Equal(Accessibility.Public, m01.DeclaredAccessibility);

            var m02 = i1.GetMember<MethodSymbol>("M02");

            Assert.True(m02.IsAbstract);
            Assert.False(m02.IsVirtual);
            Assert.True(m02.IsMetadataVirtual());
            Assert.False(m02.IsSealed);
            Assert.False(m02.IsStatic);
            Assert.False(m02.IsExtern);
            Assert.False(m02.IsAsync);
            Assert.False(m02.IsOverride);
            Assert.Equal(Accessibility.Protected, m02.DeclaredAccessibility);

            var m03 = i1.GetMember<MethodSymbol>("M03");

            Assert.True(m03.IsAbstract);
            Assert.False(m03.IsVirtual);
            Assert.True(m03.IsMetadataVirtual());
            Assert.False(m03.IsSealed);
            Assert.False(m03.IsStatic);
            Assert.False(m03.IsExtern);
            Assert.False(m03.IsAsync);
            Assert.False(m03.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, m03.DeclaredAccessibility);

            var m04 = i1.GetMember<MethodSymbol>("M04");

            Assert.True(m04.IsAbstract);
            Assert.False(m04.IsVirtual);
            Assert.True(m04.IsMetadataVirtual());
            Assert.False(m04.IsSealed);
            Assert.False(m04.IsStatic);
            Assert.False(m04.IsExtern);
            Assert.False(m04.IsAsync);
            Assert.False(m04.IsOverride);
            Assert.Equal(Accessibility.Internal, m04.DeclaredAccessibility);

            var m05 = i1.GetMember<MethodSymbol>("M05");

            Assert.False(m05.IsAbstract);
            Assert.False(m05.IsVirtual);
            Assert.False(m05.IsMetadataVirtual());
            Assert.False(m05.IsSealed);
            Assert.False(m05.IsStatic);
            Assert.False(m05.IsExtern);
            Assert.False(m05.IsAsync);
            Assert.False(m05.IsOverride);
            Assert.Equal(Accessibility.Private, m05.DeclaredAccessibility);

            var m06 = i1.GetMember<MethodSymbol>("M06");

            Assert.False(m06.IsAbstract);
            Assert.False(m06.IsVirtual);
            Assert.False(m06.IsMetadataVirtual());
            Assert.False(m06.IsSealed);
            Assert.True(m06.IsStatic);
            Assert.False(m06.IsExtern);
            Assert.False(m06.IsAsync);
            Assert.False(m06.IsOverride);
            Assert.Equal(Accessibility.Public, m06.DeclaredAccessibility);

            var m07 = i1.GetMember<MethodSymbol>("M07");

            Assert.False(m07.IsAbstract);
            Assert.True(m07.IsVirtual);
            Assert.True(m07.IsMetadataVirtual());
            Assert.False(m07.IsSealed);
            Assert.False(m07.IsStatic);
            Assert.False(m07.IsExtern);
            Assert.False(m07.IsAsync);
            Assert.False(m07.IsOverride);
            Assert.Equal(Accessibility.Public, m07.DeclaredAccessibility);

            var m08 = i1.GetMember<MethodSymbol>("M08");

            Assert.False(m08.IsAbstract);
            Assert.False(m08.IsVirtual);
            Assert.False(m08.IsMetadataVirtual());
            Assert.False(m08.IsSealed);
            Assert.False(m08.IsStatic);
            Assert.False(m08.IsExtern);
            Assert.False(m08.IsAsync);
            Assert.False(m08.IsOverride);
            Assert.Equal(Accessibility.Public, m08.DeclaredAccessibility);

            var m09 = i1.GetMember<MethodSymbol>("M09");

            Assert.True(m09.IsAbstract);
            Assert.False(m09.IsVirtual);
            Assert.True(m09.IsMetadataVirtual());
            Assert.False(m09.IsSealed);
            Assert.False(m09.IsStatic);
            Assert.False(m09.IsExtern);
            Assert.False(m09.IsAsync);
            Assert.False(m09.IsOverride);
            Assert.Equal(Accessibility.Public, m09.DeclaredAccessibility);

            var m10 = i1.GetMember<MethodSymbol>("M10");

            Assert.True(m10.IsAbstract);
            Assert.False(m10.IsVirtual);
            Assert.True(m10.IsMetadataVirtual());
            Assert.False(m10.IsSealed);
            Assert.False(m10.IsStatic);
            Assert.False(m10.IsExtern);
            Assert.False(m10.IsAsync);
            Assert.False(m10.IsOverride);
            Assert.Equal(Accessibility.Public, m10.DeclaredAccessibility);

            var m11 = i1.GetMember<MethodSymbol>("M11");

            Assert.False(m11.IsAbstract);
            Assert.True(m11.IsVirtual);
            Assert.True(m11.IsMetadataVirtual());
            Assert.False(m11.IsSealed);
            Assert.False(m11.IsStatic);
            Assert.True(m11.IsExtern);
            Assert.False(m11.IsAsync);
            Assert.False(m11.IsOverride);
            Assert.Equal(Accessibility.Public, m11.DeclaredAccessibility);

            var m12 = i1.GetMember<MethodSymbol>("M12");

            Assert.True(m12.IsAbstract);
            Assert.False(m12.IsVirtual);
            Assert.True(m12.IsMetadataVirtual());
            Assert.False(m12.IsSealed);
            Assert.False(m12.IsStatic);
            Assert.False(m12.IsExtern);
            Assert.True(m12.IsAsync);
            Assert.False(m12.IsOverride);
            Assert.Equal(Accessibility.Public, m12.DeclaredAccessibility);

            var m13 = i1.GetMember<MethodSymbol>("M13");

            Assert.True(m13.IsAbstract);
            Assert.False(m13.IsVirtual);
            Assert.True(m13.IsMetadataVirtual());
            Assert.False(m13.IsSealed);
            Assert.False(m13.IsStatic);
            Assert.False(m13.IsExtern);
            Assert.False(m13.IsAsync);
            Assert.False(m13.IsOverride);
            Assert.Equal(Accessibility.ProtectedAndInternal, m13.DeclaredAccessibility);
        }

        [Fact]
        public void MethodModifiers_02()
        {
            var source1 =
@"
public interface I1
{
    public void M01();
    protected void M02();
    protected internal void M03();
    internal void M04();
    private void M05();
    static void M06();
    virtual void M07();
    sealed void M08();
    override void M09();
    abstract void M10();
    extern void M11();
    async void M12();
    private protected void M13();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,17): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public void M01();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M01").WithArguments("public", "7.3", "8.0").WithLocation(4, 17),
                // (5,20): error CS8703: The modifier 'protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     protected void M02();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M02").WithArguments("protected", "7.3", "8.0").WithLocation(5, 20),
                // (6,29): error CS8703: The modifier 'protected internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     protected internal void M03();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M03").WithArguments("protected internal", "7.3", "8.0").WithLocation(6, 29),
                // (7,19): error CS8503: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal void M04();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M04").WithArguments("internal", "7.3", "8.0").WithLocation(7, 19),
                // (8,18): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private void M05();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M05").WithArguments("private", "7.3", "8.0").WithLocation(8, 18),
                // (9,17): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static void M06();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M06").WithArguments("static", "7.3", "8.0").WithLocation(9, 17),
                // (10,18): error CS8503: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual void M07();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M07").WithArguments("virtual", "7.3", "8.0").WithLocation(10, 18),
                // (11,17): error CS8503: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     sealed void M08();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M08").WithArguments("sealed", "7.3", "8.0").WithLocation(11, 17),
                // (12,19): error CS0106: The modifier 'override' is not valid for this item
                //     override void M09();
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M09").WithArguments("override").WithLocation(12, 19),
                // (13,19): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     abstract void M10();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M10").WithArguments("abstract", "7.3", "8.0").WithLocation(13, 19),
                // (14,17): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern void M11();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M11").WithArguments("extern", "7.3", "8.0").WithLocation(14, 17),
                // (15,16): error CS8503: The modifier 'async' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     async void M12();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M12").WithArguments("async", "7.3", "8.0").WithLocation(15, 16),
                // (15,16): error CS1994: The 'async' modifier can only be used in methods that have a body.
                //     async void M12();
                Diagnostic(ErrorCode.ERR_BadAsyncLacksBody, "M12").WithLocation(15, 16),
                // (8,18): error CS0501: 'I1.M05()' must declare a body because it is not marked abstract, extern, or partial
                //     private void M05();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M05").WithArguments("I1.M05()").WithLocation(8, 18),
                // (9,17): error CS0501: 'I1.M06()' must declare a body because it is not marked abstract, extern, or partial
                //     static void M06();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M06").WithArguments("I1.M06()").WithLocation(9, 17),
                // (10,18): error CS0501: 'I1.M07()' must declare a body because it is not marked abstract, extern, or partial
                //     virtual void M07();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M07").WithArguments("I1.M07()").WithLocation(10, 18),
                // (11,17): error CS0501: 'I1.M08()' must declare a body because it is not marked abstract, extern, or partial
                //     sealed void M08();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M08").WithArguments("I1.M08()").WithLocation(11, 17),
                // (14,17): warning CS0626: Method, operator, or accessor 'I1.M11()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void M11();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M11").WithArguments("I1.M11()").WithLocation(14, 17),
                // (16,28): error CS8703: The modifier 'private protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private protected void M13();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M13").WithArguments("private protected", "7.3", "8.0").WithLocation(16, 28)
                );

            ValidateSymbolsMethodModifiers_01(compilation1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (5,20): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected void M02();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M02").WithLocation(5, 20),
                // (6,29): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal void M03();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M03").WithLocation(6, 29),
                // (8,18): error CS0501: 'I1.M05()' must declare a body because it is not marked abstract, extern, or partial
                //     private void M05();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M05").WithArguments("I1.M05()").WithLocation(8, 18),
                // (9,17): error CS0501: 'I1.M06()' must declare a body because it is not marked abstract, extern, or partial
                //     static void M06();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M06").WithArguments("I1.M06()").WithLocation(9, 17),
                // (10,18): error CS0501: 'I1.M07()' must declare a body because it is not marked abstract, extern, or partial
                //     virtual void M07();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M07").WithArguments("I1.M07()").WithLocation(10, 18),
                // (11,17): error CS0501: 'I1.M08()' must declare a body because it is not marked abstract, extern, or partial
                //     sealed void M08();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M08").WithArguments("I1.M08()").WithLocation(11, 17),
                // (12,19): error CS0106: The modifier 'override' is not valid for this item
                //     override void M09();
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M09").WithArguments("override").WithLocation(12, 19),
                // (14,17): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern void M11();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M11").WithLocation(14, 17),
                // (14,17): warning CS0626: Method, operator, or accessor 'I1.M11()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void M11();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M11").WithArguments("I1.M11()").WithLocation(14, 17),
                // (15,16): error CS1994: The 'async' modifier can only be used in methods that have a body.
                //     async void M12();
                Diagnostic(ErrorCode.ERR_BadAsyncLacksBody, "M12").WithLocation(15, 16),
                // (16,28): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected void M13();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M13").WithLocation(16, 28)
                );

            ValidateSymbolsMethodModifiers_01(compilation2);
        }

        [Fact]
        [WorkItem(33083, "https://github.com/dotnet/roslyn/issues/33083")]
        public void MethodModifiers_03()
        {
            var source1 =
@"
public interface I1
{
    public virtual void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            ValidateMethodImplementation_011(source1);
        }

        [Fact]
        public void MethodModifiers_04()
        {
            var source1 =
@"
public interface I1
{
    public abstract void M1(); 
    void M2(); 
}

class Test1 : I1
{
    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }

    public void M2() 
    {
        System.Console.WriteLine(""M2"");
    }

    static void Main()
    {
        I1 x = new Test1();
        x.M1();
        x.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput:
@"M1
M2", symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var methodName in new[] { "M1", "M2" })
                {
                    var m1 = i1.GetMember<MethodSymbol>(methodName);

                    Assert.True(m1.IsAbstract);
                    Assert.False(m1.IsVirtual);
                    Assert.True(m1.IsMetadataVirtual());
                    Assert.False(m1.IsSealed);
                    Assert.False(m1.IsStatic);
                    Assert.False(m1.IsExtern);
                    Assert.False(m1.IsAsync);
                    Assert.False(m1.IsOverride);
                    Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
                    Assert.Same(test1.GetMember(methodName), test1.FindImplementationForInterfaceMember(m1));
                }
            }
        }

        [Fact]
        public void MethodModifiers_05()
        {
            var source1 =
@"
public interface I1
{
    public abstract void M1();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation1.VerifyDiagnostics(
                // (4,26): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract void M1();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "7.3", "8.0").WithLocation(4, 26),
                // (4,26): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract void M1();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("public", "7.3", "8.0").WithLocation(4, 26)
                );

            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
        }

        [Fact]
        public void MethodModifiers_06()
        {
            var source1 =
@"
public interface I1
{
    public static void M1() 
    {
        System.Console.WriteLine(""M1"");
    }

    internal static void M2() 
    {
        System.Console.WriteLine(""M2"");
        M3();
    }

    private static void M3() 
    {
        System.Console.WriteLine(""M3"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.M1();
        I1.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M1
M2
M3", symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var tuple in new[] { (name: "M1", access: Accessibility.Public), (name: "M2", access: Accessibility.Internal), (name: "M3", access: Accessibility.Private) })
                {
                    var m1 = i1.GetMember<MethodSymbol>(tuple.name);

                    Assert.False(m1.IsAbstract);
                    Assert.False(m1.IsVirtual);
                    Assert.False(m1.IsMetadataVirtual());
                    Assert.False(m1.IsSealed);
                    Assert.True(m1.IsStatic);
                    Assert.False(m1.IsExtern);
                    Assert.False(m1.IsAsync);
                    Assert.False(m1.IsOverride);
                    Assert.Equal(tuple.access, m1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(m1));
                }
            }
        }

        [Fact]
        public void MethodModifiers_07()
        {
            var source1 =
@"
public interface I1
{
    abstract static void M1(); 

    virtual static void M2() 
    {
    }

    sealed static void M3() 
    {
    }

    static void M4() 
    {
    }
}

class Test1 : I1
{
    void I1.M4() {}
    void I1.M1() {}
    void I1.M2() {}
    void I1.M3() {}
}

class Test2 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (4,26): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     abstract static void M1(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "9.0", "11.0").WithLocation(4, 26),
                // (6,25): error CS8703: The modifier 'virtual' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     virtual static void M2() 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M2").WithArguments("virtual", "9.0", "11.0").WithLocation(6, 25),
                // (10,24): error CS8703: The modifier 'sealed' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     sealed static void M3() 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M3").WithArguments("sealed", "9.0", "11.0").WithLocation(10, 24),
                // (27,15): error CS0535: 'Test2' does not implement interface member 'I1.M1()'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M1()").WithLocation(27, 15),
                // (22,13): error CS0539: 'Test1.M1()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M1() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("Test1.M1()").WithLocation(22, 13),
                // (23,13): error CS0539: 'Test1.M2()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M2() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M2").WithArguments("Test1.M2()").WithLocation(23, 13),
                // (24,13): error CS0539: 'Test1.M3()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M3() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M3").WithArguments("Test1.M3()").WithLocation(24, 13),
                // (21,13): error CS0539: 'Test1.M4()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M4() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M4").WithArguments("Test1.M4()").WithLocation(21, 13),
                // (19,15): error CS0535: 'Test1' does not implement interface member 'I1.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M1()").WithLocation(19, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.True(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m1));

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.False(m2.IsAbstract);
            Assert.True(m2.IsVirtual);
            Assert.True(m2.IsMetadataVirtual());
            Assert.False(m2.IsSealed);
            Assert.True(m2.IsStatic);
            Assert.False(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Public, m2.DeclaredAccessibility);
            Assert.Same(m2, test1.FindImplementationForInterfaceMember(m2));

            var m3 = i1.GetMember<MethodSymbol>("M3");

            Assert.False(m3.IsAbstract);
            Assert.False(m3.IsVirtual);
            Assert.False(m3.IsMetadataVirtual());
            Assert.False(m3.IsSealed);
            Assert.True(m3.IsStatic);
            Assert.False(m3.IsExtern);
            Assert.False(m3.IsAsync);
            Assert.False(m3.IsOverride);
            Assert.Equal(Accessibility.Public, m3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m3));
        }

        [Fact]
        public void MethodModifiers_08()
        {
            var source1 =
@"
public interface I1
{
    private void M1() 
    {
        System.Console.WriteLine(""M1"");
    }

    void M4()
    {
        System.Console.WriteLine(""M4"");
        M1();
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M4();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M4
M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");

                Assert.False(m1.IsAbstract);
                Assert.False(m1.IsVirtual);
                Assert.False(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(m1));
            }
        }

        [Fact]
        public void MethodModifiers_09()
        {
            var source1 =
@"
public interface I1
{
    abstract private void M1(); 

    virtual private void M2() 
    {
    }

    sealed private void M3() 
    {
    }
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (10,25): error CS0238: 'I1.M3()' cannot be sealed because it is not an override
                //     sealed private void M3() 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "M3").WithArguments("I1.M3()").WithLocation(10, 25),
                // (6,26): error CS0621: 'I1.M2()': virtual or abstract members cannot be private
                //     virtual private void M2() 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "M2").WithArguments("I1.M2()").WithLocation(6, 26),
                // (4,27): error CS0621: 'I1.M1()': virtual or abstract members cannot be private
                //     abstract private void M1(); 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "M1").WithArguments("I1.M1()").WithLocation(4, 27),
                // (15,15): error CS0535: 'Test1' does not implement interface member 'I1.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M1()").WithLocation(15, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m1));

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.False(m2.IsAbstract);
            Assert.True(m2.IsVirtual);
            Assert.True(m2.IsMetadataVirtual());
            Assert.False(m2.IsSealed);
            Assert.False(m2.IsStatic);
            Assert.False(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Private, m2.DeclaredAccessibility);
            Assert.Same(m2, test1.FindImplementationForInterfaceMember(m2));

            var m3 = i1.GetMember<MethodSymbol>("M3");

            Assert.False(m3.IsAbstract);
            Assert.False(m3.IsVirtual);
            Assert.False(m3.IsMetadataVirtual());
            Assert.True(m3.IsSealed);
            Assert.False(m3.IsStatic);
            Assert.False(m3.IsExtern);
            Assert.False(m3.IsAsync);
            Assert.False(m3.IsOverride);
            Assert.Equal(Accessibility.Private, m3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m3));
        }

        [Theory]
        [CombinatorialData]
        public void MethodModifiers_10_01(bool isStatic)
        {
            string declModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{   " + declModifiers + @"
    internal abstract void M1(); 

";
            if (!isStatic)
            {
                source1 +=
@"
    void M2() {M1();}
";
            }
            else
            {
                source1 +=
@"
    static void M2<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}
";

            var source2 =
@"
class Test1 : I1
{
    " + declModifiers + @"
    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        I1.M2<T>();
    }
";
            }

            source2 +=
@"
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            if (!isStatic)
            {
                compilation1.VerifyDiagnostics(
                    // (14,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(14, 17)
                    );
            }
            else
            {
                compilation1.VerifyDiagnostics(
                    // (4,28): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                    //     internal abstract void M1(); 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "9.0", "11.0").WithLocation(4, 28),
                    // (9,9): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                    //         T.M1();
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "T").WithArguments("static abstract members in interfaces", "11.0").WithLocation(9, 9)
                    );
            }

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.Internal, isStatic: isStatic);

            compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: isStatic ? TestOptions.Regular11 : TestOptions.Regular10,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, expectedOutput: !Execute(isStatic) ? null : "M1", verify: Verify(isStatic), symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, Accessibility.Internal, isStatic: isStatic)).VerifyDiagnostics();

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.Internal, isStatic: isStatic);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Internal, isStatic: isStatic);

            var source3 =
@"
class Test2 : I1
{
}
";

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation3.VerifyDiagnostics(
                    // (5,17): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(5, 17)
                    );

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.Internal, isStatic: isStatic);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular10,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation3.VerifyDiagnostics(
                    // (5,17): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(5, 17)
                    );

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.Internal, isStatic: isStatic);

                var compilation5 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation5.VerifyDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.M1()'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M1()").WithLocation(2, 15)
                    );

                ValidateI1M1NotImplemented(compilation5, "Test2");
            }
        }

        private static void ValidateI1M1NotImplemented(CSharpCompilation compilation, string className)
        {
            var test2 = compilation.GetTypeByMetadataName(className);
            var i1 = compilation.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");
            Assert.Null(test2.FindImplementationForInterfaceMember(m1));
        }

        private static void ValidateMethodModifiersImplicit_10(ModuleSymbol m, Accessibility accessibility, bool isStatic = false)
        {
            ValidateMethodModifiers_10(m, implementedByBase: false, isExplicit: false, accessibility, isStatic: isStatic);
        }

        private static void ValidateMethodModifiersExplicit_10(ModuleSymbol m, Accessibility accessibility)
        {
            ValidateMethodModifiers_10(m, implementedByBase: false, isExplicit: true, accessibility);
        }

        private static void ValidateMethodModifiersImplicitInTest2_10(ModuleSymbol m, Accessibility accessibility)
        {
            ValidateMethodModifiers_10(m, implementedByBase: true, isExplicit: false, accessibility);
        }

        private static void ValidateMethodModifiersExplicitInTest2_10(ModuleSymbol m, Accessibility accessibility)
        {
            ValidateMethodModifiers_10(m, implementedByBase: true, isExplicit: true, accessibility);
        }

        private static void ValidateMethodModifiers_10(ModuleSymbol m, bool implementedByBase, bool isExplicit, Accessibility accessibility, bool isStatic = false)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            var m1 = i1.GetMember<MethodSymbol>("M1");

            ValidateMethodModifiers_10(m1, accessibility, isStatic: isStatic);
            var implementation = (implementedByBase ? test1.BaseTypeNoUseSiteDiagnostics : test1).GetMember<MethodSymbol>((isExplicit ? "I1." : "") + "M1");
            Assert.NotNull(implementation);
            Assert.Same(implementation, test1.FindImplementationForInterfaceMember(m1));

            Assert.Equal(!isStatic, implementation.IsMetadataVirtual());
        }

        private static void ValidateMethodModifiers_10(MethodSymbol m1, Accessibility accessibility, bool isStatic = false)
        {
            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(accessibility, m1.DeclaredAccessibility);
        }

        [Fact]
        public void MethodModifiers_10_02()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallM1(new Test1());
    }

    public virtual void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";

            ValidateMethodModifiers_10_02(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (9,25): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(9, 25)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,25): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(9, 25)
                    )
                );
        }

        private static DiagnosticDescription[] ExpectedDiagnostics(params DiagnosticDescription[] array) => array;

        private void ValidateMethodModifiers_10_02(string source1, string source2,
                                                  Accessibility accessibility,
                                                  params DiagnosticDescription[] expectedIn9)
        {
            ValidateMethodModifiers_10_02(source1, source2, accessibility, expectedIn9, expectedIn9AcrossAssemblyBoundaries: expectedIn9, expectedAcrossAssemblyBoundaries: Array.Empty<DiagnosticDescription>());
        }

        private void ValidateMethodModifiers_10_02(string source1, string source2,
                                                  Accessibility accessibility,
                                                  DiagnosticDescription[] expectedIn9,
                                                  params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            ValidateMethodModifiers_10_02(source1, source2, accessibility, expectedIn9, expectedIn9AcrossAssemblyBoundaries: expectedIn9, expectedAcrossAssemblyBoundaries);
        }

        private void ValidateMethodModifiers_10_02(string source1, string source2,
                                                  Accessibility accessibility,
                                                  DiagnosticDescription[] expectedIn9,
                                                  DiagnosticDescription[] expectedIn9AcrossAssemblyBoundaries,
                                                  DiagnosticDescription[] expectedAcrossAssemblyBoundaries,
                                                  bool isStatic = false)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expectedIn9);

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, accessibility, isStatic: isStatic);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            CompileAndVerify(compilation1, expectedOutput: !Execute(isStatic) ? null : "Test1.M1", verify: Verify(isStatic), symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, accessibility, isStatic: isStatic)).VerifyDiagnostics();

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, accessibility, isStatic: isStatic);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), accessibility, isStatic: isStatic);

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9AcrossAssemblyBoundaries);

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, accessibility, isStatic: isStatic);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !Execute(isStatic) ? null : "Test1.M1", verify: Verify(isStatic), symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, accessibility, isStatic: isStatic)).VerifyDiagnostics();
                }

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, accessibility, isStatic: isStatic);
            }
        }

        [Fact]
        public void MethodModifiers_10_03()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallM1(new Test1());
    }

    void I1.M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput: "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicit_10(m, Accessibility.Internal));

            ValidateMethodModifiersExplicit_10(compilation1.SourceModule, Accessibility.Internal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Internal);

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation3.VerifyDiagnostics(
                // (9,13): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                //     void I1.M1() 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(9, 13)
                );

            ValidateMethodModifiersExplicit_10(compilation3.SourceModule, Accessibility.Internal);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation4.VerifyDiagnostics(
                // (9,13): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                //     void I1.M1() 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(9, 13)
                );

            ValidateMethodModifiersExplicit_10(compilation4.SourceModule, Accessibility.Internal);
        }

        [Fact]
        public void MethodModifiers_10_04()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}

public class Test2 : I1
{
    void I1.M1() 
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallM1(new Test1());
    }

    public void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";
            ValidateMethodModifiers_10_02(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (9,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(9, 17)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,17): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(9, 17)
                    )
                );
        }

        [Fact]
        public void MethodModifiers_10_05()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}

public class Test2 : I1
{
    void I1.M1() 
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallM1(new Test1());
    }

    public virtual void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";
            ValidateMethodModifiers_10_02(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (9,25): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(9, 25)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,25): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(9, 25)
                    )
                );
        }

        [Fact]
        public void MethodModifiers_10_06()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}

public class Test2 : I1
{
    void I1.M1() 
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";

            var source2 =
@"abstract 
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallM1(new Test3());
    }

    public abstract void M1();
}

class Test3 : Test1
{
    public override void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";
            ValidateMethodModifiers_10_02(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (9,26): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract void M1();
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(9, 26)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,26): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public abstract void M1();
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(9, 26)
                    )
                );
        }

        [Fact]
        public void MethodModifiers_10_07()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}

public class Test2 : I1
{
    void I1.M1() 
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallM1(new Test1());
    }

    public void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}

public interface I2
{
    void M1(); 
}
";
            ValidateMethodModifiers_10_02(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (9,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(9, 17)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,17): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(9, 17)
                    )
                );
        }

        [Fact]
        public void MethodModifiers_10_08()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

public class TestHelper
{
    public static void CallM1(I1 x) {x.M1();}
}

public class Test2 : I1
{
    void I1.M1() 
    {
        System.Console.WriteLine(""Test2.M1"");
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallM1(new Test1());
    }

    public virtual int M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
        return 0;
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput: "Test2.M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicitInTest2_10(m, Accessibility.Internal));

            ValidateMethodModifiersExplicitInTest2_10(compilation1.SourceModule, Accessibility.Internal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Internal);

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation3, expectedOutput: "Test2.M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicitInTest2_10(m, Accessibility.Internal));

            ValidateMethodModifiersExplicitInTest2_10(compilation3.SourceModule, Accessibility.Internal);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation4, expectedOutput: "Test2.M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicitInTest2_10(m, Accessibility.Internal));

            ValidateMethodModifiersExplicitInTest2_10(compilation4.SourceModule, Accessibility.Internal);
        }

        [Fact]
        public void MethodModifiers_10_09()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 

    void M2() {M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public virtual int M1() 
    {
        System.Console.WriteLine(""M1"");
        return 0;
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (9,15): error CS0738: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implement 'I1.M1()' because it does not have the matching return type of 'void'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "void").WithLocation(9, 15)
                );

            ValidateI1M1NotImplemented(compilation1, "Test1");

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Internal);

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation3.VerifyDiagnostics(
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implement 'I1.M1()' because it does not have the matching return type of 'void'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "void").WithLocation(2, 15)
                );

            ValidateI1M1NotImplemented(compilation3, "Test1");

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics(
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implement 'I1.M1()' because it does not have the matching return type of 'void'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "void").WithLocation(2, 15)
                );

            ValidateI1M1NotImplemented(compilation4, "Test1");
        }

        [Fact]
        public void MethodModifiers_10_10()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 

    void M2() {M1();}
}
";

            var source2 =
@"
class Test2 : I1
{
    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (11,17): error CS8704: 'Test2' does not implement interface member 'I1.M1()'. 'Test2.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //     public void M1() 
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test2", "I1.M1()", "Test2.M1()", "9.0", "10.0").WithLocation(11, 17)
                );

            ValidateMethodModifiersImplicitInTest2_10(compilation1.SourceModule, Accessibility.Internal);

            compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicitInTest2_10(m, Accessibility.Internal)).VerifyDiagnostics();

            ValidateMethodModifiersImplicitInTest2_10(compilation1.SourceModule, Accessibility.Internal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Internal);

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                foreach (var parseOptions in new[] { TestOptions.Regular9, TestOptions.Regular })
                {
                    var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                         parseOptions: parseOptions,
                                                         targetFramework: TargetFramework.NetCoreApp);

                    compilation3.VerifyDiagnostics(
                        // (4,17): error CS9044: 'Test2' does not implement interface member 'I1.M1()'. 'Test2.M1()' cannot implicitly implement an inaccessible member.
                        //     public void M1() 
                        Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test2", "I1.M1()", "Test2.M1()").WithLocation(4, 17)
                        );

                    ValidateMethodModifiersImplicitInTest2_10(compilation3.SourceModule, Accessibility.Internal);
                }
            }
        }

        [Fact]
        public void MethodModifiers_10_11()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 

    void M2() {M1();}
}

public class Test2 : I1
{
    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 assemblyName: "MethodModifiers_10_11");

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicitInTest2_10(m, Accessibility.Internal)).VerifyDiagnostics();

            ValidateMethodModifiersImplicitInTest2_10(compilation3.SourceModule, Accessibility.Internal);

            compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicitInTest2_10(m, Accessibility.Internal)).VerifyDiagnostics();

            ValidateMethodModifiersImplicitInTest2_10(compilation3.SourceModule, Accessibility.Internal);
        }

        [Fact]
        public void MethodModifiers_11()
        {
            var source1 =
@"
public interface I1
{
    internal abstract void M1(); 
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(
                // (7,15): error CS0535: 'Test1' does not implement interface member 'I1.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M1()").WithLocation(7, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Internal, m1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m1));
        }

        [Fact]
        public void MethodModifiers_12()
        {
            var source1 =
@"
public interface I1
{
    public sealed void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }

    public void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");

                Assert.False(m1.IsAbstract);
                Assert.False(m1.IsVirtual);
                Assert.False(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(m1));
            }

            CompileAndVerify(compilation1, expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate);
            Validate(compilation1.SourceModule);
        }

        [Fact]
        public void MethodModifiers_13()
        {
            var source1 =
@"
public interface I1
{
    public sealed void M1() 
    {
        System.Console.WriteLine(""M1"");
    }

    abstract sealed void M2(); 

    virtual sealed void M3() 
    {
    }

    public sealed void M4();
}

class Test1 : I1
{
    void I1.M1() {}
    void I1.M2() {}
    void I1.M3() {}
    void I1.M4() {}
}

class Test2 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (15,24): error CS0501: 'I1.M4()' must declare a body because it is not marked abstract, extern, or partial
                //     public sealed void M4();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M4").WithArguments("I1.M4()").WithLocation(15, 24),
                // (9,26): error CS0238: 'I1.M2()' cannot be sealed because it is not an override
                //     abstract sealed void M2(); 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "M2").WithArguments("I1.M2()").WithLocation(9, 26),
                // (11,25): error CS0238: 'I1.M3()' cannot be sealed because it is not an override
                //     virtual sealed void M3() 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "M3").WithArguments("I1.M3()").WithLocation(11, 25),
                // (23,13): error CS0539: 'Test1.M4()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M4() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M4").WithArguments("Test1.M4()").WithLocation(23, 13),
                // (20,13): error CS0539: 'Test1.M1()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M1() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("Test1.M1()").WithLocation(20, 13),
                // (21,13): error CS0539: 'Test1.M2()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M2() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M2").WithArguments("Test1.M2()").WithLocation(21, 13),
                // (22,13): error CS0539: 'Test1.M3()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M3() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M3").WithArguments("Test1.M3()").WithLocation(22, 13)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.False(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.False(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m1));
            Assert.Null(test2.FindImplementationForInterfaceMember(m1));

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.True(m2.IsAbstract);
            Assert.False(m2.IsVirtual);
            Assert.True(m2.IsMetadataVirtual());
            Assert.True(m2.IsSealed);
            Assert.False(m2.IsStatic);
            Assert.False(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Public, m2.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m2));
            Assert.Null(test2.FindImplementationForInterfaceMember(m2));

            var m3 = i1.GetMember<MethodSymbol>("M3");

            Assert.False(m3.IsAbstract);
            Assert.True(m3.IsVirtual);
            Assert.True(m3.IsMetadataVirtual());
            Assert.True(m3.IsSealed);
            Assert.False(m3.IsStatic);
            Assert.False(m3.IsExtern);
            Assert.False(m3.IsAsync);
            Assert.False(m3.IsOverride);
            Assert.Equal(Accessibility.Public, m3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m3));
            Assert.Null(test2.FindImplementationForInterfaceMember(m3));

            var m4 = i1.GetMember<MethodSymbol>("M4");

            Assert.False(m4.IsAbstract);
            Assert.False(m4.IsVirtual);
            Assert.False(m4.IsMetadataVirtual());
            Assert.False(m4.IsSealed);
            Assert.False(m4.IsStatic);
            Assert.False(m4.IsExtern);
            Assert.False(m4.IsAsync);
            Assert.False(m4.IsOverride);
            Assert.Equal(Accessibility.Public, m4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m4));
            Assert.Null(test2.FindImplementationForInterfaceMember(m4));
        }

        [Fact]
        public void MethodModifiers_14()
        {
            var source1 =
@"
public interface I1
{
    abstract virtual void M2(); 

    virtual abstract void M3() 
    {
    }
}

class Test1 : I1
{
    void I1.M2() {}
    void I1.M3() {}
}

class Test2 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (6,27): error CS0500: 'I1.M3()' cannot declare a body because it is marked abstract
                //     virtual abstract void M3() 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "M3").WithArguments("I1.M3()").WithLocation(6, 27),
                // (6,27): error CS0503: The abstract method 'I1.M3()' cannot be marked virtual
                //     virtual abstract void M3() 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "M3").WithArguments("method", "I1.M3()").WithLocation(6, 27),
                // (4,27): error CS0503: The abstract method 'I1.M2()' cannot be marked virtual
                //     abstract virtual void M2(); 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "M2").WithArguments("method", "I1.M2()").WithLocation(4, 27),
                // (17,15): error CS0535: 'Test2' does not implement interface member 'I1.M3()'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M3()").WithLocation(17, 15),
                // (17,15): error CS0535: 'Test2' does not implement interface member 'I1.M2()'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M2()").WithLocation(17, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var i1 = compilation1.GetTypeByMetadataName("I1");

            foreach (var methodName in new[] { "M2", "M3" })
            {
                var m2 = i1.GetMember<MethodSymbol>(methodName);

                Assert.True(m2.IsAbstract);
                Assert.True(m2.IsVirtual);
                Assert.True(m2.IsMetadataVirtual());
                Assert.False(m2.IsSealed);
                Assert.False(m2.IsStatic);
                Assert.False(m2.IsExtern);
                Assert.False(m2.IsAsync);
                Assert.False(m2.IsOverride);
                Assert.Equal(Accessibility.Public, m2.DeclaredAccessibility);
                Assert.Same(test1.GetMember("I1." + methodName), test1.FindImplementationForInterfaceMember(m2));
                Assert.Null(test2.FindImplementationForInterfaceMember(m2));
            }
        }

        [Fact]
        public void MethodModifiers_15()
        {
            var source1 =
@"
public interface I1
{
    extern void M1(); 
    virtual extern void M2(); 
    static extern void M3(); 
    private extern void M4();
    extern sealed void M5();
}

class Test1 : I1
{
}

class Test2 : I1
{
    void I1.M1() {}
    void I1.M2() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var test2 = m.GlobalNamespace.GetTypeMember("Test2");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");
                bool isSource = !(m is PEModuleSymbol);

                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.Equal(isSource, m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
                Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
                Assert.Same(test2.GetMember("I1.M1"), test2.FindImplementationForInterfaceMember(m1));

                var m2 = i1.GetMember<MethodSymbol>("M2");

                Assert.False(m2.IsAbstract);
                Assert.True(m2.IsVirtual);
                Assert.True(m2.IsMetadataVirtual());
                Assert.False(m2.IsSealed);
                Assert.False(m2.IsStatic);
                Assert.Equal(isSource, m2.IsExtern);
                Assert.False(m2.IsAsync);
                Assert.False(m2.IsOverride);
                Assert.Equal(Accessibility.Public, m2.DeclaredAccessibility);
                Assert.Same(m2, test1.FindImplementationForInterfaceMember(m2));
                Assert.Same(test2.GetMember("I1.M2"), test2.FindImplementationForInterfaceMember(m2));

                var m3 = i1.GetMember<MethodSymbol>("M3");

                Assert.False(m3.IsAbstract);
                Assert.False(m3.IsVirtual);
                Assert.False(m3.IsMetadataVirtual());
                Assert.False(m3.IsSealed);
                Assert.True(m3.IsStatic);
                Assert.Equal(isSource, m3.IsExtern);
                Assert.False(m3.IsAsync);
                Assert.False(m3.IsOverride);
                Assert.Equal(Accessibility.Public, m3.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(m3));
                Assert.Null(test2.FindImplementationForInterfaceMember(m3));

                var m4 = i1.GetMember<MethodSymbol>("M4");

                Assert.False(m4.IsAbstract);
                Assert.False(m4.IsVirtual);
                Assert.False(m4.IsMetadataVirtual());
                Assert.False(m4.IsSealed);
                Assert.False(m4.IsStatic);
                Assert.Equal(isSource, m4.IsExtern);
                Assert.False(m4.IsAsync);
                Assert.False(m4.IsOverride);
                Assert.Equal(Accessibility.Private, m4.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(m4));
                Assert.Null(test2.FindImplementationForInterfaceMember(m4));

                var m5 = i1.GetMember<MethodSymbol>("M5");

                Assert.False(m5.IsAbstract);
                Assert.False(m5.IsVirtual);
                Assert.False(m5.IsMetadataVirtual());
                Assert.False(m5.IsSealed);
                Assert.False(m5.IsStatic);
                Assert.Equal(isSource, m5.IsExtern);
                Assert.False(m5.IsAsync);
                Assert.False(m5.IsOverride);
                Assert.Equal(Accessibility.Public, m5.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(m5));
                Assert.Null(test2.FindImplementationForInterfaceMember(m5));
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (4,17): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern void M1(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("extern", "7.3", "8.0").WithLocation(4, 17),
                // (5,25): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M2").WithArguments("extern", "7.3", "8.0").WithLocation(5, 25),
                // (5,25): error CS8503: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M2").WithArguments("virtual", "7.3", "8.0").WithLocation(5, 25),
                // (6,24): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M3").WithArguments("static", "7.3", "8.0").WithLocation(6, 24),
                // (6,24): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M3").WithArguments("extern", "7.3", "8.0").WithLocation(6, 24),
                // (7,25): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern void M4();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M4").WithArguments("private", "7.3", "8.0").WithLocation(7, 25),
                // (7,25): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern void M4();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M4").WithArguments("extern", "7.3", "8.0").WithLocation(7, 25),
                // (8,24): error CS8503: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M5").WithArguments("sealed", "7.3", "8.0").WithLocation(8, 24),
                // (8,24): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M5").WithArguments("extern", "7.3", "8.0").WithLocation(8, 24),
                // (4,17): warning CS0626: Method, operator, or accessor 'I1.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void M1(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.M1()").WithLocation(4, 17),
                // (5,25): warning CS0626: Method, operator, or accessor 'I1.M2()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M2").WithArguments("I1.M2()").WithLocation(5, 25),
                // (6,24): warning CS0626: Method, operator, or accessor 'I1.M3()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M3").WithArguments("I1.M3()").WithLocation(6, 24),
                // (7,25): warning CS0626: Method, operator, or accessor 'I1.M4()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern void M4();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M4").WithArguments("I1.M4()").WithLocation(7, 25),
                // (8,24): warning CS0626: Method, operator, or accessor 'I1.M5()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M5").WithArguments("I1.M5()").WithLocation(8, 24)
                );

            Validate(compilation2.SourceModule);

            var compilation3 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular14, skipUsesIsNullable: true);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (4,17): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern void M1(); 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(4, 17),
                // (5,25): error CS8501: Target runtime doesn't support default interface implementation.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M2").WithLocation(5, 25),
                // (6,24): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "M3").WithArguments("static members in interfaces", "15.0").WithLocation(6, 24),
                // (7,25): error CS8501: Target runtime doesn't support default interface implementation.
                //     private extern void M4();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M4").WithLocation(7, 25),
                // (8,24): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M5").WithLocation(8, 24),
                // (4,17): warning CS0626: Method, operator, or accessor 'I1.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void M1(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.M1()").WithLocation(4, 17),
                // (5,25): warning CS0626: Method, operator, or accessor 'I1.M2()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M2").WithArguments("I1.M2()").WithLocation(5, 25),
                // (6,24): warning CS0626: Method, operator, or accessor 'I1.M3()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M3").WithArguments("I1.M3()").WithLocation(6, 24),
                // (7,25): warning CS0626: Method, operator, or accessor 'I1.M4()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern void M4();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M4").WithArguments("I1.M4()").WithLocation(7, 25),
                // (8,24): warning CS0626: Method, operator, or accessor 'I1.M5()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M5").WithArguments("I1.M5()").WithLocation(8, 24)
                );

            Validate(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular15, skipUsesIsNullable: true);
            Assert.False(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            var expected = new[]
            {
                // (4,17): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern void M1(); 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(4, 17),
                // (5,25): error CS8501: Target runtime doesn't support default interface implementation.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M2").WithLocation(5, 25),
                // (7,25): error CS8501: Target runtime doesn't support default interface implementation.
                //     private extern void M4();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M4").WithLocation(7, 25),
                // (8,24): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M5").WithLocation(8, 24),
                // (4,17): warning CS0626: Method, operator, or accessor 'I1.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void M1(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.M1()").WithLocation(4, 17),
                // (5,25): warning CS0626: Method, operator, or accessor 'I1.M2()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern void M2(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M2").WithArguments("I1.M2()").WithLocation(5, 25),
                // (6,24): warning CS0626: Method, operator, or accessor 'I1.M3()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M3").WithArguments("I1.M3()").WithLocation(6, 24),
                // (7,25): warning CS0626: Method, operator, or accessor 'I1.M4()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern void M4();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M4").WithArguments("I1.M4()").WithLocation(7, 25),
                // (8,24): warning CS0626: Method, operator, or accessor 'I1.M5()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M5").WithArguments("I1.M5()").WithLocation(8, 24)
            };

            compilation4.VerifyDiagnostics(expected);

            Validate(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);
            Assert.False(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation5.VerifyDiagnostics(expected);

            Validate(compilation5.SourceModule);
        }

        [Fact]
        public void MethodModifiers_16()
        {
            var source1 =
@"
public interface I1
{
    abstract extern void M1(); 
    extern void M2() {} 
    static extern void M3(); 
    private extern void M4();
    extern sealed void M5();
}

class Test1 : I1
{
}

class Test2 : I1
{
    void I1.M1() {}
    void I1.M2() {}
    void I1.M3() {}
    void I1.M4() {}
    void I1.M5() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,26): error CS0180: 'I1.M1()' cannot be both extern and abstract
                //     abstract extern void M1(); 
                Diagnostic(ErrorCode.ERR_AbstractAndExtern, "M1").WithArguments("I1.M1()").WithLocation(4, 26),
                // (5,17): error CS0179: 'I1.M2()' cannot be extern and declare a body
                //     extern void M2() {} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "M2").WithArguments("I1.M2()").WithLocation(5, 17),
                // (11,15): error CS0535: 'Test1' does not implement interface member 'I1.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M1()").WithLocation(11, 15),
                // (19,13): error CS0539: 'Test2.M3()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M3() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M3").WithArguments("Test2.M3()").WithLocation(19, 13),
                // (20,13): error CS0539: 'Test2.M4()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M4() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M4").WithArguments("Test2.M4()").WithLocation(20, 13),
                // (21,13): error CS0539: 'Test2.M5()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M5() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M5").WithArguments("Test2.M5()").WithLocation(21, 13),
                // (6,24): warning CS0626: Method, operator, or accessor 'I1.M3()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern void M3(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M3").WithArguments("I1.M3()").WithLocation(6, 24),
                // (7,25): warning CS0626: Method, operator, or accessor 'I1.M4()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern void M4();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M4").WithArguments("I1.M4()").WithLocation(7, 25),
                // (8,24): warning CS0626: Method, operator, or accessor 'I1.M5()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed void M5();
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M5").WithArguments("I1.M5()").WithLocation(8, 24)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.True(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m1));
            Assert.Same(test2.GetMember("I1.M1"), test2.FindImplementationForInterfaceMember(m1));

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.False(m2.IsAbstract);
            Assert.True(m2.IsVirtual);
            Assert.True(m2.IsMetadataVirtual());
            Assert.False(m2.IsSealed);
            Assert.False(m2.IsStatic);
            Assert.True(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Public, m2.DeclaredAccessibility);
            Assert.Same(m2, test1.FindImplementationForInterfaceMember(m2));
            Assert.Same(test2.GetMember("I1.M2"), test2.FindImplementationForInterfaceMember(m2));

            var m3 = i1.GetMember<MethodSymbol>("M3");
            Assert.Null(test2.FindImplementationForInterfaceMember(m3));

            var m4 = i1.GetMember<MethodSymbol>("M4");
            Assert.Null(test2.FindImplementationForInterfaceMember(m4));

            var m5 = i1.GetMember<MethodSymbol>("M5");
            Assert.Null(test2.FindImplementationForInterfaceMember(m5));
        }

        [Fact]
        public void MethodModifiers_17()
        {
            var source1 =
@"
public interface I1
{
    abstract void M1() {} 
    abstract private void M2() {} 
    abstract static void M3() {} 
    static extern void M4() {}
    override sealed void M5() {}
}

class Test1 : I1
{
}

class Test2 : I1
{
    void I1.M1() {}
    void I1.M2() {}
    void I1.M3() {}
    void I1.M4() {}
    void I1.M5() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,19): error CS0500: 'I1.M1()' cannot declare a body because it is marked abstract
                //     abstract void M1() {} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "M1").WithArguments("I1.M1()").WithLocation(4, 19),
                // (5,27): error CS0500: 'I1.M2()' cannot declare a body because it is marked abstract
                //     abstract private void M2() {} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "M2").WithArguments("I1.M2()").WithLocation(5, 27),
                // (6,26): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     abstract static void M3() {} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M3").WithArguments("abstract", "9.0", "11.0").WithLocation(6, 26),
                // (6,26): error CS0500: 'I1.M3()' cannot declare a body because it is marked abstract
                //     abstract static void M3() {} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "M3").WithArguments("I1.M3()").WithLocation(6, 26),
                // (7,24): error CS0179: 'I1.M4()' cannot be extern and declare a body
                //     static extern void M4() {}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "M4").WithArguments("I1.M4()").WithLocation(7, 24),
                // (8,26): error CS0106: The modifier 'override' is not valid for this item
                //     override sealed void M5() {}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M5").WithArguments("override").WithLocation(8, 26),
                // (5,27): error CS0621: 'I1.M2()': virtual or abstract members cannot be private
                //     abstract private void M2() {} 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "M2").WithArguments("I1.M2()").WithLocation(5, 27),
                // (11,15): error CS0535: 'Test1' does not implement interface member 'I1.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M1()").WithLocation(11, 15),
                // (11,15): error CS0535: 'Test1' does not implement interface member 'I1.M2()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M2()").WithLocation(11, 15),
                // (11,15): error CS0535: 'Test1' does not implement interface member 'I1.M3()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M3()").WithLocation(11, 15),
                // (18,13): error CS0122: 'I1.M2()' is inaccessible due to its protection level
                //     void I1.M2() {}
                Diagnostic(ErrorCode.ERR_BadAccess, "M2").WithArguments("I1.M2()").WithLocation(18, 13),
                // (19,13): error CS0539: 'Test2.M3()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M3() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M3").WithArguments("Test2.M3()").WithLocation(19, 13),
                // (20,13): error CS0539: 'Test2.M4()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M4() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M4").WithArguments("Test2.M4()").WithLocation(20, 13),
                // (21,13): error CS0539: 'Test2.M5()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     void I1.M5() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M5").WithArguments("Test2.M5()").WithLocation(21, 13),
                // (15,15): error CS0535: 'Test2' does not implement interface member 'I1.M3()'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M3()").WithLocation(15, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m1));
            Assert.Same(test2.GetMember("I1.M1"), test2.FindImplementationForInterfaceMember(m1));

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.True(m2.IsAbstract);
            Assert.False(m2.IsVirtual);
            Assert.True(m2.IsMetadataVirtual());
            Assert.False(m2.IsSealed);
            Assert.False(m2.IsStatic);
            Assert.False(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Private, m2.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m2));
            Assert.Same(test2.GetMember("I1.M2"), test2.FindImplementationForInterfaceMember(m2));

            var m3 = i1.GetMember<MethodSymbol>("M3");

            Assert.True(m3.IsAbstract);
            Assert.False(m3.IsVirtual);
            Assert.True(m3.IsMetadataVirtual());
            Assert.False(m3.IsSealed);
            Assert.True(m3.IsStatic);
            Assert.False(m3.IsExtern);
            Assert.False(m3.IsAsync);
            Assert.False(m3.IsOverride);
            Assert.Equal(Accessibility.Public, m3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m3));
            Assert.Null(test2.FindImplementationForInterfaceMember(m3));

            var m4 = i1.GetMember<MethodSymbol>("M4");

            Assert.False(m4.IsAbstract);
            Assert.False(m4.IsVirtual);
            Assert.False(m4.IsMetadataVirtual());
            Assert.False(m4.IsSealed);
            Assert.True(m4.IsStatic);
            Assert.True(m4.IsExtern);
            Assert.False(m4.IsAsync);
            Assert.False(m4.IsOverride);
            Assert.Equal(Accessibility.Public, m4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m4));
            Assert.Null(test2.FindImplementationForInterfaceMember(m4));

            var m5 = i1.GetMember<MethodSymbol>("M5");

            Assert.False(m5.IsAbstract);
            Assert.False(m5.IsVirtual);
            Assert.False(m5.IsMetadataVirtual());
            Assert.False(m5.IsSealed);
            Assert.False(m5.IsStatic);
            Assert.False(m5.IsExtern);
            Assert.False(m5.IsAsync);
            Assert.False(m5.IsOverride);
            Assert.Equal(Accessibility.Public, m5.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(m5));
            Assert.Null(test2.FindImplementationForInterfaceMember(m5));
        }

        [Fact]
        [WorkItem(34658, "https://github.com/dotnet/roslyn/issues/34658")]
        public void MethodModifiers_18()
        {
            var source1 =
@"
using System.Threading;
using System.Threading.Tasks;

public interface I1
{
    public static async Task M1() 
    {
        await Task.Factory.StartNew(() => System.Console.WriteLine(""M1""));
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.M1().Wait();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");

                Assert.False(m1.IsAbstract);
                Assert.False(m1.IsVirtual);
                Assert.False(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.True(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.Equal(!(m is PEModuleSymbol), m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Public, m1.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(m1));
            }
        }

        [Fact]
        public void MethodModifiers_20()
        {
            var source1 =
@"
public interface I1
{
    internal void M1()
    {
        System.Console.WriteLine(""M1"");
    }

    void M2() {M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var m1 = i1.GetMember<MethodSymbol>("M1");

                ValidateMethod(m1);
                Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
            }

            void ValidateMethod(MethodSymbol m1)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Internal, m1.DeclaredAccessibility);
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");
                ValidateMethod(m1);
            }

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation3,
                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation4,
                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation4.SourceModule);
        }

        [Fact]
        public void MethodModifiers_21()
        {
            var source1 =
@"
public interface I1
{
    private static void M1() {}

    internal static void M2() {}

    public static void M3() {}

    static void M4() {}
}

class Test1
{
    static void Main()
    {
        I1.M1();
        I1.M2();
        I1.M3();
        I1.M4();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (17,12): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                //         I1.M1();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(17, 12)
                );

            var source2 =
@"
class Test2
{
    static void Main()
    {
        I1.M1();
        I1.M2();
        I1.M3();
        I1.M4();
    }
}
";
            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(
                // (6,12): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                //         I1.M1();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(6, 12),
                // (7,12): error CS0122: 'I1.M2()' is inaccessible due to its protection level
                //         I1.M2();
                Diagnostic(ErrorCode.ERR_BadAccess, "M2").WithArguments("I1.M2()").WithLocation(7, 12)
                );
        }

        [Fact]
        public void MethodModifiers_22()
        {
            var source1 =
@"
public partial interface I1
{
    static partial void M1();

    [Test2(1)]
    static partial void M2();

    static partial void Main() 
    {
        M1();
        M2();
        new System.Action(M2).Invoke();
    }
}

public partial interface I1
{
    [Test2(2)]
    static partial void M2() 
    {
        System.Console.WriteLine(""M2"");
    }

    static partial void Main();
}

class Test1 : I1
{
}

[System.AttributeUsage(System.AttributeTargets.All, AllowMultiple = true)]
class Test2 : System.Attribute
{
    public Test2(int x) {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M2
M2");

            var i1 = compilation1.GetTypeByMetadataName("I1");

            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.False(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.False(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.True(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);
            Assert.True(m1.IsPartialMember());
            Assert.Null(m1.PartialImplementationPart);

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.False(m2.IsAbstract);
            Assert.False(m2.IsVirtual);
            Assert.False(m2.IsMetadataVirtual());
            Assert.False(m2.IsSealed);
            Assert.True(m2.IsStatic);
            Assert.False(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Private, m2.DeclaredAccessibility);
            Assert.True(m2.IsPartialMember());

            Assert.Equal(2, m2.GetAttributes().Length);
            Assert.Equal("Test2(1)", m2.GetAttributes()[0].ToString());
            Assert.Equal("Test2(2)", m2.GetAttributes()[1].ToString());

            var m2Impl = m2.PartialImplementationPart;

            Assert.False(m2Impl.IsAbstract);
            Assert.False(m2Impl.IsVirtual);
            Assert.False(m2Impl.IsMetadataVirtual());
            Assert.False(m2Impl.IsSealed);
            Assert.True(m2Impl.IsStatic);
            Assert.False(m2Impl.IsExtern);
            Assert.False(m2Impl.IsAsync);
            Assert.False(m2Impl.IsOverride);
            Assert.Equal(Accessibility.Private, m2Impl.DeclaredAccessibility);
            Assert.True(m2Impl.IsPartialMember());
            Assert.Same(m2, m2Impl.PartialDefinitionPart);

            Assert.Equal(2, m2Impl.GetAttributes().Length);
            Assert.Equal("Test2(1)", m2Impl.GetAttributes()[0].ToString());
            Assert.Equal("Test2(2)", m2Impl.GetAttributes()[1].ToString());
        }

        [Fact]
        public void MethodModifiers_23()
        {
            var source1 =
@"
public partial interface I1
{
    partial void M1();

    [Test2(1)]
    partial void M2();

    void M3() 
    {
        M1();
        M2();
        new System.Action(M2).Invoke();
    }
}

public partial interface I1
{
    [Test2(2)]
    partial void M2() 
    {
        System.Console.WriteLine(""M2"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M3();
    }
}

[System.AttributeUsage(System.AttributeTargets.All, AllowMultiple = true)]
class Test2 : System.Attribute
{
    public Test2(int x) {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M2
M2",
                verify: VerifyOnMonoOrCoreClr);

            var i1 = compilation1.GetTypeByMetadataName("I1");

            var m1 = i1.GetMember<MethodSymbol>("M1");

            Assert.False(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.False(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);
            Assert.True(m1.IsPartialMember());
            Assert.Null(m1.PartialImplementationPart);

            var m2 = i1.GetMember<MethodSymbol>("M2");

            Assert.False(m2.IsAbstract);
            Assert.False(m2.IsVirtual);
            Assert.False(m2.IsMetadataVirtual());
            Assert.False(m2.IsSealed);
            Assert.False(m2.IsStatic);
            Assert.False(m2.IsExtern);
            Assert.False(m2.IsAsync);
            Assert.False(m2.IsOverride);
            Assert.Equal(Accessibility.Private, m2.DeclaredAccessibility);
            Assert.True(m2.IsPartialMember());

            Assert.Equal(2, m2.GetAttributes().Length);
            Assert.Equal("Test2(1)", m2.GetAttributes()[0].ToString());
            Assert.Equal("Test2(2)", m2.GetAttributes()[1].ToString());

            var m2Impl = m2.PartialImplementationPart;

            Assert.False(m2Impl.IsAbstract);
            Assert.False(m2Impl.IsVirtual);
            Assert.False(m2Impl.IsMetadataVirtual());
            Assert.False(m2Impl.IsSealed);
            Assert.False(m2Impl.IsStatic);
            Assert.False(m2Impl.IsExtern);
            Assert.False(m2Impl.IsAsync);
            Assert.False(m2Impl.IsOverride);
            Assert.Equal(Accessibility.Private, m2Impl.DeclaredAccessibility);
            Assert.True(m2Impl.IsPartialMember());
            Assert.Same(m2, m2Impl.PartialDefinitionPart);

            Assert.Equal(2, m2Impl.GetAttributes().Length);
            Assert.Equal("Test2(1)", m2Impl.GetAttributes()[0].ToString());
            Assert.Equal("Test2(2)", m2Impl.GetAttributes()[1].ToString());
        }

        [Fact]
        public void MethodModifiers_24()
        {
            var source1 =
@"
public partial interface I1
{
    static partial void M1();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation1.VerifyDiagnostics(
                // (4,25): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static partial void M1();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "7.3", "8.0").WithLocation(4, 25),
                // (4,25): error CS8703: The modifier 'partial' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static partial void M1();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("partial", "7.3", "8.0").WithLocation(4, 25)
                );
        }

        [Fact]
        public void MethodModifiers_25()
        {
            var source1 =
@"
public partial interface I1
{
    partial void M1();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,18): error CS8703: The modifier 'partial' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     partial void M1();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("partial", "7.3", "8.0").WithLocation(4, 18)
                );
        }

        [Fact]
        public void MethodModifiers_26_01()
        {
            var source1 =
@"
public partial interface I1
{
    static partial int M1();
    public static partial void M2();
    internal static partial void M3();
    private static partial void M4();
    static partial void M5(out int x);
    static partial void M6(); 
    static void M7() {}
    static partial void M8() {} 
    static partial void M9(); 
    static partial void M10(); 
}

public partial interface I1
{
    static void M6() {}
    static partial void M7();
    static partial void M8() {} 
    static partial void M9(); 
    static extern partial void M10(); 
    protected static partial void M11();
    protected internal static partial void M12();
    private protected static partial void M13();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularWithExtendedPartialMethods,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,24): error CS8794: Partial method 'I1.M1()' must have accessibility modifiers because it has a non-void return type.
                //     static partial int M1();
                Diagnostic(ErrorCode.ERR_PartialMethodWithNonVoidReturnMustHaveAccessMods, "M1").WithArguments("I1.M1()").WithLocation(4, 24),
                // (5,32): error CS8793: Partial method 'I1.M2()' must have an implementation part because it has accessibility modifiers.
                //     public static partial void M2();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M2").WithArguments("I1.M2()").WithLocation(5, 32),
                // (6,34): error CS8793: Partial method 'I1.M3()' must have an implementation part because it has accessibility modifiers.
                //     internal static partial void M3();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M3").WithArguments("I1.M3()").WithLocation(6, 34),
                // (7,33): error CS8793: Partial method 'I1.M4()' must have an implementation part because it has accessibility modifiers.
                //     private static partial void M4();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M4").WithArguments("I1.M4()").WithLocation(7, 33),
                // (8,25): error CS8795: Partial method 'I1.M5(out int)' must have accessibility modifiers because it has 'out' parameters.
                //     static partial void M5(out int x);
                Diagnostic(ErrorCode.ERR_PartialMethodWithOutParamMustHaveAccessMods, "M5").WithArguments("I1.M5(out int)").WithLocation(8, 25),
                // (11,25): error CS0759: No defining declaration found for implementing declaration of partial method 'I1.M8()'
                //     static partial void M8() {}
                Diagnostic(ErrorCode.ERR_PartialMethodMustHaveLatent, "M8").WithArguments("I1.M8()").WithLocation(11, 25),
                // (18,17): error CS0111: Type 'I1' already defines a member called 'M6' with the same parameter types
                //     static void M6() {}
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M6").WithArguments("M6", "I1").WithLocation(18, 17),
                // (19,25): error CS0111: Type 'I1' already defines a member called 'M7' with the same parameter types
                //     static partial void M7();
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M7").WithArguments("M7", "I1").WithLocation(19, 25),
                // (20,25): error CS0757: A partial method may not have multiple implementing declarations
                //     static partial void M8() {}
                Diagnostic(ErrorCode.ERR_PartialMethodOnlyOneActual, "M8").WithLocation(20, 25),
                // (20,25): error CS0111: Type 'I1' already defines a member called 'M8' with the same parameter types
                //     static partial void M8() {}
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M8").WithArguments("M8", "I1").WithLocation(20, 25),
                // (21,25): error CS0756: A partial method may not have multiple defining declarations
                //     static partial void M9();
                Diagnostic(ErrorCode.ERR_PartialMethodOnlyOneLatent, "M9").WithLocation(21, 25),
                // (21,25): error CS0111: Type 'I1' already defines a member called 'M9' with the same parameter types
                //     static partial void M9();
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M9").WithArguments("M9", "I1").WithLocation(21, 25),
                // (22,32): error CS8796: Partial method 'I1.M10()' must have accessibility modifiers because it has a 'virtual', 'override', 'sealed', 'new', or 'extern' modifier.
                //     static extern partial void M10();
                Diagnostic(ErrorCode.ERR_PartialMethodWithExtendedModMustHaveAccessMods, "M10").WithArguments("I1.M10()").WithLocation(22, 32),
                // (23,35): error CS8793: Partial method 'I1.M11()' must have an implementation part because it has accessibility modifiers.
                //     protected static partial void M11();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M11").WithArguments("I1.M11()").WithLocation(23, 35),
                // (24,44): error CS8793: Partial method 'I1.M12()' must have an implementation part because it has accessibility modifiers.
                //     protected internal static partial void M12();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M12").WithArguments("I1.M12()").WithLocation(24, 44),
                // (25,43): error CS8793: Partial method 'I1.M13()' must have an implementation part because it has accessibility modifiers.
                //     private protected static partial void M13();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M13").WithArguments("I1.M13()").WithLocation(25, 43)
                );

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularWithExtendedPartialMethods,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (4,24): error CS8794: Partial method 'I1.M1()' must have accessibility modifiers because it has a non-void return type.
                //     static partial int M1();
                Diagnostic(ErrorCode.ERR_PartialMethodWithNonVoidReturnMustHaveAccessMods, "M1").WithArguments("I1.M1()").WithLocation(4, 24),
                // (5,32): error CS8793: Partial method 'I1.M2()' must have an implementation part because it has accessibility modifiers.
                //     public static partial void M2();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M2").WithArguments("I1.M2()").WithLocation(5, 32),
                // (6,34): error CS8793: Partial method 'I1.M3()' must have an implementation part because it has accessibility modifiers.
                //     internal static partial void M3();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M3").WithArguments("I1.M3()").WithLocation(6, 34),
                // (7,33): error CS8793: Partial method 'I1.M4()' must have an implementation part because it has accessibility modifiers.
                //     private static partial void M4();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M4").WithArguments("I1.M4()").WithLocation(7, 33),
                // (8,25): error CS8795: Partial method 'I1.M5(out int)' must have accessibility modifiers because it has 'out' parameters.
                //     static partial void M5(out int x);
                Diagnostic(ErrorCode.ERR_PartialMethodWithOutParamMustHaveAccessMods, "M5").WithArguments("I1.M5(out int)").WithLocation(8, 25),
                // (11,25): error CS0759: No defining declaration found for implementing declaration of partial method 'I1.M8()'
                //     static partial void M8() {}
                Diagnostic(ErrorCode.ERR_PartialMethodMustHaveLatent, "M8").WithArguments("I1.M8()").WithLocation(11, 25),
                // (18,17): error CS0111: Type 'I1' already defines a member called 'M6' with the same parameter types
                //     static void M6() {}
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M6").WithArguments("M6", "I1").WithLocation(18, 17),
                // (19,25): error CS0111: Type 'I1' already defines a member called 'M7' with the same parameter types
                //     static partial void M7();
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M7").WithArguments("M7", "I1").WithLocation(19, 25),
                // (20,25): error CS0757: A partial method may not have multiple implementing declarations
                //     static partial void M8() {}
                Diagnostic(ErrorCode.ERR_PartialMethodOnlyOneActual, "M8").WithLocation(20, 25),
                // (20,25): error CS0111: Type 'I1' already defines a member called 'M8' with the same parameter types
                //     static partial void M8() {}
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M8").WithArguments("M8", "I1").WithLocation(20, 25),
                // (21,25): error CS0756: A partial method may not have multiple defining declarations
                //     static partial void M9();
                Diagnostic(ErrorCode.ERR_PartialMethodOnlyOneLatent, "M9").WithLocation(21, 25),
                // (21,25): error CS0111: Type 'I1' already defines a member called 'M9' with the same parameter types
                //     static partial void M9();
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M9").WithArguments("M9", "I1").WithLocation(21, 25),
                // (22,32): error CS8796: Partial method 'I1.M10()' must have accessibility modifiers because it has a 'virtual', 'override', 'sealed', 'new', or 'extern' modifier.
                //     static extern partial void M10();
                Diagnostic(ErrorCode.ERR_PartialMethodWithExtendedModMustHaveAccessMods, "M10").WithArguments("I1.M10()").WithLocation(22, 32),
                // (23,35): error CS8793: Partial method 'I1.M11()' must have an implementation part because it has accessibility modifiers.
                //     protected static partial void M11();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M11").WithArguments("I1.M11()").WithLocation(23, 35),
                // (24,44): error CS8793: Partial method 'I1.M12()' must have an implementation part because it has accessibility modifiers.
                //     protected internal static partial void M12();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M12").WithArguments("I1.M12()").WithLocation(24, 44),
                // (25,43): error CS8793: Partial method 'I1.M13()' must have an implementation part because it has accessibility modifiers.
                //     private protected static partial void M13();
                Diagnostic(ErrorCode.ERR_PartialMethodWithAccessibilityModsMustHaveImplementation, "M13").WithArguments("I1.M13()").WithLocation(25, 43)
                );
        }

        [Theory]
        [CombinatorialData]
        public void MethodModifiers_26_02([CombinatorialValues("", "public", "private", "internal")] string accessibility)
        {
            var source1 =
@"
public partial interface I1
{
    " + accessibility + @"
    static void M1() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (5,17): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static void M1() {}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "M1").WithArguments("static members in interfaces", "15.0").WithLocation(5, 17)
                );

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics();

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics();
        }

        [Theory]
        [CombinatorialData]
        public void MethodModifiers_26_03([CombinatorialValues("protected", "internal protected", "private protected")] string accessibility)
        {
            var source1 =
@"
public partial interface I1
{
    " + accessibility + @"
    static void M1() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (5,17): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static void M1() {}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M1").WithLocation(5, 17)
                );

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (5,17): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static void M1() {}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M1").WithLocation(5, 17)
                );

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (5,17): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static void M1() {}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M1").WithLocation(5, 17)
                );
        }

        [Fact]
        public void MethodModifiers_27()
        {
            var source1 =
@"
partial interface I1 : I2
{
    sealed partial void M1();
    abstract partial void M2();
    virtual partial void M3();
    partial void M4();
    partial void M5();
    partial void M6();
    partial void I2.M7();
}

partial interface I1 : I2
{
    sealed partial void M4() {}
    abstract partial void M5(); 
    virtual partial void M6() {}

    partial void I2.M7() {}
}

interface I2
{
    void M7();
    partial void M8();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularWithExtendedPartialMethods,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,25): error CS8796: Partial method 'I1.M1()' must have accessibility modifiers because it has a 'virtual', 'override', 'sealed', 'new', or 'extern' modifier.
                //     sealed partial void M1();
                Diagnostic(ErrorCode.ERR_PartialMethodWithExtendedModMustHaveAccessMods, "M1").WithArguments("I1.M1()").WithLocation(4, 25),
                // (5,27): error CS0750: A partial member cannot have the 'abstract' modifier
                //     abstract partial void M2();
                Diagnostic(ErrorCode.ERR_PartialMemberCannotBeAbstract, "M2").WithLocation(5, 27),
                // (6,26): error CS8796: Partial method 'I1.M3()' must have accessibility modifiers because it has a 'virtual', 'override', 'sealed', 'new', or 'extern' modifier.
                //     virtual partial void M3();
                Diagnostic(ErrorCode.ERR_PartialMethodWithExtendedModMustHaveAccessMods, "M3").WithArguments("I1.M3()").WithLocation(6, 26),
                // (10,21): error CS0754: A partial member may not explicitly implement an interface member
                //     partial void I2.M7();
                Diagnostic(ErrorCode.ERR_PartialMemberNotExplicit, "M7").WithLocation(10, 21),
                // (15,25): error CS8796: Partial method 'I1.M4()' must have accessibility modifiers because it has a 'virtual', 'override', 'sealed', 'new', or 'extern' modifier.
                //     sealed partial void M4() {}
                Diagnostic(ErrorCode.ERR_PartialMethodWithExtendedModMustHaveAccessMods, "M4").WithArguments("I1.M4()").WithLocation(15, 25),
                // (15,25): error CS8798: Both partial member declarations must have identical combinations of 'virtual', 'override', 'sealed', and 'new' modifiers.
                //     sealed partial void M4() {}
                Diagnostic(ErrorCode.ERR_PartialMemberExtendedModDifference, "M4").WithLocation(15, 25),
                // (16,27): error CS0750: A partial member cannot have the 'abstract' modifier
                //     abstract partial void M5();
                Diagnostic(ErrorCode.ERR_PartialMemberCannotBeAbstract, "M5").WithLocation(16, 27),
                // (16,27): error CS0756: A partial method may not have multiple defining declarations
                //     abstract partial void M5();
                Diagnostic(ErrorCode.ERR_PartialMethodOnlyOneLatent, "M5").WithLocation(16, 27),
                // (16,27): error CS0111: Type 'I1' already defines a member called 'M5' with the same parameter types
                //     abstract partial void M5();
                Diagnostic(ErrorCode.ERR_MemberAlreadyExists, "M5").WithArguments("M5", "I1").WithLocation(16, 27),
                // (17,26): error CS8796: Partial method 'I1.M6()' must have accessibility modifiers because it has a 'virtual', 'override', 'sealed', 'new', or 'extern' modifier.
                //     virtual partial void M6() {}
                Diagnostic(ErrorCode.ERR_PartialMethodWithExtendedModMustHaveAccessMods, "M6").WithArguments("I1.M6()").WithLocation(17, 26),
                // (17,26): error CS8798: Both partial member declarations must have identical combinations of 'virtual', 'override', 'sealed', and 'new' modifiers.
                //     virtual partial void M6() {}
                Diagnostic(ErrorCode.ERR_PartialMemberExtendedModDifference, "M6").WithLocation(17, 26),
                // (19,21): error CS0754: A partial member may not explicitly implement an interface member
                //     partial void I2.M7() {}
                Diagnostic(ErrorCode.ERR_PartialMemberNotExplicit, "M7").WithLocation(19, 21),
                // (25,18): error CS0751: A partial member must be declared within a partial type
                //     partial void M8();
                Diagnostic(ErrorCode.ERR_PartialMemberOnlyInPartialClass, "M8").WithLocation(25, 18)
                );
        }

        [Fact]
        public void MethodModifiers_28()
        {
            var source1 =
@"
partial interface I1
{
    partial void M1();

    class C : I1
    {
        void I1.M1() {}
    }
}

public partial interface I1
{
    partial void M1() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (8,17): error CS0539: 'I1.C.M1()' in explicit interface declaration is not found among members of the interface that can be implemented
                //         void I1.M1() {}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.C.M1()").WithLocation(8, 17)
                );
        }

        [Fact]
        public void MethodModifiers_29()
        {
            var source1 =
@"
public partial interface I1
{
    partial void M1();
    static partial void M2();

    void M3() 
    {
        new System.Action(M1).Invoke();
        new System.Action(M2).Invoke();
    }

    partial static void M4();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (9,27): error CS0762: Cannot create delegate from method 'I1.M1()' because it is a partial method without an implementing declaration
                //         new System.Action(M1).Invoke();
                Diagnostic(ErrorCode.ERR_PartialMethodToDelegate, "M1").WithArguments("I1.M1()").WithLocation(9, 27),
                // (10,27): error CS0762: Cannot create delegate from method 'I1.M2()' because it is a partial method without an implementing declaration
                //         new System.Action(M2).Invoke();
                Diagnostic(ErrorCode.ERR_PartialMethodToDelegate, "M2").WithArguments("I1.M2()").WithLocation(10, 27),
                // (13,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', or a method return type.
                //     partial static void M4();
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(13, 5)
                );
        }

        [Fact]
        public void MethodModifiers_30()
        {
            var source1 =
@"
public partial interface I1
{
    static partial void Main();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // error CS5001: Program does not contain a static 'Main' method suitable for an entry point
                Diagnostic(ErrorCode.ERR_NoEntryPoint).WithLocation(1, 1)
                );
        }

        [Fact]
        public void MethodModifiers_31()
        {
            var source1 =
@"
public partial interface I1
{
    static partial void M1<T>() where T : struct;
    static partial void M1<T>() {}

    static partial void M2(params int[] x);
    static partial void M2(int[] x) {}

    static partial void M3();
    partial void M3() {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (5,25): error CS0761: Partial method declarations of 'I1.M1<T>()' have inconsistent constraints for type parameter 'T'
                //     static partial void M1<T>() {}
                Diagnostic(ErrorCode.ERR_PartialMethodInconsistentConstraints, "M1").WithArguments("I1.M1<T>()", "T").WithLocation(5, 25),
                // (8,25): error CS0758: Both partial member declarations must use a params parameter or neither may use a params parameter
                //     static partial void M2(int[] x) {}
                Diagnostic(ErrorCode.ERR_PartialMemberParamsDifference, "M2").WithLocation(8, 25),
                // (11,18): error CS0763: Both partial member declarations must be static or neither may be static
                //     partial void M3() {}
                Diagnostic(ErrorCode.ERR_PartialMemberStaticDifference, "M3").WithLocation(11, 18)
                );
        }

        [Fact]
        public void MethodModifiers_32()
        {
            var source1 =
@"
partial interface I1
{
    partial void M1();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation1.VerifyDiagnostics();

            var source2 =
@"
partial interface I1
{
    partial void M1() {}
}
";
            var compilation2 = CreateCompilation(source1 + source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (9,18): error CS8701: Target runtime doesn't support default interface implementation.
                //     partial void M1() {}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(9, 18)
                );
        }

        [Fact]
        public void MethodModifiers_33()
        {
            var source0 =
@"
public interface I1
{
    protected static void M1() 
    {
        System.Console.WriteLine(""M1"");
    }

    protected internal static void M2() 
    {
        System.Console.WriteLine(""M2"");
    }

    private protected static void M3() 
    {
        System.Console.WriteLine(""M3"");
    }
}
";
            var source1 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.M1();
        I1.M2();
        I1.M3();
    }
}
";
            var compilation1 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M1
M2
M3",
                symbolValidator: validate,
                verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

            validate(compilation1.SourceModule);

            var source2 =
@"
class Test1
{
    static void Main()
    {
        I1.M2();
    }
}
";

            var compilation2 = CreateCompilation(source0 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M2",
                verify: VerifyOnMonoOrCoreClr);

            var compilation3 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            var source3 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.M1();
        I1.M2();
    }
}
";

            var source4 =
@"
class Test1
{
    static void Main()
    {
        I1.M1();
        I1.M2();
        I1.M3();
    }
}
";

            foreach (var reference in new[] { compilation3.ToMetadataReference(), compilation3.EmitToImageReference() })
            {
                var compilation4 = CreateCompilation(source3, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                CompileAndVerify(compilation4, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M1
M2",
                    verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

                var compilation5 = CreateCompilation(source4, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation5.VerifyDiagnostics(
                    // (6,12): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                    //         I1.M1();
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(6, 12),
                    // (7,12): error CS0122: 'I1.M2()' is inaccessible due to its protection level
                    //         I1.M2();
                    Diagnostic(ErrorCode.ERR_BadAccess, "M2").WithArguments("I1.M2()").WithLocation(7, 12),
                    // (8,12): error CS0122: 'I1.M3()' is inaccessible due to its protection level
                    //         I1.M3();
                    Diagnostic(ErrorCode.ERR_BadAccess, "M3").WithArguments("I1.M3()").WithLocation(8, 12)
                    );

                var compilation6 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation6.VerifyDiagnostics(
                    // (8,12): error CS0122: 'I1.M3()' is inaccessible due to its protection level
                    //         I1.M3();
                    Diagnostic(ErrorCode.ERR_BadAccess, "M3").WithArguments("I1.M3()").WithLocation(8, 12)
                    );
            }

            void validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var tuple in new[] { (name: "M1", access: Accessibility.Protected), (name: "M2", access: Accessibility.ProtectedOrInternal), (name: "M3", access: Accessibility.ProtectedAndInternal) })
                {
                    var m1 = i1.GetMember<MethodSymbol>(tuple.name);

                    Assert.False(m1.IsAbstract);
                    Assert.False(m1.IsVirtual);
                    Assert.False(m1.IsMetadataVirtual());
                    Assert.False(m1.IsSealed);
                    Assert.True(m1.IsStatic);
                    Assert.False(m1.IsExtern);
                    Assert.False(m1.IsAsync);
                    Assert.False(m1.IsOverride);
                    Assert.Equal(tuple.access, m1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(m1));
                }
            }
        }

        [Fact]
        public void MethodModifiers_34()
        {
            var source1 =
@"
public interface I1
{
    protected abstract void M1(); 
    public void M2() => M1();
}
";

            var source21 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }

    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source21 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            var expected = new DiagnosticDescription[] {
                // (10,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //     public void M1() 
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(10, 17),
                // (7,11): error CS1540: Cannot access protected member 'I1.M1()' via a qualifier of type 'I1'; the qualifier must be of type 'Test1' (or derived from it)
                //         x.M1();
                Diagnostic(ErrorCode.ERR_BadProtectedAccess, "M1").WithArguments("I1.M1()", "I1", "Test1").WithLocation(7, 11)
                };

            compilation1.VerifyDiagnostics(expected);

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.Protected);

            var source22 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            compilation1 = CreateCompilation(source22 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, Accessibility.Protected)).VerifyDiagnostics();

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.Protected);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Protected);

            var source3 =
@"
class Test2 : I1
{
}
";

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source21, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(expected);

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.Protected);

                compilation3 = CreateCompilation(source22, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, Accessibility.Protected)).VerifyDiagnostics();
                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.Protected);

                var compilation5 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation5.VerifyDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.M1()'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M1()").WithLocation(2, 15)
                    );

                ValidateI1M1NotImplemented(compilation5, "Test2");
            }
        }

        [Fact]
        public void MethodModifiers_35()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract void M1(); 
    public void M2() => M1();
}
";

            var source21 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }

    public virtual void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var source22 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public virtual void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var compilation1 = CreateCompilation(source21 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (10,25): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //     public virtual void M1() 
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(10, 25)
                );

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.ProtectedOrInternal);

            compilation1 = CreateCompilation(source21 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, Accessibility.ProtectedOrInternal)).VerifyDiagnostics();

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.ProtectedOrInternal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.ProtectedOrInternal);

            var source3 =
@"
class Test2 : I1
{
}
";

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source21, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(
                    // (10,25): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(10, 25),
                    // (7,11): error CS1540: Cannot access protected member 'I1.M1()' via a qualifier of type 'I1'; the qualifier must be of type 'Test1' (or derived from it)
                    //         x.M1();
                    Diagnostic(ErrorCode.ERR_BadProtectedAccess, "M1").WithArguments("I1.M1()", "I1", "Test1").WithLocation(7, 11)
                    );

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.ProtectedOrInternal);

                compilation3 = CreateCompilation(source22, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, Accessibility.ProtectedOrInternal)).VerifyDiagnostics();
                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.ProtectedOrInternal);

                var compilation5 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation5.VerifyDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.M1()'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M1()").WithLocation(2, 15)
                    );

                ValidateI1M1NotImplemented(compilation5, "Test2");
            }
        }

        [Fact]
        public void MethodModifiers_36()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract void M1(); 
    public void M2() => M1();
}
";

            var source21 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M1();
    }

    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var source22 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public void M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source21 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (10,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //     public void M1() 
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(10, 17),
                // (7,11): error CS1540: Cannot access protected member 'I1.M1()' via a qualifier of type 'I1'; the qualifier must be of type 'Test1' (or derived from it)
                //         x.M1();
                Diagnostic(ErrorCode.ERR_BadProtectedAccess, "M1").WithArguments("I1.M1()", "I1", "Test1").WithLocation(7, 11)
                );

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.ProtectedAndInternal);

            compilation1 = CreateCompilation(source22 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "M1", verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersImplicit_10(m, Accessibility.ProtectedAndInternal)).VerifyDiagnostics();

            ValidateMethodModifiersImplicit_10(compilation1.SourceModule, Accessibility.ProtectedAndInternal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.ProtectedAndInternal);

            var source3 =
@"
class Test2 : I1
{
}
";

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source21, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(
                    // (10,17): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(10, 17),
                    // (7,11): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                    //         x.M1();
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(7, 11)
                    );

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.ProtectedAndInternal);

                compilation3 = CreateCompilation(source22, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation3.VerifyDiagnostics(
                    // (10,17): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(10, 17)
                    );

                ValidateMethodModifiersImplicit_10(compilation3.SourceModule, Accessibility.ProtectedAndInternal);

                var compilation5 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation5.VerifyDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.M1()'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.M1()").WithLocation(2, 15)
                    );

                ValidateI1M1NotImplemented(compilation5, "Test2");
            }
        }

        [Fact]
        public void MethodModifiers_37()
        {
            var source1 =
@"
public interface I1
{
    protected abstract void M1(); 

    static void M2(I1 x) => x.M1();
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        I1.M2(x);
    }

    void I1.M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null, verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicit_10(m, Accessibility.Protected));

            ValidateMethodModifiersExplicit_10(compilation1.SourceModule, Accessibility.Protected);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.Protected);

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var targetFramework = ExecutionConditionUtil.IsMonoOrCoreClr ? TargetFramework.Net50 : TargetFramework.NetFramework;
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: targetFramework);

                compilation3.VerifyDiagnostics();

                CompileAndVerify(compilation3, expectedOutput: "M1", symbolValidator: (m) => ValidateMethodModifiersExplicit_10(m, Accessibility.Protected));

                ValidateMethodModifiersExplicit_10(compilation3.SourceModule, Accessibility.Protected);
            }
        }

        [Fact]
        public void MethodModifiers_38()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract void M1(); 

    static void M2(I1 x) => x.M1();
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        I1.M2(x);
    }

    void I1.M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null, verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicit_10(m, Accessibility.ProtectedOrInternal));

            ValidateMethodModifiersExplicit_10(compilation1.SourceModule, Accessibility.ProtectedOrInternal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.ProtectedOrInternal);

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var targetFramework = ExecutionConditionUtil.IsMonoOrCoreClr ? TargetFramework.Net50 : TargetFramework.NetFramework;
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: targetFramework);

                compilation3.VerifyDiagnostics();

                CompileAndVerify(compilation3, expectedOutput: "M1", symbolValidator: (m) => ValidateMethodModifiersExplicit_10(m, Accessibility.ProtectedOrInternal));

                ValidateMethodModifiersExplicit_10(compilation3.SourceModule, Accessibility.ProtectedOrInternal);
            }
        }

        [Fact]
        public void MethodModifiers_39()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract void M1(); 

    static void M2(I1 x) => x.M1();
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        I1.M2(x);
    }

    void I1.M1() 
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null, verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateMethodModifiersExplicit_10(m, Accessibility.ProtectedAndInternal));

            ValidateMethodModifiersExplicit_10(compilation1.SourceModule, Accessibility.ProtectedAndInternal);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation2.VerifyDiagnostics();

            ValidateMethodModifiers_10(compilation2.GetTypeByMetadataName("I1").GetMember<MethodSymbol>("M1"), Accessibility.ProtectedAndInternal);

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.Net50);

                compilation3.VerifyDiagnostics(
                    // (10,13): error CS0122: 'I1.M1()' is inaccessible due to its protection level
                    //     void I1.M1() 
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.M1()").WithLocation(10, 13)
                    );

                ValidateMethodModifiersExplicit_10(compilation3.SourceModule, Accessibility.ProtectedAndInternal);
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodModifiers_40(bool isStatic)
        {
            string declModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{   " + declModifiers + @"
    protected abstract void M1(); 

";
            if (!isStatic)
            {
                source1 +=
@"
    public void M2() => M1();
";
            }
            else
            {
                source1 +=
@"
    public static void M2<T>() where T : I1
    {
        T.M1();
    }
";
            }

            source1 +=
@"
}

public class Test2 : I1
{   " + declModifiers + @"
    void I1.M1() 
    {
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    " + declModifiers + @"
    public void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<T>() where T : I1
    {
        I1.M2<T>();
    }
";
            }

            source2 +=
@"
}
";

            ValidateMethodModifiers_10_02(source1, source2, Accessibility.Protected,
                expectedIn9: !isStatic ?
                    ExpectedDiagnostics(
                        // (5,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                        //     public void M1() 
                        Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(5, 17)
                        ) :
                    ExpectedDiagnostics(
                        // (24,29): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                        //     protected abstract void M1(); 
                        Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "9.0", "11.0").WithLocation(24, 29),
                        // (29,9): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                        //         T.M1();
                        Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "T").WithArguments("static abstract members in interfaces", "11.0").WithLocation(29, 9),
                        // (36,13): error CS8703: The modifier 'static' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                        //     void I1.M1() 
                        Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "9.0", "11.0").WithLocation(36, 13)
                        ),
                expectedIn9AcrossAssemblyBoundaries: !isStatic ?
                    ExpectedDiagnostics(
                        // (5,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                        //     public void M1() 
                        Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(5, 17)
                        ) :
                    ExpectedDiagnostics(
                        // (5,17): error CS8706: 'Test1.M1()' cannot implement interface member 'I1.M1()' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version '11.0' or greater.
                        //     public void M1() 
                        Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "M1").WithArguments("Test1.M1()", "I1.M1()", "Test1", "static abstract members in interfaces", "9.0", "11.0").WithLocation(5, 17)
                        ),
                expectedAcrossAssemblyBoundaries: Array.Empty<DiagnosticDescription>(),
                isStatic: isStatic
                );
        }

        [Fact]
        public void MethodModifiers_41()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract void M1(); 
    public void M2() => M1();
}

public class Test2 : I1
{
    void I1.M1() 
    {
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";
            ValidateMethodModifiers_10_02(source1, source2, Accessibility.ProtectedOrInternal,
                // (10,17): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //     public void M1() 
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(10, 17)
                );
        }

        [Fact]
        public void MethodModifiers_42()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract void M1(); 
    public void M2() => M1();
}

public class Test2 : I1
{
    void I1.M1() 
    {
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public virtual void M1() 
    {
        System.Console.WriteLine(""Test1.M1"");
    }
}
";

            ValidateMethodModifiers_10_02(source1, source2, Accessibility.ProtectedAndInternal,
                expectedIn9: ExpectedDiagnostics(
                    // (10,25): error CS8704: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()", "9.0", "10.0").WithLocation(10, 25)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (10,25): error CS9044: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implicitly implement an inaccessible member.
                    //     public virtual void M1() 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "M1").WithArguments("Test1", "I1.M1()", "Test1.M1()").WithLocation(10, 25)
                    )
                );
        }

        [Fact]
        public void MethodModifiers_43()
        {
            var source1 =
@"
public interface I1
{
    protected void M1()
    {
        System.Console.WriteLine(""M1"");
    }

    void M2() {M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: validate1);

            validate1(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");
                validateMethod(m1);
            }

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                CompileAndVerify(compilation3,
                    expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                    verify: VerifyOnMonoOrCoreClr,
                    symbolValidator: validate1);

                validate1(compilation3.SourceModule);
            }

            void validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var m1 = i1.GetMember<MethodSymbol>("M1");

                validateMethod(m1);
                Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
            }

            void validateMethod(MethodSymbol m1)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Protected, m1.DeclaredAccessibility);
            }
        }

        [Fact]
        public void MethodModifiers_44()
        {
            var source1 =
@"
public interface I1
{
    protected internal void M1()
    {
        System.Console.WriteLine(""M1"");
    }

    void M2() {M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: validate1);

            validate1(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");
                validateMethod(m1);
            }

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                CompileAndVerify(compilation3,
                    expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                    verify: VerifyOnMonoOrCoreClr,
                    symbolValidator: validate1);

                validate1(compilation3.SourceModule);
            }

            void validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var m1 = i1.GetMember<MethodSymbol>("M1");

                validateMethod(m1);
                Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
            }

            void validateMethod(MethodSymbol m1)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.ProtectedOrInternal, m1.DeclaredAccessibility);
            }
        }

        [Fact]
        public void MethodModifiers_45()
        {
            var source1 =
@"
public interface I1
{
    private protected void M1()
    {
        System.Console.WriteLine(""M1"");
    }

    void M2() {M1();}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: validate1);

            validate1(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var m1 = i1.GetMember<MethodSymbol>("M1");
                validateMethod(m1);
            }

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                CompileAndVerify(compilation3,
                    expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "M1" : null,
                    verify: VerifyOnMonoOrCoreClr,
                    symbolValidator: validate1);

                validate1(compilation3.SourceModule);
            }

            void validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var m1 = i1.GetMember<MethodSymbol>("M1");

                validateMethod(m1);
                Assert.Same(m1, test1.FindImplementationForInterfaceMember(m1));
            }

            void validateMethod(MethodSymbol m1)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.ProtectedAndInternal, m1.DeclaredAccessibility);
            }
        }

        [Fact]
        public void ImplicitThisIsAllowed_03()
        {
            var source1 =
@"
public interface I1
{
    public int F1;

    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    }

    int P1
    {
        get
        {
            System.Console.WriteLine(""I1.get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""I1.set_P1"");
    }

    event System.Action E1
    {
        add => System.Console.WriteLine(""I1.add_E1"");
        remove => System.Console.WriteLine(""I1.remove_E1"");
    }

    public interface I2 : I1
    {
        void M2() 
        {
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            F1 = 0;
        }
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,16): error CS0525: Interfaces cannot contain fields
                //     public int F1;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F1").WithLocation(4, 16)
                );
        }

        [Fact]
        public void ImplicitThisIsAllowed_04()
        {
            var source1 =
@"
public interface I1
{
    public int F1;

    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    }

    int P1
    {
        get
        {
            System.Console.WriteLine(""I1.get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""I1.set_P1"");
    }

    event System.Action E1
    {
        add => System.Console.WriteLine(""I1.add_E1"");
        remove => System.Console.WriteLine(""I1.remove_E1"");
    }

    public interface I2
    {
        void M2() 
        {
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            F1 = 0;
        }
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (4,16): error CS0525: Interfaces cannot contain fields
                //     public int F1;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F1").WithLocation(4, 16),
                // (31,13): error CS0120: An object reference is required for the non-static field, method, or property 'I1.M1()'
                //             M1();
                Diagnostic(ErrorCode.ERR_ObjectRequired, "M1").WithArguments("I1.M1()").WithLocation(31, 13),
                // (32,13): error CS0120: An object reference is required for the non-static field, method, or property 'I1.P1'
                //             P1 = P1;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "P1").WithArguments("I1.P1").WithLocation(32, 13),
                // (32,18): error CS0120: An object reference is required for the non-static field, method, or property 'I1.P1'
                //             P1 = P1;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "P1").WithArguments("I1.P1").WithLocation(32, 18),
                // (33,13): error CS0120: An object reference is required for the non-static field, method, or property 'I1.E1'
                //             E1 += null;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "E1").WithArguments("I1.E1").WithLocation(33, 13),
                // (34,13): error CS0120: An object reference is required for the non-static field, method, or property 'I1.E1'
                //             E1 -= null;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "E1").WithArguments("I1.E1").WithLocation(34, 13),
                // (35,13): error CS0120: An object reference is required for the non-static field, method, or property 'I1.F1'
                //             F1 = 0;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "F1").WithArguments("I1.F1").WithLocation(35, 13)
                );
        }

        [Fact]
        public void ImplicitThisIsAllowed_05()
        {
            var source1 =
@"
public class C1
{
    public int F1;

    void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    }

    int P1
    {
        get
        {
            System.Console.WriteLine(""I1.get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""I1.set_P1"");
    }

    event System.Action E1
    {
        add => System.Console.WriteLine(""I1.add_E1"");
        remove => System.Console.WriteLine(""I1.remove_E1"");
    }

    public interface I2
    {
        void M2() 
        {
            M1();
            P1 = P1;
            E1 += null;
            E1 -= null;
            F1 = 0;
        }
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (31,13): error CS0120: An object reference is required for the non-static field, method, or property 'C1.M1()'
                //             M1();
                Diagnostic(ErrorCode.ERR_ObjectRequired, "M1").WithArguments("C1.M1()").WithLocation(31, 13),
                // (32,13): error CS0120: An object reference is required for the non-static field, method, or property 'C1.P1'
                //             P1 = P1;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "P1").WithArguments("C1.P1").WithLocation(32, 13),
                // (32,18): error CS0120: An object reference is required for the non-static field, method, or property 'C1.P1'
                //             P1 = P1;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "P1").WithArguments("C1.P1").WithLocation(32, 18),
                // (33,13): error CS0120: An object reference is required for the non-static field, method, or property 'C1.E1'
                //             E1 += null;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "E1").WithArguments("C1.E1").WithLocation(33, 13),
                // (34,13): error CS0120: An object reference is required for the non-static field, method, or property 'C1.E1'
                //             E1 -= null;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "E1").WithArguments("C1.E1").WithLocation(34, 13),
                // (35,13): error CS0120: An object reference is required for the non-static field, method, or property 'C1.F1'
                //             F1 = 0;
                Diagnostic(ErrorCode.ERR_ObjectRequired, "F1").WithArguments("C1.F1").WithLocation(35, 13)
                );
        }

        [Fact]
        public void PropertyModifiers_01()
        {
            var source1 =
@"
public interface I1
{
    public int P01 {get; set;}
    protected int P02 {get;}
    protected internal int P03 {set;}
    internal int P04 {get;}
    private int P05 {set;}
    static int P06 {get;}
    virtual int P07 {set;}
    sealed int P08 {get;}
    override int P09 {set;}
    abstract int P10 {get;}
    extern int P11 {get; set;}

    int P12 { public get; set;}
    int P13 { get; protected set;}
    int P14 { protected internal get; set;}
    int P15 { get; internal set;}
    int P16 { private get; set;}
    int P17 { private get;}

    private protected int P18 {get;}
    int P19 { get; private protected set;}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (8,22): error CS0501: 'I1.P05.set' must declare a body because it is not marked abstract, extern, or partial
                //     private int P05 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P05.set").WithLocation(8, 22),
                // (10,22): error CS0501: 'I1.P07.set' must declare a body because it is not marked abstract, extern, or partial
                //     virtual int P07 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P07.set").WithLocation(10, 22),
                // (11,21): error CS0501: 'I1.P08.get' must declare a body because it is not marked abstract, extern, or partial
                //     sealed int P08 {get;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P08.get").WithLocation(11, 21),
                // (12,18): error CS0106: The modifier 'override' is not valid for this item
                //     override int P09 {set;}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 18),
                // (16,22): error CS0273: The accessibility modifier of the 'I1.P12.get' accessor must be more restrictive than the property or indexer 'I1.P12'
                //     int P12 { public get; set;}
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I1.P12.get", "I1.P12").WithLocation(16, 22),
                // (20,23): error CS0442: 'I1.P16.get': abstract properties cannot have private accessors
                //     int P16 { private get; set;}
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I1.P16.get").WithLocation(20, 23),
                // (21,9): error CS0276: 'I1.P17': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int P17 { private get;}
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P17").WithArguments("I1.P17").WithLocation(21, 9),
                // (14,21): warning CS0626: Method, operator, or accessor 'I1.P11.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.P11.get").WithLocation(14, 21),
                // (14,26): warning CS0626: Method, operator, or accessor 'I1.P11.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I1.P11.set").WithLocation(14, 26)
                );

            ValidateSymbolsPropertyModifiers_01(compilation1);
        }

        private static void ValidateSymbolsPropertyModifiers_01(CSharpCompilation compilation1)
        {
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p01 = i1.GetMember<PropertySymbol>("P01");

            Assert.True(p01.IsAbstract);
            Assert.False(p01.IsVirtual);
            Assert.False(p01.IsSealed);
            Assert.False(p01.IsStatic);
            Assert.False(p01.IsExtern);
            Assert.False(p01.IsOverride);
            Assert.Equal(Accessibility.Public, p01.DeclaredAccessibility);

            ValidateP01Accessor(p01.GetMethod);
            ValidateP01Accessor(p01.SetMethod);
            void ValidateP01Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p02 = i1.GetMember<PropertySymbol>("P02");
            var p02get = p02.GetMethod;

            Assert.True(p02.IsAbstract);
            Assert.False(p02.IsVirtual);
            Assert.False(p02.IsSealed);
            Assert.False(p02.IsStatic);
            Assert.False(p02.IsExtern);
            Assert.False(p02.IsOverride);
            Assert.Equal(Accessibility.Protected, p02.DeclaredAccessibility);

            Assert.True(p02get.IsAbstract);
            Assert.False(p02get.IsVirtual);
            Assert.True(p02get.IsMetadataVirtual());
            Assert.False(p02get.IsSealed);
            Assert.False(p02get.IsStatic);
            Assert.False(p02get.IsExtern);
            Assert.False(p02get.IsAsync);
            Assert.False(p02get.IsOverride);
            Assert.Equal(Accessibility.Protected, p02get.DeclaredAccessibility);

            var p03 = i1.GetMember<PropertySymbol>("P03");
            var p03set = p03.SetMethod;

            Assert.True(p03.IsAbstract);
            Assert.False(p03.IsVirtual);
            Assert.False(p03.IsSealed);
            Assert.False(p03.IsStatic);
            Assert.False(p03.IsExtern);
            Assert.False(p03.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, p03.DeclaredAccessibility);

            Assert.True(p03set.IsAbstract);
            Assert.False(p03set.IsVirtual);
            Assert.True(p03set.IsMetadataVirtual());
            Assert.False(p03set.IsSealed);
            Assert.False(p03set.IsStatic);
            Assert.False(p03set.IsExtern);
            Assert.False(p03set.IsAsync);
            Assert.False(p03set.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, p03set.DeclaredAccessibility);

            var p04 = i1.GetMember<PropertySymbol>("P04");
            var p04get = p04.GetMethod;

            Assert.True(p04.IsAbstract);
            Assert.False(p04.IsVirtual);
            Assert.False(p04.IsSealed);
            Assert.False(p04.IsStatic);
            Assert.False(p04.IsExtern);
            Assert.False(p04.IsOverride);
            Assert.Equal(Accessibility.Internal, p04.DeclaredAccessibility);

            Assert.True(p04get.IsAbstract);
            Assert.False(p04get.IsVirtual);
            Assert.True(p04get.IsMetadataVirtual());
            Assert.False(p04get.IsSealed);
            Assert.False(p04get.IsStatic);
            Assert.False(p04get.IsExtern);
            Assert.False(p04get.IsAsync);
            Assert.False(p04get.IsOverride);
            Assert.Equal(Accessibility.Internal, p04get.DeclaredAccessibility);

            var p05 = i1.GetMember<PropertySymbol>("P05");
            var p05set = p05.SetMethod;

            Assert.False(p05.IsAbstract);
            Assert.False(p05.IsVirtual);
            Assert.False(p05.IsSealed);
            Assert.False(p05.IsStatic);
            Assert.False(p05.IsExtern);
            Assert.False(p05.IsOverride);
            Assert.Equal(Accessibility.Private, p05.DeclaredAccessibility);

            Assert.False(p05set.IsAbstract);
            Assert.False(p05set.IsVirtual);
            Assert.False(p05set.IsMetadataVirtual());
            Assert.False(p05set.IsSealed);
            Assert.False(p05set.IsStatic);
            Assert.False(p05set.IsExtern);
            Assert.False(p05set.IsAsync);
            Assert.False(p05set.IsOverride);
            Assert.Equal(Accessibility.Private, p05set.DeclaredAccessibility);

            var p06 = i1.GetMember<PropertySymbol>("P06");
            var p06get = p06.GetMethod;

            Assert.False(p06.IsAbstract);
            Assert.False(p06.IsVirtual);
            Assert.False(p06.IsSealed);
            Assert.True(p06.IsStatic);
            Assert.False(p06.IsExtern);
            Assert.False(p06.IsOverride);
            Assert.Equal(Accessibility.Public, p06.DeclaredAccessibility);

            Assert.False(p06get.IsAbstract);
            Assert.False(p06get.IsVirtual);
            Assert.False(p06get.IsMetadataVirtual());
            Assert.False(p06get.IsSealed);
            Assert.True(p06get.IsStatic);
            Assert.False(p06get.IsExtern);
            Assert.False(p06get.IsAsync);
            Assert.False(p06get.IsOverride);
            Assert.Equal(Accessibility.Public, p06get.DeclaredAccessibility);

            var p07 = i1.GetMember<PropertySymbol>("P07");
            var p07set = p07.SetMethod;

            Assert.False(p07.IsAbstract);
            Assert.True(p07.IsVirtual);
            Assert.False(p07.IsSealed);
            Assert.False(p07.IsStatic);
            Assert.False(p07.IsExtern);
            Assert.False(p07.IsOverride);
            Assert.Equal(Accessibility.Public, p07.DeclaredAccessibility);

            Assert.False(p07set.IsAbstract);
            Assert.True(p07set.IsVirtual);
            Assert.True(p07set.IsMetadataVirtual());
            Assert.False(p07set.IsSealed);
            Assert.False(p07set.IsStatic);
            Assert.False(p07set.IsExtern);
            Assert.False(p07set.IsAsync);
            Assert.False(p07set.IsOverride);
            Assert.Equal(Accessibility.Public, p07set.DeclaredAccessibility);

            var p08 = i1.GetMember<PropertySymbol>("P08");
            var p08get = p08.GetMethod;

            Assert.False(p08.IsAbstract);
            Assert.False(p08.IsVirtual);
            Assert.False(p08.IsSealed);
            Assert.False(p08.IsStatic);
            Assert.False(p08.IsExtern);
            Assert.False(p08.IsOverride);
            Assert.Equal(Accessibility.Public, p08.DeclaredAccessibility);

            Assert.False(p08get.IsAbstract);
            Assert.False(p08get.IsVirtual);
            Assert.False(p08get.IsMetadataVirtual());
            Assert.False(p08get.IsSealed);
            Assert.False(p08get.IsStatic);
            Assert.False(p08get.IsExtern);
            Assert.False(p08get.IsAsync);
            Assert.False(p08get.IsOverride);
            Assert.Equal(Accessibility.Public, p08get.DeclaredAccessibility);

            var p09 = i1.GetMember<PropertySymbol>("P09");
            var p09set = p09.SetMethod;

            Assert.True(p09.IsAbstract);
            Assert.False(p09.IsVirtual);
            Assert.False(p09.IsSealed);
            Assert.False(p09.IsStatic);
            Assert.False(p09.IsExtern);
            Assert.False(p09.IsOverride);
            Assert.Equal(Accessibility.Public, p09.DeclaredAccessibility);

            Assert.True(p09set.IsAbstract);
            Assert.False(p09set.IsVirtual);
            Assert.True(p09set.IsMetadataVirtual());
            Assert.False(p09set.IsSealed);
            Assert.False(p09set.IsStatic);
            Assert.False(p09set.IsExtern);
            Assert.False(p09set.IsAsync);
            Assert.False(p09set.IsOverride);
            Assert.Equal(Accessibility.Public, p09set.DeclaredAccessibility);

            var p10 = i1.GetMember<PropertySymbol>("P10");
            var p10get = p10.GetMethod;

            Assert.True(p10.IsAbstract);
            Assert.False(p10.IsVirtual);
            Assert.False(p10.IsSealed);
            Assert.False(p10.IsStatic);
            Assert.False(p10.IsExtern);
            Assert.False(p10.IsOverride);
            Assert.Equal(Accessibility.Public, p10.DeclaredAccessibility);

            Assert.True(p10get.IsAbstract);
            Assert.False(p10get.IsVirtual);
            Assert.True(p10get.IsMetadataVirtual());
            Assert.False(p10get.IsSealed);
            Assert.False(p10get.IsStatic);
            Assert.False(p10get.IsExtern);
            Assert.False(p10get.IsAsync);
            Assert.False(p10get.IsOverride);
            Assert.Equal(Accessibility.Public, p10get.DeclaredAccessibility);

            var p11 = i1.GetMember<PropertySymbol>("P11");

            Assert.False(p11.IsAbstract);
            Assert.True(p11.IsVirtual);
            Assert.False(p11.IsSealed);
            Assert.False(p11.IsStatic);
            Assert.True(p11.IsExtern);
            Assert.False(p11.IsOverride);
            Assert.Equal(Accessibility.Public, p11.DeclaredAccessibility);

            ValidateP11Accessor(p11.GetMethod);
            ValidateP11Accessor(p11.SetMethod);
            void ValidateP11Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p12 = i1.GetMember<PropertySymbol>("P12");

            Assert.True(p12.IsAbstract);
            Assert.False(p12.IsVirtual);
            Assert.False(p12.IsSealed);
            Assert.False(p12.IsStatic);
            Assert.False(p12.IsExtern);
            Assert.False(p12.IsOverride);
            Assert.Equal(Accessibility.Public, p12.DeclaredAccessibility);

            ValidateP12Accessor(p12.GetMethod);
            ValidateP12Accessor(p12.SetMethod);
            void ValidateP12Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p13 = i1.GetMember<PropertySymbol>("P13");

            Assert.True(p13.IsAbstract);
            Assert.False(p13.IsVirtual);
            Assert.False(p13.IsSealed);
            Assert.False(p13.IsStatic);
            Assert.False(p13.IsExtern);
            Assert.False(p13.IsOverride);
            Assert.Equal(Accessibility.Public, p13.DeclaredAccessibility);

            ValidateP13Accessor(p13.GetMethod, Accessibility.Public);
            ValidateP13Accessor(p13.SetMethod, Accessibility.Protected);
            void ValidateP13Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p14 = i1.GetMember<PropertySymbol>("P14");

            Assert.True(p14.IsAbstract);
            Assert.False(p14.IsVirtual);
            Assert.False(p14.IsSealed);
            Assert.False(p14.IsStatic);
            Assert.False(p14.IsExtern);
            Assert.False(p14.IsOverride);
            Assert.Equal(Accessibility.Public, p14.DeclaredAccessibility);

            ValidateP14Accessor(p14.GetMethod, Accessibility.ProtectedOrInternal);
            ValidateP14Accessor(p14.SetMethod, Accessibility.Public);
            void ValidateP14Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p15 = i1.GetMember<PropertySymbol>("P15");

            Assert.True(p15.IsAbstract);
            Assert.False(p15.IsVirtual);
            Assert.False(p15.IsSealed);
            Assert.False(p15.IsStatic);
            Assert.False(p15.IsExtern);
            Assert.False(p15.IsOverride);
            Assert.Equal(Accessibility.Public, p15.DeclaredAccessibility);

            ValidateP15Accessor(p15.GetMethod, Accessibility.Public);
            ValidateP15Accessor(p15.SetMethod, Accessibility.Internal);
            void ValidateP15Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p16 = i1.GetMember<PropertySymbol>("P16");

            Assert.True(p16.IsAbstract);
            Assert.False(p16.IsVirtual);
            Assert.False(p16.IsSealed);
            Assert.False(p16.IsStatic);
            Assert.False(p16.IsExtern);
            Assert.False(p16.IsOverride);
            Assert.Equal(Accessibility.Public, p16.DeclaredAccessibility);

            ValidateP16Accessor(p16.GetMethod, Accessibility.Private);
            ValidateP16Accessor(p16.SetMethod, Accessibility.Public);
            void ValidateP16Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p17 = i1.GetMember<PropertySymbol>("P17");
            var p17get = p17.GetMethod;

            Assert.True(p17.IsAbstract);
            Assert.False(p17.IsVirtual);
            Assert.False(p17.IsSealed);
            Assert.False(p17.IsStatic);
            Assert.False(p17.IsExtern);
            Assert.False(p17.IsOverride);
            Assert.Equal(Accessibility.Public, p17.DeclaredAccessibility);

            Assert.True(p17get.IsAbstract);
            Assert.False(p17get.IsVirtual);
            Assert.True(p17get.IsMetadataVirtual());
            Assert.False(p17get.IsSealed);
            Assert.False(p17get.IsStatic);
            Assert.False(p17get.IsExtern);
            Assert.False(p17get.IsAsync);
            Assert.False(p17get.IsOverride);
            Assert.Equal(Accessibility.Private, p17get.DeclaredAccessibility);

            var p18 = i1.GetMember<PropertySymbol>("P18");
            var p18get = p18.GetMethod;

            Assert.True(p18.IsAbstract);
            Assert.False(p18.IsVirtual);
            Assert.False(p18.IsSealed);
            Assert.False(p18.IsStatic);
            Assert.False(p18.IsExtern);
            Assert.False(p18.IsOverride);
            Assert.Equal(Accessibility.ProtectedAndInternal, p18.DeclaredAccessibility);

            Assert.True(p18get.IsAbstract);
            Assert.False(p18get.IsVirtual);
            Assert.True(p18get.IsMetadataVirtual());
            Assert.False(p18get.IsSealed);
            Assert.False(p18get.IsStatic);
            Assert.False(p18get.IsExtern);
            Assert.False(p18get.IsAsync);
            Assert.False(p18get.IsOverride);
            Assert.Equal(Accessibility.ProtectedAndInternal, p18get.DeclaredAccessibility);

            var p19 = i1.GetMember<PropertySymbol>("P19");

            Assert.True(p19.IsAbstract);
            Assert.False(p19.IsVirtual);
            Assert.False(p19.IsSealed);
            Assert.False(p19.IsStatic);
            Assert.False(p19.IsExtern);
            Assert.False(p19.IsOverride);
            Assert.Equal(Accessibility.Public, p19.DeclaredAccessibility);

            ValidateP13Accessor(p19.GetMethod, Accessibility.Public);
            ValidateP13Accessor(p19.SetMethod, Accessibility.ProtectedAndInternal);
        }

        [Fact]
        public void PropertyModifiers_02()
        {
            var source1 =
@"
public interface I1
{
    public int P01 {get; set;}
    protected int P02 {get;}
    protected internal int P03 {set;}
    internal int P04 {get;}
    private int P05 {set;}
    static int P06 {get;}
    virtual int P07 {set;}
    sealed int P08 {get;}
    override int P09 {set;}
    abstract int P10 {get;}
    extern int P11 {get; set;}

    int P12 { public get; set;}
    int P13 { get; protected set;}
    int P14 { protected internal get; set;}
    int P15 { get; internal set;}
    int P16 { private get; set;}
    int P17 { private get;}

    private protected int P18 {get;}
    int P19 { get; private protected set;}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,16): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public int P01 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P01").WithArguments("public", "7.3", "8.0").WithLocation(4, 16),
                // (5,19): error CS8703: The modifier 'protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     protected int P02 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P02").WithArguments("protected", "7.3", "8.0").WithLocation(5, 19),
                // (6,28): error CS8703: The modifier 'protected internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     protected internal int P03 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P03").WithArguments("protected internal", "7.3", "8.0").WithLocation(6, 28),
                // (7,18): error CS8503: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal int P04 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P04").WithArguments("internal", "7.3", "8.0").WithLocation(7, 18),
                // (8,17): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private int P05 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P05").WithArguments("private", "7.3", "8.0").WithLocation(8, 17),
                // (8,22): error CS0501: 'I1.P05.set' must declare a body because it is not marked abstract, extern, or partial
                //     private int P05 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P05.set").WithLocation(8, 22),
                // (9,16): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static int P06 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P06").WithArguments("static", "7.3", "8.0").WithLocation(9, 16),
                // (10,17): error CS8503: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual int P07 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P07").WithArguments("virtual", "7.3", "8.0").WithLocation(10, 17),
                // (10,22): error CS0501: 'I1.P07.set' must declare a body because it is not marked abstract, extern, or partial
                //     virtual int P07 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P07.set").WithLocation(10, 22),
                // (11,16): error CS8503: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     sealed int P08 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P08").WithArguments("sealed", "7.3", "8.0").WithLocation(11, 16),
                // (11,21): error CS0501: 'I1.P08.get' must declare a body because it is not marked abstract, extern, or partial
                //     sealed int P08 {get;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P08.get").WithLocation(11, 21),
                // (12,18): error CS0106: The modifier 'override' is not valid for this item
                //     override int P09 {set;}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 18),
                // (13,18): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     abstract int P10 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P10").WithArguments("abstract", "7.3", "8.0").WithLocation(13, 18),
                // (14,16): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P11").WithArguments("extern", "7.3", "8.0").WithLocation(14, 16),
                // (16,22): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P12 { public get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("public", "7.3", "8.0").WithLocation(16, 22),
                // (16,22): error CS0273: The accessibility modifier of the 'I1.P12.get' accessor must be more restrictive than the property or indexer 'I1.P12'
                //     int P12 { public get; set;}
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I1.P12.get", "I1.P12").WithLocation(16, 22),
                // (17,30): error CS8703: The modifier 'protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P13 { get; protected set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("protected", "7.3", "8.0").WithLocation(17, 30),
                // (18,34): error CS8703: The modifier 'protected internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P14 { protected internal get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("protected internal", "7.3", "8.0").WithLocation(18, 34),
                // (19,29): error CS8503: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P15 { get; internal set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("internal", "7.3", "8.0").WithLocation(19, 29),
                // (20,23): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P16 { private get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("private", "7.3", "8.0").WithLocation(20, 23),
                // (20,23): error CS0442: 'I1.P16.get': abstract properties cannot have private accessors
                //     int P16 { private get; set;}
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I1.P16.get").WithLocation(20, 23),
                // (21,23): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P17 { private get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("private", "7.3", "8.0").WithLocation(21, 23),
                // (21,9): error CS0276: 'I1.P17': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int P17 { private get;}
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P17").WithArguments("I1.P17").WithLocation(21, 9),
                // (23,27): error CS8703: The modifier 'private protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private protected int P18 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P18").WithArguments("private protected", "7.3", "8.0").WithLocation(23, 27),
                // (24,38): error CS8703: The modifier 'private protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     int P19 { get; private protected set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("private protected", "7.3", "8.0").WithLocation(24, 38),
                // (14,21): warning CS0626: Method, operator, or accessor 'I1.P11.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.P11.get").WithLocation(14, 21),
                // (14,26): warning CS0626: Method, operator, or accessor 'I1.P11.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I1.P11.set").WithLocation(14, 26)
                );

            ValidateSymbolsPropertyModifiers_01(compilation1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (5,19): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected int P02 {get;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P02").WithLocation(5, 19),
                // (6,28): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal int P03 {set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P03").WithLocation(6, 28),
                // (8,22): error CS0501: 'I1.P05.set' must declare a body because it is not marked abstract, extern, or partial
                //     private int P05 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P05.set").WithLocation(8, 22),
                // (9,21): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int P06 {get;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(9, 21),
                // (10,22): error CS0501: 'I1.P07.set' must declare a body because it is not marked abstract, extern, or partial
                //     virtual int P07 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P07.set").WithLocation(10, 22),
                // (11,21): error CS0501: 'I1.P08.get' must declare a body because it is not marked abstract, extern, or partial
                //     sealed int P08 {get;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P08.get").WithLocation(11, 21),
                // (12,18): error CS0106: The modifier 'override' is not valid for this item
                //     override int P09 {set;}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 18),
                // (14,21): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(14, 21),
                // (14,21): warning CS0626: Method, operator, or accessor 'I1.P11.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.P11.get").WithLocation(14, 21),
                // (14,26): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(14, 26),
                // (14,26): warning CS0626: Method, operator, or accessor 'I1.P11.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I1.P11.set").WithLocation(14, 26),
                // (16,22): error CS0273: The accessibility modifier of the 'I1.P12.get' accessor must be more restrictive than the property or indexer 'I1.P12'
                //     int P12 { public get; set;}
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I1.P12.get", "I1.P12").WithLocation(16, 22),
                // (17,30): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     int P13 { get; protected set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(17, 30),
                // (18,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     int P14 { protected internal get; set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "get").WithLocation(18, 34),
                // (20,23): error CS0442: 'I1.P16.get': abstract properties cannot have private accessors
                //     int P16 { private get; set;}
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I1.P16.get").WithLocation(20, 23),
                // (21,9): error CS0276: 'I1.P17': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int P17 { private get;}
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P17").WithArguments("I1.P17").WithLocation(21, 9),
                // (23,27): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected int P18 {get;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P18").WithLocation(23, 27),
                // (24,38): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     int P19 { get; private protected set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(24, 38)
                );

            ValidateSymbolsPropertyModifiers_01(compilation2);

            var expected = new[]
            {
                // (5,19): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected int P02 {get;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P02").WithLocation(5, 19),
                // (6,28): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal int P03 {set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P03").WithLocation(6, 28),
                // (8,22): error CS0501: 'I1.P05.set' must declare a body because it is not marked abstract, extern, or partial
                //     private int P05 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P05.set").WithLocation(8, 22),
                // (10,22): error CS0501: 'I1.P07.set' must declare a body because it is not marked abstract, extern, or partial
                //     virtual int P07 {set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.P07.set").WithLocation(10, 22),
                // (11,21): error CS0501: 'I1.P08.get' must declare a body because it is not marked abstract, extern, or partial
                //     sealed int P08 {get;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P08.get").WithLocation(11, 21),
                // (12,18): error CS0106: The modifier 'override' is not valid for this item
                //     override int P09 {set;}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 18),
                // (14,21): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(14, 21),
                // (14,21): warning CS0626: Method, operator, or accessor 'I1.P11.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.P11.get").WithLocation(14, 21),
                // (14,26): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(14, 26),
                // (14,26): warning CS0626: Method, operator, or accessor 'I1.P11.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P11 {get; set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I1.P11.set").WithLocation(14, 26),
                // (16,22): error CS0273: The accessibility modifier of the 'I1.P12.get' accessor must be more restrictive than the property or indexer 'I1.P12'
                //     int P12 { public get; set;}
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I1.P12.get", "I1.P12").WithLocation(16, 22),
                // (17,30): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     int P13 { get; protected set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(17, 30),
                // (18,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     int P14 { protected internal get; set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "get").WithLocation(18, 34),
                // (20,23): error CS0442: 'I1.P16.get': abstract properties cannot have private accessors
                //     int P16 { private get; set;}
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I1.P16.get").WithLocation(20, 23),
                // (21,9): error CS0276: 'I1.P17': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int P17 { private get;}
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P17").WithArguments("I1.P17").WithLocation(21, 9),
                // (23,27): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected int P18 {get;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P18").WithLocation(23, 27),
                // (24,38): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     int P19 { get; private protected set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(24, 38)
            };

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(expected);

            ValidateSymbolsPropertyModifiers_01(compilation2);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(expected);

            ValidateSymbolsPropertyModifiers_01(compilation2);
        }

        [Fact]
        public void PropertyModifiers_03()
        {
            ValidatePropertyImplementation_101(@"
public interface I1
{
    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1.P1;
    }
}
");

            ValidatePropertyImplementation_101(@"
public interface I1
{
    public virtual int P1 
    {
        get => Test1.GetP1();
    }
}

class Test1 : I1
{
    public static int GetP1()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1.P1;
    }
}
");

            ValidatePropertyImplementation_101(@"
public interface I1
{
    public virtual int P1 => Test1.GetP1();
}

class Test1 : I1
{
    public static int GetP1()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1.P1;
    }
}
");

            ValidatePropertyImplementation_102(@"
public interface I1
{
    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 = i1.P1;
    }
}
");

            ValidatePropertyImplementation_102(@"
public interface I1
{
    public virtual int P1 
    {
        get => Test1.GetP1();
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{
    public static int GetP1()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 = i1.P1;
    }
}
");

            ValidatePropertyImplementation_103(@"
public interface I1
{
    public virtual int P1 
    {
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 = 1;
    }
}
");

            ValidatePropertyImplementation_103(@"
public interface I1
{
    public virtual int P1 
    {
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 = 1;
    }
}
");
        }

        [Fact]
        public void PropertyModifiers_04()
        {
            var source1 =
@"
public interface I1
{
    public virtual int P1 { get; } = 0; 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyEmitDiagnostics(
                // (4,24): error CS8053: Instance properties in interfaces cannot have initializers.
                //     public virtual int P1 { get; } = 0; 
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P1").WithLocation(4, 24),
                // (4,29): error CS0501: 'I1.P1.get' must declare a body because it is not marked abstract, extern, or partial
                //     public virtual int P1 { get; } = 0; 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P1.get").WithLocation(4, 29)
                );

            ValidatePropertyModifiers_04(compilation1, "P1");
        }

        private static void ValidatePropertyModifiers_04(CSharpCompilation compilation1, string propertyName)
        {
            var i1 = compilation1.GlobalNamespace.GetTypeMember("I1");
            var p1 = i1.GetMember<PropertySymbol>(propertyName);
            var p1get = p1.GetMethod;

            Assert.False(p1.IsAbstract);
            Assert.True(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);

            Assert.False(p1get.IsAbstract);
            Assert.True(p1get.IsVirtual);
            Assert.True(p1get.IsMetadataVirtual());
            Assert.False(p1get.IsSealed);
            Assert.False(p1get.IsStatic);
            Assert.False(p1get.IsExtern);
            Assert.False(p1get.IsAsync);
            Assert.False(p1get.IsOverride);
            Assert.Equal(Accessibility.Public, p1get.DeclaredAccessibility);
        }

        [Fact]
        public void PropertyModifiers_05()
        {
            var source1 =
@"
public interface I1
{
    public abstract int P1 {get; set;} 
}
public interface I2
{
    int P2 {get; set;} 
}

class Test1 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""set_P1"");
    }
}
class Test2 : I2
{
    public int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        set => System.Console.WriteLine(""set_P2"");
    }

    static void Main()
    {
        I1 x = new Test1();
        x.P1 = x.P1;
        I2 y = new Test2();
        y.P2 = y.P2;
    }
}
";

            ValidatePropertyModifiers_05(source1);
        }

        private void ValidatePropertyModifiers_05(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput:
@"get_P1
set_P1
get_P2
set_P2", symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                for (int i = 1; i <= 2; i++)
                {
                    var test1 = m.GlobalNamespace.GetTypeMember("Test" + i);
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleProperty(i1);
                    var test1P1 = GetSingleProperty(test1);

                    Assert.True(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                    Assert.Same(test1P1, test1.FindImplementationForInterfaceMember(p1));

                    ValidateAccessor(p1.GetMethod, test1P1.GetMethod);
                    ValidateAccessor(p1.SetMethod, test1P1.SetMethod);

                    void ValidateAccessor(MethodSymbol accessor, MethodSymbol implementation)
                    {
                        Assert.True(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.True(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                        Assert.Same(implementation, test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        private static PropertySymbol GetSingleProperty(NamedTypeSymbol container)
        {
            return container.GetMembers().OfType<PropertySymbol>().Single();
        }

        private static PropertySymbol GetSingleProperty(CSharpCompilation compilation, string containerName)
        {
            return GetSingleProperty(compilation.GetTypeByMetadataName(containerName));
        }

        private static PropertySymbol GetSingleProperty(ModuleSymbol m, string containerName)
        {
            return GetSingleProperty(m.GlobalNamespace.GetTypeMember(containerName));
        }

        [Fact]
        public void PropertyModifiers_06()
        {
            var source1 =
@"
public interface I1
{
    public abstract int P1 {get; set;} 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation1.VerifyDiagnostics(
                // (4,25): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract int P1 {get; set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("abstract", "7.3", "8.0").WithLocation(4, 25),
                // (4,25): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract int P1 {get; set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("public", "7.3", "8.0").WithLocation(4, 25)
                );

            ValidatePropertyModifiers_06(compilation1, "P1");
        }

        private static void ValidatePropertyModifiers_06(CSharpCompilation compilation1, string propertyName)
        {
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<PropertySymbol>(propertyName);

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);

            ValidateAccessor(p1.GetMethod);
            ValidateAccessor(p1.SetMethod);

            void ValidateAccessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }
        }

        [Fact]
        public void PropertyModifiers_07()
        {
            var source1 =
@"
public interface I1
{
    public static int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    internal static int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return P3;
        }
        set
        {
            System.Console.WriteLine(""set_P2"");
            P3 = value;
        }
    }

    private static int P3 
    {
        get => Test1.GetP3();
        set => System.Console.WriteLine(""set_P3"");
    }

    internal static int P4 => Test1.GetP4();

    internal static int P5 
    {
        get
        {
            System.Console.WriteLine(""get_P5"");
            return 0;
        }
    }

    internal static int P6 
    {
        get => Test1.GetP6();
    }

    internal static int P7 
    {
        set
        {
            System.Console.WriteLine(""set_P7"");
        }
    }

    internal static int P8 
    {
        set => System.Console.WriteLine(""set_P8"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.P1 = I1.P1;
        I1.P2 = I1.P2;
        var x = I1.P4;
        x = I1.P5;
        x = I1.P6;
        I1.P7 = x;
        I1.P8 = x;
    }

    public static int GetP3()
    {
        System.Console.WriteLine(""get_P3"");
        return 0;
    }

    public static int GetP4()
    {
        System.Console.WriteLine(""get_P4"");
        return 0;
    }

    public static int GetP6()
    {
        System.Console.WriteLine(""get_P6"");
        return 0;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);
            var expected =
@"get_P1
set_P1
get_P2
get_P3
set_P2
set_P3
get_P4
get_P5
get_P6
set_P7
set_P8";

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expected, symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular14, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (6,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         get
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(6, 9),
                // (11,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         set 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(11, 9),
                // (19,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         get
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(19, 9),
                // (24,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         set
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(24, 9),
                // (33,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         get => Test1.GetP3();
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(33, 9),
                // (34,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         set => System.Console.WriteLine("set_P3");
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(34, 9),
                // (37,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int P4 => Test1.GetP4();
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "Test1.GetP4()").WithArguments("static members in interfaces", "15.0").WithLocation(37, 31),
                // (41,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         get
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(41, 9),
                // (50,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         get => Test1.GetP6();
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(50, 9),
                // (55,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         set
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(55, 9),
                // (63,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         set => System.Console.WriteLine("set_P8");
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(63, 9)
                );

            Validate(compilation2.SourceModule);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: expected, symbolValidator: Validate);

            Validate(compilation2.SourceModule);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: expected, symbolValidator: Validate);

            Validate(compilation2.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var tuple in new[] { (name: "P1", access: Accessibility.Public),
                                              (name: "P2", access: Accessibility.Internal),
                                              (name: "P3", access: Accessibility.Private),
                                              (name: "P4", access: Accessibility.Internal),
                                              (name: "P5", access: Accessibility.Internal),
                                              (name: "P6", access: Accessibility.Internal),
                                              (name: "P7", access: Accessibility.Internal),
                                              (name: "P8", access: Accessibility.Internal)})
                {
                    var p1 = i1.GetMember<PropertySymbol>(tuple.name);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.True(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(tuple.access, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    switch (tuple.name)
                    {
                        case "P7":
                        case "P8":
                            Assert.Null(p1.GetMethod);
                            ValidateAccessor(p1.SetMethod);
                            break;
                        case "P4":
                        case "P5":
                        case "P6":
                            Assert.Null(p1.SetMethod);
                            ValidateAccessor(p1.GetMethod);
                            break;
                        default:
                            ValidateAccessor(p1.GetMethod);
                            ValidateAccessor(p1.SetMethod);
                            break;
                    }

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.True(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(tuple.access, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        [Fact]
        public void PropertyModifiers_08()
        {
            var source1 =
@"
public interface I1
{
    abstract static int P1 {get;} 

    virtual static int P2 {set {}} 
    
    sealed static int P3 => 0; 

    static int P4 {get;} = 0;  
}

class Test1 : I1
{
    int I1.P1 => 0;
    int I1.P2 {set {}}
    int I1.P3 => 0;
    int I1.P4 {set {}}
}

class Test2 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (4,25): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     abstract static int P1 {get;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("abstract", "9.0", "11.0").WithLocation(4, 25),
                // (6,24): error CS8703: The modifier 'virtual' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     virtual static int P2 {set {}} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P2").WithArguments("virtual", "9.0", "11.0").WithLocation(6, 24),
                // (8,23): error CS8703: The modifier 'sealed' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     sealed static int P3 => 0; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("sealed", "9.0", "11.0").WithLocation(8, 23),
                // (13,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(13, 15),
                // (15,12): error CS0539: 'Test1.P1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I1.P1 => 0;
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P1").WithArguments("Test1.P1").WithLocation(15, 12),
                // (16,12): error CS0539: 'Test1.P2' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I1.P2 {set {}}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P2").WithArguments("Test1.P2").WithLocation(16, 12),
                // (17,12): error CS0539: 'Test1.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I1.P3 => 0;
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test1.P3").WithLocation(17, 12),
                // (18,12): error CS0539: 'Test1.P4' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I1.P4 {set {}}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P4").WithArguments("Test1.P4").WithLocation(18, 12),
                // (21,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(21, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<PropertySymbol>("P1");
            var p1get = p1.GetMethod;

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.True(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));

            Assert.True(p1get.IsAbstract);
            Assert.False(p1get.IsVirtual);
            Assert.True(p1get.IsMetadataVirtual());
            Assert.False(p1get.IsSealed);
            Assert.True(p1get.IsStatic);
            Assert.False(p1get.IsExtern);
            Assert.False(p1get.IsAsync);
            Assert.False(p1get.IsOverride);
            Assert.Equal(Accessibility.Public, p1get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1get));

            var p2 = i1.GetMember<PropertySymbol>("P2");
            var p2set = p2.SetMethod;

            Assert.False(p2.IsAbstract);
            Assert.True(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.True(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
            Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));

            Assert.False(p2set.IsAbstract);
            Assert.True(p2set.IsVirtual);
            Assert.True(p2set.IsMetadataVirtual());
            Assert.False(p2set.IsSealed);
            Assert.True(p2set.IsStatic);
            Assert.False(p2set.IsExtern);
            Assert.False(p2set.IsAsync);
            Assert.False(p2set.IsOverride);
            Assert.Equal(Accessibility.Public, p2set.DeclaredAccessibility);
            Assert.Same(p2set, test1.FindImplementationForInterfaceMember(p2set));

            var p3 = i1.GetMember<PropertySymbol>("P3");
            var p3get = p3.GetMethod;

            Assert.False(p3.IsAbstract);
            Assert.False(p3.IsVirtual);
            Assert.False(p3.IsSealed);
            Assert.True(p3.IsStatic);
            Assert.False(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));

            Assert.False(p3get.IsAbstract);
            Assert.False(p3get.IsVirtual);
            Assert.False(p3get.IsMetadataVirtual());
            Assert.False(p3get.IsSealed);
            Assert.True(p3get.IsStatic);
            Assert.False(p3get.IsExtern);
            Assert.False(p3get.IsAsync);
            Assert.False(p3get.IsOverride);
            Assert.Equal(Accessibility.Public, p3get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3get));
        }

        [Fact]
        public void PropertyModifiers_09()
        {
            var source1 =
@"
public interface I1
{
    private int P1
    {
        get
        { 
            System.Console.WriteLine(""get_P1"");
            return 0;
        }           
    }
    sealed void M()
    {
        var x = P1;
    }
}
public interface I2
{
    private int P2 
    {
        get
        { 
            System.Console.WriteLine(""get_P2"");
            return 0;
        }           
        set
        { 
            System.Console.WriteLine(""set_P2"");
        }           
    }
    sealed void M()
    {
        P2 = P2;
    }
}
public interface I3
{
    private int P3 
    {
        set
        { 
            System.Console.WriteLine(""set_P3"");
        }           
    }
    sealed void M()
    {
        P3 = 0;
    }
}
public interface I4
{
    private int P4 
    {
        get => GetP4();
    }

    private int GetP4()
    { 
        System.Console.WriteLine(""get_P4"");
        return 0;
    }           
    sealed void M()
    {
        var x = P4;
    }
}
public interface I5
{
    private int P5 
    {
        get => GetP5();
        set => System.Console.WriteLine(""set_P5"");
    }

    private int GetP5()
    { 
        System.Console.WriteLine(""get_P5"");
        return 0;
    }           
    sealed void M()
    {
        P5 = P5;
    }
}
public interface I6
{
    private int P6 
    {
        set => System.Console.WriteLine(""set_P6"");
    }
    sealed void M()
    {
        P6 = 0;
    }
}
public interface I7
{
    private int P7 => GetP7();

    private int GetP7()
    { 
        System.Console.WriteLine(""get_P7"");
        return 0;
    }           
    sealed void M()
    {
        var x = P7;
    }
}

class Test1 : I1, I2, I3, I4, I5, I6, I7
{
    static void Main()
    {
        I1 x1 = new Test1();
        x1.M();
        I2 x2 = new Test1();
        x2.M();
        I3 x3 = new Test1();
        x3.M();
        I4 x4 = new Test1();
        x4.M();
        I5 x5 = new Test1();
        x5.M();
        I6 x6 = new Test1();
        x6.M();
        I7 x7 = new Test1();
        x7.M();
    }
}
";

            ValidatePropertyModifiers_09(source1);
        }

        private void ValidatePropertyModifiers_09(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
get_P2
set_P2
set_P3
get_P4
get_P5
set_P5
set_P6
get_P7
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");

                for (int i = 1; i <= 7; i++)
                {
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleProperty(i1);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Private, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    switch (i)
                    {
                        case 3:
                        case 6:
                            Assert.Null(p1.GetMethod);
                            ValidateAccessor(p1.SetMethod);
                            break;
                        case 1:
                        case 4:
                        case 7:
                            Assert.Null(p1.SetMethod);
                            ValidateAccessor(p1.GetMethod);
                            break;
                        default:
                            ValidateAccessor(p1.GetMethod);
                            ValidateAccessor(p1.SetMethod);
                            break;
                    }

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        [Fact]
        public void PropertyModifiers_10()
        {
            var source1 =
@"
public interface I1
{
    abstract private int P1 { get; } 

    virtual private int P2 => 0;

    sealed private int P3 
    {
        get => 0;
        set {}
    }

    private int P4 {get;} = 0;
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyEmitDiagnostics(
                // (4,26): error CS0621: 'I1.P1': virtual or abstract members cannot be private
                //     abstract private int P1 { get; } 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "P1").WithArguments("I1.P1").WithLocation(4, 26),
                // (6,25): error CS0621: 'I1.P2': virtual or abstract members cannot be private
                //     virtual private int P2 => 0;
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "P2").WithArguments("I1.P2").WithLocation(6, 25),
                // (8,24): error CS0238: 'I1.P3' cannot be sealed because it is not an override
                //     sealed private int P3 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "P3").WithArguments("I1.P3").WithLocation(8, 24),
                // (14,17): error CS8053: Instance properties in interfaces cannot have initializers.
                //     private int P4 {get;} = 0;
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P4").WithLocation(14, 17),
                // (14,21): error CS0501: 'I1.P4.get' must declare a body because it is not marked abstract, extern, or partial
                //     private int P4 {get;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P4.get").WithLocation(14, 21),
                // (17,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(17, 15)
                );

            ValidatePropertyModifiers_10(compilation1);
        }

        private static void ValidatePropertyModifiers_10(CSharpCompilation compilation1)
        {
            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMembers().OfType<PropertySymbol>().ElementAt(0);
            var p1get = p1.GetMethod;

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Private, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));

            Assert.True(p1get.IsAbstract);
            Assert.False(p1get.IsVirtual);
            Assert.True(p1get.IsMetadataVirtual());
            Assert.False(p1get.IsSealed);
            Assert.False(p1get.IsStatic);
            Assert.False(p1get.IsExtern);
            Assert.False(p1get.IsAsync);
            Assert.False(p1get.IsOverride);
            Assert.Equal(Accessibility.Private, p1get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1get));

            var p2 = i1.GetMembers().OfType<PropertySymbol>().ElementAt(1);
            var p2get = p2.GetMethod;

            Assert.False(p2.IsAbstract);
            Assert.True(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Private, p2.DeclaredAccessibility);
            Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));

            Assert.False(p2get.IsAbstract);
            Assert.True(p2get.IsVirtual);
            Assert.True(p2get.IsMetadataVirtual());
            Assert.False(p2get.IsSealed);
            Assert.False(p2get.IsStatic);
            Assert.False(p2get.IsExtern);
            Assert.False(p2get.IsAsync);
            Assert.False(p2get.IsOverride);
            Assert.Equal(Accessibility.Private, p2get.DeclaredAccessibility);
            Assert.Same(p2get, test1.FindImplementationForInterfaceMember(p2get));

            var p3 = i1.GetMembers().OfType<PropertySymbol>().ElementAt(2);

            Assert.False(p3.IsAbstract);
            Assert.False(p3.IsVirtual);
            Assert.True(p3.IsSealed);
            Assert.False(p3.IsStatic);
            Assert.False(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Private, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));

            ValidateP3Accessor(p3.GetMethod);
            ValidateP3Accessor(p3.SetMethod);
            void ValidateP3Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.True(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
            }

            var p4 = i1.GetMembers().OfType<PropertySymbol>().ElementAt(3);
            var p4get = p4.GetMethod;

            Assert.False(p4.IsAbstract);
            Assert.False(p4.IsVirtual);
            Assert.False(p4.IsSealed);
            Assert.False(p4.IsStatic);
            Assert.False(p4.IsExtern);
            Assert.False(p4.IsOverride);
            Assert.Equal(Accessibility.Private, p4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4));

            Assert.False(p4get.IsAbstract);
            Assert.False(p4get.IsVirtual);
            Assert.False(p4get.IsMetadataVirtual());
            Assert.False(p4get.IsSealed);
            Assert.False(p4get.IsStatic);
            Assert.False(p4get.IsExtern);
            Assert.False(p4get.IsAsync);
            Assert.False(p4get.IsOverride);
            Assert.Equal(Accessibility.Private, p4get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4get));
        }

        [Fact]
        public void PropertyModifiers_11_01()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_01(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9),
                    // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(17, 9)
                    ),
                expectedNoImplementation: ExpectedDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(12, 9),
                    // (17,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(17, 9)
                    )
                );
        }

        private void ValidatePropertyModifiers_11_01(string source1, string source2, Accessibility accessibility,
                                                  DiagnosticDescription[] expectedIn9,
                                                  params DiagnosticDescription[] expectedNoImplementation)
        {
            ValidatePropertyModifiers_11_01(source1, source2, accessibility, expectedIn9, expectedNoImplementation, expectedAcrossAssemblyBoundaries: Array.Empty<DiagnosticDescription>());
        }

        private void ValidatePropertyModifiers_11_01(string source1, string source2, Accessibility accessibility,
                                                     DiagnosticDescription[] expectedIn9,
                                                     DiagnosticDescription[] expectedNoImplementation,
                                                     params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expectedIn9);

            ValidatePropertyModifiers_11(compilation1.SourceModule, accessibility);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyModifiers_11(m, accessibility)).VerifyDiagnostics();

            ValidatePropertyModifiers_11(compilation1.SourceModule, accessibility);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var p1 = GetSingleProperty(i1);
                var p1get = p1.GetMethod;
                var p1set = p1.SetMethod;

                ValidatePropertyModifiers_11(p1, accessibility);
                ValidatePropertyAccessorModifiers_11(p1get, accessibility);
                ValidatePropertyAccessorModifiers_11(p1set, accessibility);
            }

            var source3 =
@"
class Test2 : I1
{
}
";

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                ValidatePropertyModifiers_11(compilation3.SourceModule, accessibility);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyModifiers_11(m, accessibility)).VerifyDiagnostics();
                }

                ValidatePropertyModifiers_11(compilation3.SourceModule, accessibility);

                var compilation5 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation5.VerifyDiagnostics(expectedNoImplementation);

                ValidatePropertyNotImplemented_11(compilation5, "Test2");
            }
        }

        private static void ValidatePropertyModifiers_11(ModuleSymbol m, Accessibility accessibility)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
            var p1 = GetSingleProperty(i1);
            var test1P1 = GetSingleProperty(test1);
            var p1get = p1.GetMethod;
            var p1set = p1.SetMethod;

            ValidatePropertyModifiers_11(p1, accessibility);
            ValidatePropertyAccessorModifiers_11(p1get, accessibility);
            ValidatePropertyAccessorModifiers_11(p1set, accessibility);
            Assert.Same(test1P1, test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(test1P1.GetMethod, test1.FindImplementationForInterfaceMember(p1get));
            Assert.Same(test1P1.SetMethod, test1.FindImplementationForInterfaceMember(p1set));
        }

        private static void ValidatePropertyModifiers_11(PropertySymbol p1, Accessibility accessibility)
        {
            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(accessibility, p1.DeclaredAccessibility);
        }

        private static void ValidatePropertyAccessorModifiers_11(MethodSymbol m1, Accessibility accessibility)
        {
            Assert.True(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.True(m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(accessibility, m1.DeclaredAccessibility);
        }

        private static void ValidatePropertyNotImplemented_11(CSharpCompilation compilation, string className)
        {
            var test2 = compilation.GetTypeByMetadataName(className);
            var i1 = compilation.GetTypeByMetadataName("I1");
            var p1 = GetSingleProperty(i1);
            Assert.Null(test2.FindImplementationForInterfaceMember(p1));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1.SetMethod));
        }

        [Fact]
        public void PropertyModifiers_11_02()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        private void ValidatePropertyModifiers_11_02(string source1, string source2,
                                                     DiagnosticDescription[] expectedIn9,
                                                     params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expectedIn9);

            ValidatePropertyImplementation_11(compilation1.SourceModule);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyImplementation_11(m)).VerifyDiagnostics();

            ValidatePropertyImplementation_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                ValidatePropertyImplementation_11(compilation3.SourceModule);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyImplementation_11(m)).VerifyDiagnostics();
                }

                ValidatePropertyImplementation_11(compilation3.SourceModule);
            }
        }

        private static void ValidatePropertyImplementation_11(ModuleSymbol m)
        {
            ValidatePropertyImplementation_11(m, implementedByBase: false);
        }

        private static void ValidatePropertyImplementationByBase_11(ModuleSymbol m)
        {
            ValidatePropertyImplementation_11(m, implementedByBase: true);
        }

        private static void ValidatePropertyImplementation_11(ModuleSymbol m, bool implementedByBase)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            var p1 = GetSingleProperty(i1);
            var test1P1 = GetSingleProperty(implementedByBase ? test1.BaseTypeNoUseSiteDiagnostics : test1);
            var p1get = p1.GetMethod;
            var p1set = p1.SetMethod;

            Assert.Same(test1P1, test1.FindImplementationForInterfaceMember(p1));

            Assert.Same(test1P1.GetMethod, test1.FindImplementationForInterfaceMember(p1get));
            Assert.Same(test1P1.SetMethod, test1.FindImplementationForInterfaceMember(p1set));

            var i2 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I2").SingleOrDefault();
            Assert.True(test1P1.GetMethod.IsMetadataVirtual());
            Assert.True(test1P1.SetMethod.IsMetadataVirtual());
        }

        [Fact]
        public void PropertyModifiers_11_03()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //     int I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(9, 12)
                );
        }

        private void ValidatePropertyModifiers_11_03(string source1, string source2, TargetFramework targetFramework,
                                                  params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 targetFramework: targetFramework,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput: !(targetFramework == TargetFramework.Standard || ExecutionConditionUtil.IsMonoOrCoreClr) ? null :
@"get_P1
set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidatePropertyImplementation_11);

            ValidatePropertyImplementation_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 targetFramework: targetFramework,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     targetFramework: targetFramework,
                                                     parseOptions: TestOptions.Regular);

                compilation3.VerifyDiagnostics(expected);

                if (expected.Length == 0)
                {
                    CompileAndVerify(compilation3, expectedOutput: !(targetFramework == TargetFramework.Standard || ExecutionConditionUtil.IsMonoOrCoreClr) ? null :
@"get_P1
set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidatePropertyImplementation_11);
                }

                ValidatePropertyImplementation_11(compilation3.SourceModule);
            }
        }

        [Fact]
        public void PropertyModifiers_11_04()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_11_05()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_11_06()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract int P1 {get; set;} 
}

class Test3 : Test1
{
    public override int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,29): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(9, 29),
                    // (9,34): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(9, 34)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,29): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(9, 29),
                    // (9,34): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(9, 34)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_11_07()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    int P1 {get; set;} 
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_11_08()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long P1 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_08(source1, source2);
        }

        private void ValidatePropertyModifiers_11_08(string source1, string source2)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput:
@"Test2.get_P1
Test2.set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidatePropertyImplementationByBase_11);

            ValidatePropertyImplementationByBase_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation3, expectedOutput:
@"Test2.get_P1
Test2.set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidatePropertyImplementationByBase_11);

            ValidatePropertyImplementationByBase_11(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation4, expectedOutput:
@"Test2.get_P1
Test2.set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidatePropertyImplementationByBase_11);

            ValidatePropertyImplementationByBase_11(compilation4.SourceModule);
        }

        [Fact]
        public void PropertyModifiers_11_09()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long P1 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.P1'. 'Test1.P1' cannot implement 'I1.P1' because it does not have the matching return type of 'int'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.P1", "Test1.P1", "int").WithLocation(2, 15)
                );
        }

        private void ValidatePropertyModifiers_11_09(string source1, string source2,
                                                  params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(expected);

            ValidatePropertyNotImplemented_11(compilation1, "Test1");

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation3.VerifyDiagnostics(expected);

            ValidatePropertyNotImplemented_11(compilation3, "Test1");

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation4.VerifyDiagnostics(expected);

            ValidatePropertyNotImplemented_11(compilation4, "Test1");
        }

        [Fact]
        public void PropertyModifiers_11_10()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (6,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.get'. 'Test2.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.P1.get", "Test2.P1.get", "9.0", "10.0").WithLocation(6, 9),
                    // (11,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.set'. 'Test2.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.P1.set", "Test2.P1.set", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (6,9): error CS9044: 'Test2' does not implement interface member 'I1.P1.get'. 'Test2.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test2", "I1.P1.get", "Test2.P1.get").WithLocation(6, 9),
                    // (11,9): error CS9044: 'Test2' does not implement interface member 'I1.P1.set'. 'Test2.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test2", "I1.P1.set", "Test2.P1.set").WithLocation(11, 9)
                    )
                );
        }

        private void ValidatePropertyModifiers_11_10(string source1, string source2,
                                                     DiagnosticDescription[] expectedIn9,
                                                     params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(expectedIn9);

            ValidatePropertyImplementationByBase_11(compilation1.SourceModule);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyImplementationByBase_11(m)).VerifyDiagnostics();

            ValidatePropertyImplementationByBase_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                ValidatePropertyImplementationByBase_11(compilation3.SourceModule);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyImplementationByBase_11(m)).VerifyDiagnostics();
                }

                ValidatePropertyImplementationByBase_11(compilation3.SourceModule);
            }
        }

        [Fact]
        public void PropertyModifiers_11_11()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
public class Test2 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_11(source1, source2,
                // (15,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.get'. 'Test2.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.P1.get", "Test2.P1.get", "9.0", "10.0").WithLocation(15, 9),
                // (20,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.set'. 'Test2.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         set
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.P1.set", "Test2.P1.set", "9.0", "10.0").WithLocation(20, 9)
                );
        }

        private void ValidatePropertyModifiers_11_11(string source1, string source2,
                                                     params DiagnosticDescription[] expectedIn9)
        {
            ValidatePropertyModifiers_11_11(source1, source2, expectedIn9, expectedAcrossAssemblyBoundaries: Array.Empty<DiagnosticDescription>());
        }

        private void ValidatePropertyModifiers_11_11(string source1, string source2,
                                                     DiagnosticDescription[] expectedIn9,
                                                     params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 assemblyName: "PropertyModifiers_11_11");

            compilation2.VerifyDiagnostics(expectedIn9);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp,
                                             assemblyName: "PropertyModifiers_11_11");

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            if (expectedAcrossAssemblyBoundaries.Length != 0)
            {
                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
            }
            else
            {
                CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                 verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidatePropertyImplementationByBase_11(m)).VerifyDiagnostics();
            }

            ValidatePropertyImplementationByBase_11(compilation3.SourceModule);
        }

        [Fact]
        public void PropertyModifiers_12()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int P1 {get; set;} 
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(
                // (7,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(7, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<PropertySymbol>("P1");
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test1.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Null(test1.FindImplementationForInterfaceMember(p1.SetMethod));
        }

        [Fact]
        public void PropertyModifiers_13()
        {
            var source1 =
@"
public interface I1
{
    public sealed int P1
    {
        get
        { 
            System.Console.WriteLine(""get_P1"");
            return 0;
        }           
    }
}
public interface I2
{
    public sealed int P2 
    {
        get
        { 
            System.Console.WriteLine(""get_P2"");
            return 0;
        }           
        set
        { 
            System.Console.WriteLine(""set_P2"");
        }           
    }
}
public interface I3
{
    public sealed int P3 
    {
        set
        { 
            System.Console.WriteLine(""set_P3"");
        }           
    }
}
public interface I4
{
    public sealed int P4 
    {
        get => GetP4();
    }

    private int GetP4()
    { 
        System.Console.WriteLine(""get_P4"");
        return 0;
    }           
}
public interface I5
{
    public sealed int P5 
    {
        get => GetP5();
        set => System.Console.WriteLine(""set_P5"");
    }

    private int GetP5()
    { 
        System.Console.WriteLine(""get_P5"");
        return 0;
    }           
}
public interface I6
{
    public sealed int P6 
    {
        set => System.Console.WriteLine(""set_P6"");
    }
}
public interface I7
{
    public sealed int P7 => GetP7();

    private int GetP7()
    { 
        System.Console.WriteLine(""get_P7"");
        return 0;
    }           
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        var x = i1.P1;
        I2 i2 = new Test2();
        i2.P2 = i2.P2;
        I3 i3 = new Test3();
        i3.P3 = x;
        I4 i4 = new Test4();
        x = i4.P4;
        I5 i5 = new Test5();
        i5.P5 = i5.P5;
        I6 i6 = new Test6();
        i6.P6 = x;
        I7 i7 = new Test7();
        x = i7.P7;
    }

    public int P1 => throw null;
}
class Test2 : I2
{
    public int P2 
    {
        get => throw null;          
        set => throw null;         
    }
}
class Test3 : I3
{
    public int P3 
    {
        set => throw null;      
    }
}
class Test4 : I4
{
    public int P4 
    {
        get => throw null;
    }
}
class Test5 : I5
{
    public int P5 
    {
        get => throw null;
        set => throw null;
    }
}
class Test6 : I6
{
    public int P6 
    {
        set => throw null;
    }
}
class Test7 : I7
{
    public int P7 => throw null;
}
";

            ValidatePropertyModifiers_13(source1);
        }

        private void ValidatePropertyModifiers_13(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate(ModuleSymbol m)
            {
                for (int i = 1; i <= 7; i++)
                {
                    var test1 = m.GlobalNamespace.GetTypeMember("Test" + i);
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleProperty(i1);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    switch (i)
                    {
                        case 3:
                        case 6:
                            Assert.Null(p1.GetMethod);
                            ValidateAccessor(p1.SetMethod);
                            break;
                        case 1:
                        case 4:
                        case 7:
                            Assert.Null(p1.SetMethod);
                            ValidateAccessor(p1.GetMethod);
                            break;
                        default:
                            ValidateAccessor(p1.GetMethod);
                            ValidateAccessor(p1.SetMethod);
                            break;
                    }

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
get_P2
set_P2
set_P3
get_P4
get_P5
set_P5
set_P6
get_P7
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate);
            Validate(compilation1.SourceModule);
        }

        [Fact]
        public void PropertyModifiers_14()
        {
            var source1 =
@"
public interface I1
{
    public sealed int P1 {get;} = 0; 
}
public interface I2
{
    abstract sealed int P2 {get;} 
}
public interface I3
{
    virtual sealed int P3 
    {
        set {}
    }
}

class Test1 : I1, I2, I3
{
    int I1.P1 { get => throw null; }
    int I2.P2 { get => throw null; }
    int I3.P3 { set => throw null; }
}

class Test2 : I1, I2, I3
{}
";
            ValidatePropertyModifiers_14(source1,
                // (4,23): error CS8053: Instance properties in interfaces cannot have initializers.
                //     public sealed int P1 {get;} = 0; 
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P1").WithLocation(4, 23),
                // (4,27): error CS0501: 'I1.P1.get' must declare a body because it is not marked abstract, extern, or partial
                //     public sealed int P1 {get;} = 0; 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.P1.get").WithLocation(4, 27),
                // (8,25): error CS0238: 'I2.P2' cannot be sealed because it is not an override
                //     abstract sealed int P2 {get;} 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "P2").WithArguments("I2.P2").WithLocation(8, 25),
                // (12,24): error CS0238: 'I3.P3' cannot be sealed because it is not an override
                //     virtual sealed int P3 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "P3").WithArguments("I3.P3").WithLocation(12, 24),
                // (20,12): error CS0539: 'Test1.P1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I1.P1 { get => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P1").WithArguments("Test1.P1").WithLocation(20, 12),
                // (21,12): error CS0539: 'Test1.P2' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I2.P2 { get => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P2").WithArguments("Test1.P2").WithLocation(21, 12),
                // (22,12): error CS0539: 'Test1.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I3.P3 { set => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test1.P3").WithLocation(22, 12)
                );
        }

        private void ValidatePropertyModifiers_14(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyEmitDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var p1 = GetSingleProperty(compilation1, "I1");
            var p1get = p1.GetMethod;

            Assert.False(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1));

            Assert.False(p1get.IsAbstract);
            Assert.False(p1get.IsVirtual);
            Assert.False(p1get.IsMetadataVirtual());
            Assert.False(p1get.IsSealed);
            Assert.False(p1get.IsStatic);
            Assert.False(p1get.IsExtern);
            Assert.False(p1get.IsAsync);
            Assert.False(p1get.IsOverride);
            Assert.Equal(Accessibility.Public, p1get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1get));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1get));

            var p2 = GetSingleProperty(compilation1, "I2");
            var test1P2 = test1.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I2.")).Single();
            var p2get = p2.GetMethod;

            Assert.True(p2.IsAbstract);
            Assert.False(p2.IsVirtual);
            Assert.True(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p2));
            Assert.Null(test2.FindImplementationForInterfaceMember(p2));

            Assert.True(p2get.IsAbstract);
            Assert.False(p2get.IsVirtual);
            Assert.True(p2get.IsMetadataVirtual());
            Assert.True(p2get.IsSealed);
            Assert.False(p2get.IsStatic);
            Assert.False(p2get.IsExtern);
            Assert.False(p2get.IsAsync);
            Assert.False(p2get.IsOverride);
            Assert.Equal(Accessibility.Public, p2get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p2get));
            Assert.Null(test2.FindImplementationForInterfaceMember(p2get));

            var p3 = GetSingleProperty(compilation1, "I3");
            var test1P3 = test1.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I3.")).Single();
            var p3set = p3.SetMethod;

            Assert.False(p3.IsAbstract);
            Assert.True(p3.IsVirtual);
            Assert.True(p3.IsSealed);
            Assert.False(p3.IsStatic);
            Assert.False(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3));

            Assert.False(p3set.IsAbstract);
            Assert.True(p3set.IsVirtual);
            Assert.True(p3set.IsMetadataVirtual());
            Assert.True(p3set.IsSealed);
            Assert.False(p3set.IsStatic);
            Assert.False(p3set.IsExtern);
            Assert.False(p3set.IsAsync);
            Assert.False(p3set.IsOverride);
            Assert.Equal(Accessibility.Public, p3set.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3set));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3set));
        }

        [Fact]
        public void PropertyModifiers_15()
        {
            var source1 =
@"
public interface I0
{
    abstract virtual int P0 { get; set; }
}
public interface I1
{
    abstract virtual int P1 { get { throw null; } }
}
public interface I2
{
    virtual abstract int P2 
    {
        get { throw null; }
        set { throw null; }
    }
}
public interface I3
{
    abstract virtual int P3 { set { throw null; } }
}
public interface I4
{
    abstract virtual int P4 { get => throw null; }
}
public interface I5
{
    abstract virtual int P5 
    {
        get => throw null;
        set => throw null;
    }
}
public interface I6
{
    abstract virtual int P6 { set => throw null; }
}
public interface I7
{
    abstract virtual int P7 => throw null;
}
public interface I8
{
    abstract virtual int P8 {get;} = 0;
}

class Test1 : I0, I1, I2, I3, I4, I5, I6, I7, I8
{
    int I0.P0 
    {
        get { throw null; }
        set { throw null; }
    }
    int I1.P1 
    {
        get { throw null; }
    }
    int I2.P2 
    {
        get { throw null; }
        set { throw null; }
    }
    int I3.P3 
    {
        set { throw null; }
    }
    int I4.P4 
    {
        get { throw null; }
    }
    int I5.P5 
    {
        get { throw null; }
        set { throw null; }
    }
    int I6.P6 
    {
        set { throw null; }
    }
    int I7.P7 
    {
        get { throw null; }
    }
    int I8.P8 
    {
        get { throw null; }
    }
}

class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
{}
";
            ValidatePropertyModifiers_15(source1,
                // (4,26): error CS0503: The abstract property 'I0.P0' cannot be marked virtual
                //     abstract virtual int P0 { get; set; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P0").WithArguments("property", "I0.P0").WithLocation(4, 26),
                // (8,26): error CS0503: The abstract property 'I1.P1' cannot be marked virtual
                //     abstract virtual int P1 { get { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P1").WithArguments("property", "I1.P1").WithLocation(8, 26),
                // (8,31): error CS0500: 'I1.P1.get' cannot declare a body because it is marked abstract
                //     abstract virtual int P1 { get { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I1.P1.get").WithLocation(8, 31),
                // (12,26): error CS0503: The abstract property 'I2.P2' cannot be marked virtual
                //     virtual abstract int P2 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P2").WithArguments("property", "I2.P2").WithLocation(12, 26),
                // (14,9): error CS0500: 'I2.P2.get' cannot declare a body because it is marked abstract
                //         get { throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I2.P2.get").WithLocation(14, 9),
                // (15,9): error CS0500: 'I2.P2.set' cannot declare a body because it is marked abstract
                //         set { throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I2.P2.set").WithLocation(15, 9),
                // (20,26): error CS0503: The abstract property 'I3.P3' cannot be marked virtual
                //     abstract virtual int P3 { set { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P3").WithArguments("property", "I3.P3").WithLocation(20, 26),
                // (20,31): error CS0500: 'I3.P3.set' cannot declare a body because it is marked abstract
                //     abstract virtual int P3 { set { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I3.P3.set").WithLocation(20, 31),
                // (24,26): error CS0503: The abstract property 'I4.P4' cannot be marked virtual
                //     abstract virtual int P4 { get => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P4").WithArguments("property", "I4.P4").WithLocation(24, 26),
                // (24,31): error CS0500: 'I4.P4.get' cannot declare a body because it is marked abstract
                //     abstract virtual int P4 { get => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I4.P4.get").WithLocation(24, 31),
                // (28,26): error CS0503: The abstract property 'I5.P5' cannot be marked virtual
                //     abstract virtual int P5 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P5").WithArguments("property", "I5.P5").WithLocation(28, 26),
                // (30,9): error CS0500: 'I5.P5.get' cannot declare a body because it is marked abstract
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I5.P5.get").WithLocation(30, 9),
                // (31,9): error CS0500: 'I5.P5.set' cannot declare a body because it is marked abstract
                //         set => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I5.P5.set").WithLocation(31, 9),
                // (36,26): error CS0503: The abstract property 'I6.P6' cannot be marked virtual
                //     abstract virtual int P6 { set => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P6").WithArguments("property", "I6.P6").WithLocation(36, 26),
                // (36,31): error CS0500: 'I6.P6.set' cannot declare a body because it is marked abstract
                //     abstract virtual int P6 { set => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I6.P6.set").WithLocation(36, 31),
                // (40,26): error CS0503: The abstract property 'I7.P7' cannot be marked virtual
                //     abstract virtual int P7 => throw null;
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P7").WithArguments("property", "I7.P7").WithLocation(40, 26),
                // (40,32): error CS0500: 'I7.P7.get' cannot declare a body because it is marked abstract
                //     abstract virtual int P7 => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "throw null").WithArguments("I7.P7.get").WithLocation(40, 32),
                // (44,26): error CS0503: The abstract property 'I8.P8' cannot be marked virtual
                //     abstract virtual int P8 {get;} = 0;
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P8").WithArguments("property", "I8.P8").WithLocation(44, 26),
                // (44,26): error CS8053: Instance properties in interfaces cannot have initializers.
                //     abstract virtual int P8 {get;} = 0;
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P8").WithLocation(44, 26),
                // (90,15): error CS0535: 'Test2' does not implement interface member 'I0.P0'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I0").WithArguments("Test2", "I0.P0").WithLocation(90, 15),
                // (90,19): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(90, 19),
                // (90,23): error CS0535: 'Test2' does not implement interface member 'I2.P2'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test2", "I2.P2").WithLocation(90, 23),
                // (90,27): error CS0535: 'Test2' does not implement interface member 'I3.P3'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test2", "I3.P3").WithLocation(90, 27),
                // (90,31): error CS0535: 'Test2' does not implement interface member 'I4.P4'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test2", "I4.P4").WithLocation(90, 31),
                // (90,35): error CS0535: 'Test2' does not implement interface member 'I5.P5'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I5").WithArguments("Test2", "I5.P5").WithLocation(90, 35),
                // (90,39): error CS0535: 'Test2' does not implement interface member 'I6.P6'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I6").WithArguments("Test2", "I6.P6").WithLocation(90, 39),
                // (90,43): error CS0535: 'Test2' does not implement interface member 'I7.P7'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I7").WithArguments("Test2", "I7.P7").WithLocation(90, 43),
                // (90,47): error CS0535: 'Test2' does not implement interface member 'I8.P8'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I8").WithArguments("Test2", "I8.P8").WithLocation(90, 47)
                );
        }

        private void ValidatePropertyModifiers_15(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");

            for (int i = 0; i <= 8; i++)
            {
                var i1 = compilation1.GetTypeByMetadataName("I" + i);
                var p2 = GetSingleProperty(i1);
                var test1P2 = test1.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith(i1.Name)).Single();

                Assert.True(p2.IsAbstract);
                Assert.True(p2.IsVirtual);
                Assert.False(p2.IsSealed);
                Assert.False(p2.IsStatic);
                Assert.False(p2.IsExtern);
                Assert.False(p2.IsOverride);
                Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
                Assert.Same(test1P2, test1.FindImplementationForInterfaceMember(p2));
                Assert.Null(test2.FindImplementationForInterfaceMember(p2));

                switch (i)
                {
                    case 3:
                    case 6:
                        Assert.Null(p2.GetMethod);
                        ValidateAccessor(p2.SetMethod, test1P2.SetMethod);
                        break;
                    case 1:
                    case 4:
                    case 7:
                    case 8:
                        Assert.Null(p2.SetMethod);
                        ValidateAccessor(p2.GetMethod, test1P2.GetMethod);
                        break;
                    default:
                        ValidateAccessor(p2.GetMethod, test1P2.GetMethod);
                        ValidateAccessor(p2.SetMethod, test1P2.SetMethod);
                        break;
                }

                void ValidateAccessor(MethodSymbol accessor, MethodSymbol implementedBy)
                {
                    Assert.True(accessor.IsAbstract);
                    Assert.True(accessor.IsVirtual);
                    Assert.True(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.False(accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                    Assert.Same(implementedBy, test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
                }
            }
        }

        [Fact]
        public void PropertyModifiers_16()
        {
            var source1 =
@"
public interface I1
{
    extern int P1 {get;} 
}
public interface I2
{
    virtual extern int P2 {set;}
}
public interface I3
{
    static extern int P3 {get; set;} 
}
public interface I4
{
    private extern int P4 {get;}
}
public interface I5
{
    extern sealed int P5 {set;}
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    int I1.P1 => 0;
    int I2.P2 { set {} }
}
";
            ValidatePropertyModifiers_16(source1,
                new DiagnosticDescription[]
                {
                // (4,16): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern int P1 {get;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("extern", "7.3", "8.0").WithLocation(4, 16),
                // (8,24): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern int P2 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P2").WithArguments("extern", "7.3", "8.0").WithLocation(8, 24),
                // (8,24): error CS8503: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern int P2 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P2").WithArguments("virtual", "7.3", "8.0").WithLocation(8, 24),
                // (12,23): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("static", "7.3", "8.0").WithLocation(12, 23),
                // (12,23): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("extern", "7.3", "8.0").WithLocation(12, 23),
                // (16,24): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern int P4 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P4").WithArguments("private", "7.3", "8.0").WithLocation(16, 24),
                // (16,24): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern int P4 {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P4").WithArguments("extern", "7.3", "8.0").WithLocation(16, 24),
                // (20,23): error CS8503: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed int P5 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P5").WithArguments("sealed", "7.3", "8.0").WithLocation(20, 23),
                // (20,23): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed int P5 {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P5").WithArguments("extern", "7.3", "8.0").WithLocation(20, 23),
                // (8,28): warning CS0626: Method, operator, or accessor 'I2.P2.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern int P2 {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I2.P2.set").WithLocation(8, 28),
                // (12,27): warning CS0626: Method, operator, or accessor 'I3.P3.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I3.P3.get").WithLocation(12, 27),
                // (12,32): warning CS0626: Method, operator, or accessor 'I3.P3.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I3.P3.set").WithLocation(12, 32),
                // (16,28): warning CS0626: Method, operator, or accessor 'I4.P4.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern int P4 {get;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I4.P4.get").WithLocation(16, 28),
                // (20,27): warning CS0626: Method, operator, or accessor 'I5.P5.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed int P5 {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I5.P5.set").WithLocation(20, 27),
                // (4,20): warning CS0626: Method, operator, or accessor 'I1.P1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.P1.get").WithLocation(4, 20)
                },
                // (4,20): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern int P1 {get;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(4, 20),
                // (8,28): error CS8501: Target runtime doesn't support default interface implementation.
                //     virtual extern int P2 {set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(8, 28),
                // (16,28): error CS8501: Target runtime doesn't support default interface implementation.
                //     private extern int P4 {get;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(16, 28),
                // (20,27): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern sealed int P5 {set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(20, 27),
                // (8,28): warning CS0626: Method, operator, or accessor 'I2.P2.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern int P2 {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I2.P2.set").WithLocation(8, 28),
                // (12,27): warning CS0626: Method, operator, or accessor 'I3.P3.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I3.P3.get").WithLocation(12, 27),
                // (12,32): warning CS0626: Method, operator, or accessor 'I3.P3.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I3.P3.set").WithLocation(12, 32),
                // (16,28): warning CS0626: Method, operator, or accessor 'I4.P4.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern int P4 {get;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I4.P4.get").WithLocation(16, 28),
                // (20,27): warning CS0626: Method, operator, or accessor 'I5.P5.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed int P5 {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I5.P5.set").WithLocation(20, 27),
                // (4,20): warning CS0626: Method, operator, or accessor 'I1.P1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int P1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.P1.get").WithLocation(4, 20)
                );
        }

        private void ValidatePropertyModifiers_16(string source1, DiagnosticDescription[] expected1, params DiagnosticDescription[] expected2)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var test2 = m.GlobalNamespace.GetTypeMember("Test2");
                bool isSource = !(m is PEModuleSymbol);
                var p1 = GetSingleProperty(m, "I1");
                var test2P1 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I1.")).Single();
                var p1get = p1.GetMethod;

                Assert.False(p1.IsAbstract);
                Assert.True(p1.IsVirtual);
                Assert.False(p1.IsSealed);
                Assert.False(p1.IsStatic);
                Assert.Equal(isSource, p1.IsExtern);
                Assert.False(p1.IsOverride);
                Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
                Assert.Same(test2P1, test2.FindImplementationForInterfaceMember(p1));

                Assert.False(p1get.IsAbstract);
                Assert.True(p1get.IsVirtual);
                Assert.True(p1get.IsMetadataVirtual());
                Assert.False(p1get.IsSealed);
                Assert.False(p1get.IsStatic);
                Assert.Equal(isSource, p1get.IsExtern);
                Assert.False(p1get.IsAsync);
                Assert.False(p1get.IsOverride);
                Assert.Equal(Accessibility.Public, p1get.DeclaredAccessibility);
                Assert.Same(p1get, test1.FindImplementationForInterfaceMember(p1get));
                Assert.Same(test2P1.GetMethod, test2.FindImplementationForInterfaceMember(p1get));

                var p2 = GetSingleProperty(m, "I2");
                var test2P2 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I2.")).Single();
                var p2set = p2.SetMethod;

                Assert.False(p2.IsAbstract);
                Assert.True(p2.IsVirtual);
                Assert.False(p2.IsSealed);
                Assert.False(p2.IsStatic);
                Assert.Equal(isSource, p2.IsExtern);
                Assert.False(p2.IsOverride);
                Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
                Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));
                Assert.Same(test2P2, test2.FindImplementationForInterfaceMember(p2));

                Assert.False(p2set.IsAbstract);
                Assert.True(p2set.IsVirtual);
                Assert.True(p2set.IsMetadataVirtual());
                Assert.False(p2set.IsSealed);
                Assert.False(p2set.IsStatic);
                Assert.Equal(isSource, p2set.IsExtern);
                Assert.False(p2set.IsAsync);
                Assert.False(p2set.IsOverride);
                Assert.Equal(Accessibility.Public, p2set.DeclaredAccessibility);
                Assert.Same(p2set, test1.FindImplementationForInterfaceMember(p2set));
                Assert.Same(test2P2.SetMethod, test2.FindImplementationForInterfaceMember(p2set));

                var i3 = m.ContainingAssembly.GetTypeByMetadataName("I3");

                if ((object)i3 != null)
                {
                    var p3 = GetSingleProperty(i3);

                    Assert.False(p3.IsAbstract);
                    Assert.False(p3.IsVirtual);
                    Assert.False(p3.IsSealed);
                    Assert.True(p3.IsStatic);
                    Assert.Equal(isSource, p3.IsExtern);
                    Assert.False(p3.IsOverride);
                    Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p3));
                    Assert.Null(test2.FindImplementationForInterfaceMember(p3));

                    ValidateP3Accessor(p3.GetMethod);
                    ValidateP3Accessor(p3.SetMethod);
                    void ValidateP3Accessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.True(accessor.IsStatic);
                        Assert.Equal(isSource, accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                        Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
                    }
                }

                var p4 = GetSingleProperty(m, "I4");
                var p4get = p4.GetMethod;

                Assert.False(p4.IsAbstract);
                Assert.False(p4.IsVirtual);
                Assert.False(p4.IsSealed);
                Assert.False(p4.IsStatic);
                Assert.Equal(isSource, p4.IsExtern);
                Assert.False(p4.IsOverride);
                Assert.Equal(Accessibility.Private, p4.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p4));
                Assert.Null(test2.FindImplementationForInterfaceMember(p4));

                Assert.False(p4get.IsAbstract);
                Assert.False(p4get.IsVirtual);
                Assert.False(p4get.IsMetadataVirtual());
                Assert.False(p4get.IsSealed);
                Assert.False(p4get.IsStatic);
                Assert.Equal(isSource, p4get.IsExtern);
                Assert.False(p4get.IsAsync);
                Assert.False(p4get.IsOverride);
                Assert.Equal(Accessibility.Private, p4get.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p4get));
                Assert.Null(test2.FindImplementationForInterfaceMember(p4get));

                var p5 = GetSingleProperty(m, "I5");
                var p5set = p5.SetMethod;

                Assert.False(p5.IsAbstract);
                Assert.False(p5.IsVirtual);
                Assert.False(p5.IsSealed);
                Assert.False(p5.IsStatic);
                Assert.Equal(isSource, p5.IsExtern);
                Assert.False(p5.IsOverride);
                Assert.Equal(Accessibility.Public, p5.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p5));
                Assert.Null(test2.FindImplementationForInterfaceMember(p5));

                Assert.False(p5set.IsAbstract);
                Assert.False(p5set.IsVirtual);
                Assert.False(p5set.IsMetadataVirtual());
                Assert.False(p5set.IsSealed);
                Assert.False(p5set.IsStatic);
                Assert.Equal(isSource, p5set.IsExtern);
                Assert.False(p5set.IsAsync);
                Assert.False(p5set.IsOverride);
                Assert.Equal(Accessibility.Public, p5set.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p5set));
                Assert.Null(test2.FindImplementationForInterfaceMember(p5set));
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(expected1);

            Validate(compilation2.SourceModule);

            var compilation3 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(expected2);

            Validate(compilation3.SourceModule);
        }

        [Fact]
        public void PropertyModifiers_17()
        {
            var source1 =
@"
public interface I1
{
    abstract extern int P1 {get;} 
}
public interface I2
{
    extern int P2 => 0; 
}
public interface I3
{
    static extern int P3 {get => 0; set => throw null;} 
}
public interface I4
{
    private extern int P4 { get {throw null;} set {throw null;}}
}
public interface I5
{
    extern sealed int P5 {get;} = 0;
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    int I1.P1 => 0;
    int I2.P2 => 0;
    int I3.P3 { get => 0; set => throw null;}
    int I4.P4 { get => 0; set => throw null;}
    int I5.P5 => 0;
}
";
            ValidatePropertyModifiers_17(source1,
                // (4,25): error CS0180: 'I1.P1' cannot be both extern and abstract
                //     abstract extern int P1 {get;} 
                Diagnostic(ErrorCode.ERR_AbstractAndExtern, "P1").WithArguments("I1.P1").WithLocation(4, 25),
                // (8,22): error CS0179: 'I2.P2.get' cannot be extern and declare a body
                //     extern int P2 => 0; 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "0").WithArguments("I2.P2.get").WithLocation(8, 22),
                // (12,27): error CS0179: 'I3.P3.get' cannot be extern and declare a body
                //     static extern int P3 {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "get").WithArguments("I3.P3.get").WithLocation(12, 27),
                // (12,37): error CS0179: 'I3.P3.set' cannot be extern and declare a body
                //     static extern int P3 {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I3.P3.set").WithLocation(12, 37),
                // (16,29): error CS0179: 'I4.P4.get' cannot be extern and declare a body
                //     private extern int P4 { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "get").WithArguments("I4.P4.get").WithLocation(16, 29),
                // (16,47): error CS0179: 'I4.P4.set' cannot be extern and declare a body
                //     private extern int P4 { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I4.P4.set").WithLocation(16, 47),
                // (20,23): error CS8053: Instance properties in interfaces cannot have initializers.
                //     extern sealed int P5 {get;} = 0;
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P5").WithLocation(20, 23),
                // (23,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(23, 15),
                // (31,12): error CS0539: 'Test2.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I3.P3 { get => 0; set => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test2.P3").WithLocation(31, 12),
                // (32,12): error CS0539: 'Test2.P4' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I4.P4 { get => 0; set => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P4").WithArguments("Test2.P4").WithLocation(32, 12),
                // (33,12): error CS0539: 'Test2.P5' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I5.P5 => 0;
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P5").WithArguments("Test2.P5").WithLocation(33, 12),
                // (20,27): warning CS0626: Method, operator, or accessor 'I5.P5.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed int P5 {get;} = 0;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I5.P5.get").WithLocation(20, 27)
                );
        }

        private void ValidatePropertyModifiers_17(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var p1 = GetSingleProperty(compilation1, "I1");
            var test2P1 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I1.")).Single();
            var p1get = p1.GetMethod;

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.True(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(test2P1, test2.FindImplementationForInterfaceMember(p1));

            Assert.True(p1get.IsAbstract);
            Assert.False(p1get.IsVirtual);
            Assert.True(p1get.IsMetadataVirtual());
            Assert.False(p1get.IsSealed);
            Assert.False(p1get.IsStatic);
            Assert.True(p1get.IsExtern);
            Assert.False(p1get.IsAsync);
            Assert.False(p1get.IsOverride);
            Assert.Equal(Accessibility.Public, p1get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1get));
            Assert.Same(test2P1.GetMethod, test2.FindImplementationForInterfaceMember(p1get));

            var p2 = GetSingleProperty(compilation1, "I2");
            var test2P2 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I2.")).Single();
            var p2get = p2.GetMethod;

            Assert.False(p2.IsAbstract);
            Assert.True(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.True(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
            Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));
            Assert.Same(test2P2, test2.FindImplementationForInterfaceMember(p2));

            Assert.False(p2get.IsAbstract);
            Assert.True(p2get.IsVirtual);
            Assert.True(p2get.IsMetadataVirtual());
            Assert.False(p2get.IsSealed);
            Assert.False(p2get.IsStatic);
            Assert.True(p2get.IsExtern);
            Assert.False(p2get.IsAsync);
            Assert.False(p2get.IsOverride);
            Assert.Equal(Accessibility.Public, p2get.DeclaredAccessibility);
            Assert.Same(p2get, test1.FindImplementationForInterfaceMember(p2get));
            Assert.Same(test2P2.GetMethod, test2.FindImplementationForInterfaceMember(p2get));

            var p3 = GetSingleProperty(compilation1, "I3");
            var test2P3 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I3.")).Single();

            Assert.False(p3.IsAbstract);
            Assert.Equal(p3.IsIndexer, p3.IsVirtual);
            Assert.False(p3.IsSealed);
            Assert.Equal(!p3.IsIndexer, p3.IsStatic);
            Assert.True(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Same(p3.IsIndexer ? p3 : null, test1.FindImplementationForInterfaceMember(p3));
            Assert.Same(p3.IsIndexer ? test2P3 : null, test2.FindImplementationForInterfaceMember(p3));

            ValidateP3Accessor(p3.GetMethod, p3.IsIndexer ? test2P3.GetMethod : null);
            ValidateP3Accessor(p3.SetMethod, p3.IsIndexer ? test2P3.SetMethod : null);
            void ValidateP3Accessor(MethodSymbol accessor, MethodSymbol test2Implementation)
            {
                Assert.False(accessor.IsAbstract);
                Assert.Equal(p3.IsIndexer, accessor.IsVirtual);
                Assert.Equal(p3.IsIndexer, accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.Equal(!p3.IsIndexer, accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Same(p3.IsIndexer ? accessor : null, test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(test2Implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p4 = GetSingleProperty(compilation1, "I4");

            Assert.False(p4.IsAbstract);
            Assert.False(p4.IsVirtual);
            Assert.False(p4.IsSealed);
            Assert.False(p4.IsStatic);
            Assert.True(p4.IsExtern);
            Assert.False(p4.IsOverride);
            Assert.Equal(Accessibility.Private, p4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4));
            Assert.Null(test2.FindImplementationForInterfaceMember(p4));

            ValidateP4Accessor(p4.GetMethod);
            ValidateP4Accessor(p4.SetMethod);
            void ValidateP4Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }

            var p5 = GetSingleProperty(compilation1, "I5");
            var p5get = p5.GetMethod;

            Assert.False(p5.IsAbstract);
            Assert.False(p5.IsVirtual);
            Assert.False(p5.IsSealed);
            Assert.False(p5.IsStatic);
            Assert.True(p5.IsExtern);
            Assert.False(p5.IsOverride);
            Assert.Equal(Accessibility.Public, p5.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p5));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5));

            Assert.False(p5get.IsAbstract);
            Assert.False(p5get.IsVirtual);
            Assert.False(p5get.IsMetadataVirtual());
            Assert.False(p5get.IsSealed);
            Assert.False(p5get.IsStatic);
            Assert.True(p5get.IsExtern);
            Assert.False(p5get.IsAsync);
            Assert.False(p5get.IsOverride);
            Assert.Equal(Accessibility.Public, p5get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p5get));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5get));
        }

        [Fact]
        public void PropertyModifiers_18()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get => 0; set => throw null;} 
}
public interface I2
{
    abstract private int P2 => 0; 
}
public interface I3
{
    static extern int P3 {get; set;} 
}
public interface I4
{
    abstract static int P4 { get {throw null;} set {throw null;}}
}
public interface I5
{
    override sealed int P5 {get;} = 0;
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    int I1.P1 { get => 0; set => throw null;}
    int I2.P2 => 0;
    int I3.P3 { get => 0; set => throw null;}
    int I4.P4 { get => 0; set => throw null;}
    int I5.P5 => 0;
}
";
            ValidatePropertyModifiers_18(source1,
                // (8,32): error CS0500: 'I2.P2.get' cannot declare a body because it is marked abstract
                //     abstract private int P2 => 0; 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "0").WithArguments("I2.P2.get").WithLocation(8, 32),
                // (4,22): error CS0500: 'I1.P1.get' cannot declare a body because it is marked abstract
                //     abstract int P1 {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I1.P1.get").WithLocation(4, 22),
                // (4,32): error CS0500: 'I1.P1.set' cannot declare a body because it is marked abstract
                //     abstract int P1 {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I1.P1.set").WithLocation(4, 32),
                // (20,25): error CS0106: The modifier 'override' is not valid for this item
                //     override sealed int P5 {get;} = 0;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P5").WithArguments("override").WithLocation(20, 25),
                // (20,29): error CS0501: 'I5.P5.get' must declare a body because it is not marked abstract, extern, or partial
                //     override sealed int P5 {get;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I5.P5.get").WithLocation(20, 29),
                // (16,25): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     abstract static int P4 { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P4").WithArguments("abstract", "9.0", "11.0").WithLocation(16, 25),
                // (16,30): error CS0500: 'I4.P4.get' cannot declare a body because it is marked abstract
                //     abstract static int P4 { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I4.P4.get").WithLocation(16, 30),
                // (16,48): error CS0500: 'I4.P4.set' cannot declare a body because it is marked abstract
                //     abstract static int P4 { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I4.P4.set").WithLocation(16, 48),
                // (8,26): error CS0621: 'I2.P2': virtual or abstract members cannot be private
                //     abstract private int P2 => 0; 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "P2").WithArguments("I2.P2").WithLocation(8, 26),
                // (20,25): error CS8053: Instance properties in interfaces cannot have initializers.
                //     override sealed int P5 {get;} = 0;
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "P5").WithLocation(20, 25),
                // (12,32): warning CS0626: Method, operator, or accessor 'I3.P3.set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I3.P3.set").WithLocation(12, 32),
                // (12,27): warning CS0626: Method, operator, or accessor 'I3.P3.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int P3 {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I3.P3.get").WithLocation(12, 27),
                // (30,12): error CS0122: 'I2.P2' is inaccessible due to its protection level
                //     int I2.P2 => 0;
                Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I2.P2").WithLocation(30, 12),
                // (31,12): error CS0539: 'Test2.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I3.P3 { get => 0; set => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test2.P3").WithLocation(31, 12),
                // (32,12): error CS0539: 'Test2.P4' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I4.P4 { get => 0; set => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P4").WithArguments("Test2.P4").WithLocation(32, 12),
                // (33,12): error CS0539: 'Test2.P5' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I5.P5 => 0;
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P5").WithArguments("Test2.P5").WithLocation(33, 12),
                // (23,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(23, 15),
                // (23,19): error CS0535: 'Test1' does not implement interface member 'I2.P2'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I2.P2").WithLocation(23, 19),
                // (23,27): error CS0535: 'Test1' does not implement interface member 'I4.P4'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test1", "I4.P4").WithLocation(23, 27),
                // (27,27): error CS0535: 'Test2' does not implement interface member 'I4.P4'
                // class Test2 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test2", "I4.P4").WithLocation(27, 27)
                );
        }

        private void ValidatePropertyModifiers_18(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var p1 = GetSingleProperty(compilation1, "I1");
            var test2P1 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I1.")).Single();

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(test2P1, test2.FindImplementationForInterfaceMember(p1));

            ValidateP1Accessor(p1.GetMethod, test2P1.GetMethod);
            ValidateP1Accessor(p1.SetMethod, test2P1.SetMethod);
            void ValidateP1Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p2 = GetSingleProperty(compilation1, "I2");
            var test2P2 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I2.")).Single();
            var p2get = p2.GetMethod;

            Assert.True(p2.IsAbstract);
            Assert.False(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Private, p2.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p2));
            Assert.Same(test2P2, test2.FindImplementationForInterfaceMember(p2));

            Assert.True(p2get.IsAbstract);
            Assert.False(p2get.IsVirtual);
            Assert.True(p2get.IsMetadataVirtual());
            Assert.False(p2get.IsSealed);
            Assert.False(p2get.IsStatic);
            Assert.False(p2get.IsExtern);
            Assert.False(p2get.IsAsync);
            Assert.False(p2get.IsOverride);
            Assert.Equal(Accessibility.Private, p2get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p2get));
            Assert.Same(test2P2.GetMethod, test2.FindImplementationForInterfaceMember(p2get));

            var p3 = GetSingleProperty(compilation1, "I3");
            var test2P3 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I3.")).Single();

            Assert.False(p3.IsAbstract);
            Assert.Equal(p3.IsIndexer, p3.IsVirtual);
            Assert.False(p3.IsSealed);
            Assert.Equal(!p3.IsIndexer, p3.IsStatic);
            Assert.True(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Same(p3.IsIndexer ? p3 : null, test1.FindImplementationForInterfaceMember(p3));
            Assert.Same(p3.IsIndexer ? test2P3 : null, test2.FindImplementationForInterfaceMember(p3));

            ValidateP3Accessor(p3.GetMethod, p3.IsIndexer ? test2P3.GetMethod : null);
            ValidateP3Accessor(p3.SetMethod, p3.IsIndexer ? test2P3.SetMethod : null);
            void ValidateP3Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.False(accessor.IsAbstract);
                Assert.Equal(p3.IsIndexer, accessor.IsVirtual);
                Assert.Equal(p3.IsIndexer, accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.Equal(!p3.IsIndexer, accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Same(p3.IsIndexer ? accessor : null, test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p4 = GetSingleProperty(compilation1, "I4");
            var test2P4 = test2.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I4.")).Single();

            Assert.True(p4.IsAbstract);
            Assert.False(p4.IsVirtual);
            Assert.False(p4.IsSealed);
            Assert.Equal(!p4.IsIndexer, p4.IsStatic);
            Assert.False(p4.IsExtern);
            Assert.False(p4.IsOverride);
            Assert.Equal(Accessibility.Public, p4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4));
            Assert.Same(p4.IsIndexer ? test2P4 : null, test2.FindImplementationForInterfaceMember(p4));

            ValidateP4Accessor(p4.GetMethod, p4.IsIndexer ? test2P4.GetMethod : null);
            ValidateP4Accessor(p4.SetMethod, p4.IsIndexer ? test2P4.SetMethod : null);
            void ValidateP4Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.Equal(!p4.IsIndexer, accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p5 = GetSingleProperty(compilation1, "I5");
            var p5get = p5.GetMethod;

            Assert.False(p5.IsAbstract);
            Assert.False(p5.IsVirtual);
            Assert.False(p5.IsSealed);
            Assert.False(p5.IsStatic);
            Assert.False(p5.IsExtern);
            Assert.False(p5.IsOverride);
            Assert.Equal(Accessibility.Public, p5.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p5));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5));

            Assert.False(p5get.IsAbstract);
            Assert.False(p5get.IsVirtual);
            Assert.False(p5get.IsMetadataVirtual());
            Assert.False(p5get.IsSealed);
            Assert.False(p5get.IsStatic);
            Assert.False(p5get.IsExtern);
            Assert.False(p5get.IsAsync);
            Assert.False(p5get.IsOverride);
            Assert.Equal(Accessibility.Public, p5get.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p5get));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5get));
        }

        [Fact]
        public void PropertyModifiers_20()
        {
            var source1 =
@"
public interface I1
{
    internal int P1
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {P1 = P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidatePropertyModifiers_20(source1, source2, Accessibility.Internal);
        }

        private void ValidatePropertyModifiers_20(string source1, string source2, Accessibility accessibility)
        {
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var p1 = GetSingleProperty(i1);
                var p1get = p1.GetMethod;
                var p1set = p1.SetMethod;

                ValidateProperty(p1);
                ValidateMethod(p1get);
                ValidateMethod(p1set);
                Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
                Assert.Same(p1get, test1.FindImplementationForInterfaceMember(p1get));
                Assert.Same(p1set, test1.FindImplementationForInterfaceMember(p1set));
            }

            void ValidateProperty(PropertySymbol p1)
            {
                Assert.False(p1.IsAbstract);
                Assert.True(p1.IsVirtual);
                Assert.False(p1.IsSealed);
                Assert.False(p1.IsStatic);
                Assert.False(p1.IsExtern);
                Assert.False(p1.IsOverride);
                Assert.Equal(accessibility, p1.DeclaredAccessibility);
            }

            void ValidateMethod(MethodSymbol m1)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(accessibility, m1.DeclaredAccessibility);
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var p1 = GetSingleProperty(i1);
                var p1get = p1.GetMethod;
                var p1set = p1.SetMethod;

                ValidateProperty(p1);
                ValidateMethod(p1get);
                ValidateMethod(p1set);
            }

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation3,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation4,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation4.SourceModule);
        }

        [Fact]
        public void PropertyModifiers_21()
        {
            var source1 =
@"
public interface I1
{
    private static int P1 { get => throw null; set => throw null; }

    internal static int P2 { get => throw null; set => throw null; }

    public static int P3 { get => throw null; set => throw null; }

    static int P4 { get => throw null; set => throw null; }
}

class Test1
{
    static void Main()
    {
        int x;
        x = I1.P1;
        I1.P1 = x;
        x = I1.P2;
        I1.P2 = x;
        x = I1.P3;
        I1.P3 = x;
        x = I1.P4;
        I1.P4 = x;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (18,16): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         x = I1.P1;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(18, 16),
                // (19,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         I1.P1 = x;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(19, 12)
                );

            var source2 =
@"
class Test2
{
    static void Main()
    {
        int x;
        x = I1.P1;
        I1.P1 = x;
        x = I1.P2;
        I1.P2 = x;
        x = I1.P3;
        I1.P3 = x;
        x = I1.P4;
        I1.P4 = x;
    }
}
";
            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(
                // (7,16): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         x = I1.P1;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(7, 16),
                // (8,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         I1.P1 = x;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(8, 12),
                // (9,16): error CS0122: 'I1.P2' is inaccessible due to its protection level
                //         x = I1.P2;
                Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(9, 16),
                // (10,12): error CS0122: 'I1.P2' is inaccessible due to its protection level
                //         I1.P2 = x;
                Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(10, 12)
                );
        }

        [Fact]
        public void PropertyModifiers_22()
        {
            var source1 =
@"
public interface I1
{
    public int P1 
    {
        internal get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
public interface I2
{
    int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        internal set
        {
            System.Console.WriteLine(""set_P2"");
        }
    }
}
public interface I3
{
    int P3 
    {
        internal get => Test1.GetP3();
        set => System.Console.WriteLine(""set_P3"");
    }
}
public interface I4
{
    int P4
    {
        get => Test1.GetP4();
        internal set => System.Console.WriteLine(""set_P4"");
    }
}

class Test1 : I1, I2, I3, I4
{
    static void Main()
    {
        I1 i1 = new Test1();
        I2 i2 = new Test1();
        I3 i3 = new Test1();
        I4 i4 = new Test1();

        i1.P1 = i1.P1;
        i2.P2 = i2.P2;
        i3.P3 = i3.P3;
        i4.P4 = i4.P4;
    }

    public static int GetP3()
    {
        System.Console.WriteLine(""get_P3"");
        return 0;
    }

    public static int GetP4()
    {
        System.Console.WriteLine(""get_P4"");
        return 0;
    }
}
";

            ValidatePropertyModifiers_22(source1, Accessibility.Internal);
        }

        private void ValidatePropertyModifiers_22(string source1, Accessibility accessibility)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2
get_P3
set_P3
get_P4
set_P4
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");

                for (int i = 1; i <= 4; i++)
                {
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleProperty(i1);

                    Assert.False(p1.IsAbstract);
                    Assert.True(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                    Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));

                    switch (i)
                    {
                        case 1:
                        case 3:
                            ValidateAccessor(p1.GetMethod, accessibility);
                            ValidateAccessor(p1.SetMethod, Accessibility.Public);
                            break;
                        case 2:
                        case 4:
                            ValidateAccessor(p1.GetMethod, Accessibility.Public);
                            ValidateAccessor(p1.SetMethod, accessibility);
                            break;
                        default:
                            Assert.False(true);
                            break;
                    }

                    void ValidateAccessor(MethodSymbol accessor, Accessibility access)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.Equal(accessor.DeclaredAccessibility != Accessibility.Private, accessor.IsVirtual);
                        Assert.Equal(accessor.DeclaredAccessibility != Accessibility.Private, accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(access, accessor.DeclaredAccessibility);
                        Assert.Same(accessor.DeclaredAccessibility == Accessibility.Private ? null : accessor, test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        [Fact]
        public void PropertyModifiers_23_00()
        {
            var source1 =
@"
public interface I1
{}
public interface I3
{
    int P3
    {
        private get 
        {
            System.Console.WriteLine(""get_P3"");
            return 0;
        } 
        set {System.Console.WriteLine(""set_P3"");}
    }

    void M2()
    {
        P3 = P3;
    }
}
public interface I4
{
    int P4
    {
        get {System.Console.WriteLine(""get_P4""); return 0;} 
        private set {System.Console.WriteLine(""set_P4"");}
    }

    void M2()
    {
        P4 = P4;
    }
}
public interface I5
{
    int P5
    {
        private get => GetP5();
        set => System.Console.WriteLine(""set_P5"");
    }

    private int GetP5()
    {
        System.Console.WriteLine(""get_P5"");
        return 0;
    }

    void M2()
    {
        P5 = P5;
    }
}
public interface I6
{
    int P6
    {
        get => GetP6();
        private set => System.Console.WriteLine(""set_P6"");
    }

    private int GetP6()
    {
        System.Console.WriteLine(""get_P6"");
        return 0;
    }

    void M2()
    {
        P6 = P6;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public int P3 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public int P4 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
class Test5 : I5
{
    public int P5 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public int P6 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2);

            var source3 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public int P3 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public int P4 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test5 : I5
{
    public int P5 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public int P6 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source3);

            var source4 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public virtual int P3 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public virtual int P4 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
class Test5 : I5
{
    public virtual int P5 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public virtual int P6 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source4);

            var source5 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public virtual int P3 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public virtual int P4 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test5 : I5
{
    public virtual int P5 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public virtual int P6 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source5);

            var source6 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    int I3.P3 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    int I4.P4 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test5 : I5
{
    int I5.P5 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    int I6.P6 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source6);

            var source7 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test33();
        I4 i4 = new Test44();
        I5 i5 = new Test55();
        I6 i6 = new Test66();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
interface Test3 : I3
{
    int I3.P3 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test33 : Test3 {}
interface Test4 : I4
{
    int I4.P4 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test44 : Test4 {}
interface Test5 : I5
{
    int I5.P5 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test55 : Test5 {}
interface Test6 : I6
{
    int I6.P6 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
class Test66 : Test6 {}
";
            ValidatePropertyModifiers_23(source1, source7);
        }

        private void ValidatePropertyModifiers_23(string source1, string source2)
        {
            foreach (var metadataImportOptions in new[] { MetadataImportOptions.All, MetadataImportOptions.Public })
            {
                var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe.WithMetadataImportOptions(metadataImportOptions),
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics();

                Validate1(compilation1.SourceModule);

                void Validate1(ModuleSymbol m)
                {
                    var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                    var im = test1.InterfacesNoUseSiteDiagnostics().Single().ContainingModule;

                    ValidateProperty23(GetSingleProperty(im, "I3"), false, Accessibility.Private, Accessibility.Public, m.GlobalNamespace.GetTypeMember("Test3"));
                    ValidateProperty23(GetSingleProperty(im, "I4"), false, Accessibility.Public, Accessibility.Private, m.GlobalNamespace.GetTypeMember("Test4"));
                    ValidateProperty23(GetSingleProperty(im, "I5"), false, Accessibility.Private, Accessibility.Public, m.GlobalNamespace.GetTypeMember("Test5"));
                    ValidateProperty23(GetSingleProperty(im, "I6"), false, Accessibility.Public, Accessibility.Private, m.GlobalNamespace.GetTypeMember("Test6"));
                }

                var expectedOutput =
@"
get_P3
Test3.set_P3
Test4.get_P4
set_P4
get_P5
Test5.set_P5
Test6.get_P6
set_P6
";

                CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate1);

                var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(metadataImportOptions),
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation2.VerifyDiagnostics();

                ValidateProperty23(GetSingleProperty(compilation2, "I3"), false, Accessibility.Private, Accessibility.Public);
                ValidateProperty23(GetSingleProperty(compilation2, "I4"), false, Accessibility.Public, Accessibility.Private);
                ValidateProperty23(GetSingleProperty(compilation2, "I5"), false, Accessibility.Private, Accessibility.Public);
                ValidateProperty23(GetSingleProperty(compilation2, "I6"), false, Accessibility.Public, Accessibility.Private);

                foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
                {
                    var compilation3 = CreateCompilation(source2, new[] { reference },
                                                         options: TestOptions.DebugExe.WithMetadataImportOptions(metadataImportOptions),
                                                         parseOptions: TestOptions.Regular,
                                                         targetFramework: TargetFramework.NetCoreApp);
                    Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                    compilation3.VerifyDiagnostics();
                    Validate1(compilation3.SourceModule);

                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate1);
                }
            }
        }

        private static void ValidateProperty23(PropertySymbol p1, bool isAbstract, Accessibility getAccess, Accessibility setAccess, NamedTypeSymbol test1 = null)
        {
            Assert.Equal(isAbstract, p1.IsAbstract);
            Assert.NotEqual(isAbstract, p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);

            PropertySymbol implementingProperty = null;
            if ((object)test1 != null)
            {
                implementingProperty = GetSingleProperty(test1);
                Assert.Same(implementingProperty, test1.FindImplementationForInterfaceMember(p1));
                if (implementingProperty.GetMethod?.ExplicitInterfaceImplementations.Length > 0 ||
                    implementingProperty.SetMethod?.ExplicitInterfaceImplementations.Length > 0)
                {
                    Assert.Same(p1, implementingProperty.ExplicitInterfaceImplementations.Single());
                }
                else
                {
                    Assert.Empty(implementingProperty.ExplicitInterfaceImplementations);
                }
            }

            ValidateMethod23(p1, p1.GetMethod, isAbstract, getAccess, test1, implementingProperty?.GetMethod);
            ValidateMethod23(p1, p1.SetMethod, isAbstract, setAccess, test1, implementingProperty?.SetMethod);
        }

        private static void ValidateMethod23(PropertySymbol p1, MethodSymbol m1, bool isAbstract, Accessibility access, NamedTypeSymbol test1, MethodSymbol implementingMethod)
        {
            if (m1 is null)
            {
                Assert.Equal(Accessibility.Private, access);
                Assert.Equal(MetadataImportOptions.Public, ((PEModuleSymbol)p1.ContainingModule).ImportOptions);
                return;
            }

            Assert.Equal(isAbstract, m1.IsAbstract);
            Assert.NotEqual(isAbstract || access == Accessibility.Private, m1.IsVirtual);
            Assert.Equal(isAbstract || access != Accessibility.Private, m1.IsMetadataVirtual());
            Assert.False(m1.IsSealed);
            Assert.False(m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(access, m1.DeclaredAccessibility);

            if ((object)test1 != null)
            {
                Assert.Same(access != Accessibility.Private ? implementingMethod : null, test1.FindImplementationForInterfaceMember(m1));
            }
        }

        [Fact]
        public void PropertyModifiers_23_01()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 

    void M2()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.Internal, Accessibility.Public,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(12, 9)
                    )
                );
        }

        private void ValidatePropertyModifiers_23(string source1, string source2, Accessibility getAccess, Accessibility setAccess, params DiagnosticDescription[] expectedIn9)
        {
            ValidatePropertyModifiers_23(source1, source2, getAccess, setAccess, expectedIn9, expectedAcrossAssemblyBoundaries: Array.Empty<DiagnosticDescription>());
        }

        private void ValidatePropertyModifiers_23(string source1, string source2, Accessibility getAccess, Accessibility setAccess, DiagnosticDescription[] expectedIn9, params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expectedIn9);

            Validate1(compilation1.SourceModule);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => Validate1(m)).VerifyDiagnostics();

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var im = test1.InterfacesNoUseSiteDiagnostics().Single().ContainingModule;

                ValidateProperty23(GetSingleProperty(im, "I1"), true, getAccess, setAccess, test1);
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            ValidateProperty23(GetSingleProperty(compilation2, "I1"), true, getAccess, setAccess);

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference },
                                                     options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                Validate1(compilation3.SourceModule);

                compilation3 = CreateCompilation(source2, new[] { reference },
                                                 options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => Validate1(m)).VerifyDiagnostics();
                }

                Validate1(compilation3.SourceModule);
            }
        }

        [Fact]
        public void PropertyModifiers_23_02()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_03()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,12): error CS0122: 'I1.P1.get' is inaccessible due to its protection level
                //     int I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1.get").WithLocation(9, 12)
                );
        }

        [Fact]
        public void PropertyModifiers_23_04()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_05()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_06()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract int P1 {get; set;} 
}

class Test3 : Test1
{
    public override int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,29): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(9, 29)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,29): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(9, 29)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_07()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    int P1 {get; set;} 
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_08()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long P1 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_08(source1, source2);
        }

        [Fact]
        public void PropertyModifiers_23_09()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long P1 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.P1'. 'Test1.P1' cannot implement 'I1.P1' because it does not have the matching return type of 'int'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.P1", "Test1.P1", "int").WithLocation(2, 15)
                );
        }

        [Fact]
        public void PropertyModifiers_23_10()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (6,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.get'. 'Test2.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.P1.get", "Test2.P1.get", "9.0", "10.0").WithLocation(6, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (6,9): error CS9044: 'Test2' does not implement interface member 'I1.P1.get'. 'Test2.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test2", "I1.P1.get", "Test2.P1.get").WithLocation(6, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_11()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {internal get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
public class Test2 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_11(source1, source2,
                // (15,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.get'. 'Test2.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.P1.get", "Test2.P1.get", "9.0", "10.0").WithLocation(15, 9)
                );
        }

        [Fact]
        public void PropertyModifiers_23_51()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 

    void M2()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.Public, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(17, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (17,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(17, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_52()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_53()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,12): error CS0122: 'I1.P1.set' is inaccessible due to its protection level
                //     int I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1.set").WithLocation(9, 12)
                );
        }

        [Fact]
        public void PropertyModifiers_23_54()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_55()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_56()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract int P1 {get; set;} 
}

class Test3 : Test1
{
    public override int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,34): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(9, 34)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,34): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //     public abstract int P1 {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(9, 34)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_57()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    int P1 {get; set;} 
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_58()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}

public class Test2 : I1
{
    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long P1 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_08(source1, source2);
        }

        [Fact]
        public void PropertyModifiers_23_59()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long P1 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.P1'. 'Test1.P1' cannot implement 'I1.P1' because it does not have the matching return type of 'int'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.P1", "Test1.P1", "int").WithLocation(2, 15)
                );
        }

        [Fact]
        public void PropertyModifiers_23_60()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.set'. 'Test2.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.P1.set", "Test2.P1.set", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test2' does not implement interface member 'I1.P1.set'. 'Test2.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test2", "I1.P1.set", "Test2.P1.set").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_23_61()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; internal set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
public class Test2 : I1
{
    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_11(source1, source2,
                // (20,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.set'. 'Test2.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         set
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.P1.set", "Test2.P1.set", "9.0", "10.0").WithLocation(20, 9)
                );
        }

        [Fact]
        public void PropertyModifiers_24()
        {
            var source1 =
@"
public interface I1
{
    int P1
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        internal set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {P1 = P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            ValidatePropertyModifiers_24(source1, source2);
        }

        private void ValidatePropertyModifiers_24(string source1, string source2)
        {
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var p1 = GetSingleProperty(i1);
                var p1get = p1.GetMethod;
                var p1set = p1.SetMethod;

                ValidateProperty(p1);
                ValidateMethod(p1get, Accessibility.Public);
                ValidateMethod(p1set, Accessibility.Internal);
                Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
                Assert.Same(p1get, test1.FindImplementationForInterfaceMember(p1get));
                Assert.Same(p1set, test1.FindImplementationForInterfaceMember(p1set));
            }

            void ValidateProperty(PropertySymbol p1)
            {
                Assert.False(p1.IsAbstract);
                Assert.True(p1.IsVirtual);
                Assert.False(p1.IsSealed);
                Assert.False(p1.IsStatic);
                Assert.False(p1.IsExtern);
                Assert.False(p1.IsOverride);
                Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            }

            void ValidateMethod(MethodSymbol m1, Accessibility access)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(access, m1.DeclaredAccessibility);
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var p1 = GetSingleProperty(i1);
                var p1get = p1.GetMethod;
                var p1set = p1.SetMethod;

                ValidateProperty(p1);
                ValidateMethod(p1get, Accessibility.Public);
                ValidateMethod(p1set, Accessibility.Internal);
            }

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation3,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation4,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation4.SourceModule);
        }

        [Fact]
        public void PropertyModifiers_25()
        {
            var source1 =
@"
public interface I1
{
    static int P1 { private get => throw null; set => throw null; }

    static int P2 { internal get => throw null; set => throw null; }

    public static int P3 { get => throw null; private set => throw null; }

    static int P4 { get => throw null; internal set => throw null; }
}

class Test1
{
    static void Main()
    {
        int x;
        x = I1.P1;
        I1.P1 = x;
        x = I1.P2;
        I1.P2 = x;
        x = I1.P3;
        I1.P3 = x;
        x = I1.P4;
        I1.P4 = x;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (18,13): error CS0271: The property or indexer 'I1.P1' cannot be used in this context because the get accessor is inaccessible
                //         x = I1.P1;
                Diagnostic(ErrorCode.ERR_InaccessibleGetter, "I1.P1").WithArguments("I1.P1").WithLocation(18, 13),
                // (23,9): error CS0272: The property or indexer 'I1.P3' cannot be used in this context because the set accessor is inaccessible
                //         I1.P3 = x;
                Diagnostic(ErrorCode.ERR_InaccessibleSetter, "I1.P3").WithArguments("I1.P3").WithLocation(23, 9)
                );

            var source2 =
@"
class Test2
{
    static void Main()
    {
        int x;
        x = I1.P1;
        I1.P1 = x;
        x = I1.P2;
        I1.P2 = x;
        x = I1.P3;
        I1.P3 = x;
        x = I1.P4;
        I1.P4 = x;
    }
}
";
            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(
                // (7,13): error CS0271: The property or indexer 'I1.P1' cannot be used in this context because the get accessor is inaccessible
                //         x = I1.P1;
                Diagnostic(ErrorCode.ERR_InaccessibleGetter, "I1.P1").WithArguments("I1.P1").WithLocation(7, 13),
                // (9,13): error CS0271: The property or indexer 'I1.P2' cannot be used in this context because the get accessor is inaccessible
                //         x = I1.P2;
                Diagnostic(ErrorCode.ERR_InaccessibleGetter, "I1.P2").WithArguments("I1.P2").WithLocation(9, 13),
                // (12,9): error CS0272: The property or indexer 'I1.P3' cannot be used in this context because the set accessor is inaccessible
                //         I1.P3 = x;
                Diagnostic(ErrorCode.ERR_InaccessibleSetter, "I1.P3").WithArguments("I1.P3").WithLocation(12, 9),
                // (14,9): error CS0272: The property or indexer 'I1.P4' cannot be used in this context because the set accessor is inaccessible
                //         I1.P4 = x;
                Diagnostic(ErrorCode.ERR_InaccessibleSetter, "I1.P4").WithArguments("I1.P4").WithLocation(14, 9)
                );
        }

        [Fact]
        public void PropertyModifiers_26()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 { private get; set; }
    abstract int P2 { get; private set; }
    abstract int P3 { internal get; }
    static int P4 {internal get;} = 0;
    static int P5 { internal get {throw null;} }
    static int P6 { internal set {throw null;} }
    static int P7 { internal get => throw null; }
    static int P8 { internal set => throw null; }
    static int P9 { internal get {throw null;} private set {throw null;}}
    static int P10 { internal get => throw null; private set => throw null;}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,31): error CS0442: 'I1.P1.get': abstract properties cannot have private accessors
                //     abstract int P1 { private get; set; }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I1.P1.get").WithLocation(4, 31),
                // (5,36): error CS0442: 'I1.P2.set': abstract properties cannot have private accessors
                //     abstract int P2 { get; private set; }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "set").WithArguments("I1.P2.set").WithLocation(5, 36),
                // (6,18): error CS0276: 'I1.P3': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     abstract int P3 { internal get; }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P3").WithArguments("I1.P3").WithLocation(6, 18),
                // (7,16): error CS0276: 'I1.P4': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     static int P4 {internal get;} = 0;
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P4").WithArguments("I1.P4").WithLocation(7, 16),
                // (8,16): error CS0276: 'I1.P5': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     static int P5 { internal get {throw null;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P5").WithArguments("I1.P5").WithLocation(8, 16),
                // (9,16): error CS0276: 'I1.P6': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     static int P6 { internal set {throw null;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P6").WithArguments("I1.P6").WithLocation(9, 16),
                // (10,16): error CS0276: 'I1.P7': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     static int P7 { internal get => throw null; }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P7").WithArguments("I1.P7").WithLocation(10, 16),
                // (11,16): error CS0276: 'I1.P8': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     static int P8 { internal set => throw null; }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "P8").WithArguments("I1.P8").WithLocation(11, 16),
                // (12,16): error CS0274: Cannot specify accessibility modifiers for both accessors of the property or indexer 'I1.P9'
                //     static int P9 { internal get {throw null;} private set {throw null;}}
                Diagnostic(ErrorCode.ERR_DuplicatePropertyAccessMods, "P9").WithArguments("I1.P9").WithLocation(12, 16),
                // (13,16): error CS0274: Cannot specify accessibility modifiers for both accessors of the property or indexer 'I1.P10'
                //     static int P10 { internal get => throw null; private set => throw null;}
                Diagnostic(ErrorCode.ERR_DuplicatePropertyAccessMods, "P10").WithArguments("I1.P10").WithLocation(13, 16)
                );
        }

        [Fact]
        public void PropertyModifiers_27()
        {
            var source1 =
@"
public interface I1
{
    int P3
    {
        private get {throw null;} 
        set {}
    }

    int P4
    {
        get {throw null;} 
        private set {}
    }
}

class Test1 : I1
{
    int I1.P3
    {
        get {throw null;} 
        set {}
    }

    int I1.P4
    {
        get {throw null;} 
        set {}
    }
}

interface ITest1 : I1
{
    int I1.P3
    {
        get {throw null;} 
        set {}
    }

    int I1.P4
    {
        get {throw null;} 
        set {}
    }
}

public interface I2
{
    int P5
    {
        private get {throw null;} 
        set {}
    }

    int P6
    {
        get {throw null;} 
        private set {}
    }

    class Test3 : I2
    {
        int I2.P5
        {
            get {throw null;} 
            set {}
        }

        int I2.P6
        {
            get {throw null;} 
            set {}
        }
    }

    interface ITest3 : I2
    {
        int I2.P5
        {
            get {throw null;} 
            set {}
        }

        int I2.P6
        {
            get {throw null;} 
            set {}
        }
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            // https://github.com/dotnet/roslyn/issues/34455: The wording "accessor not found in interface member" is somewhat misleading
            //                                                in this scenario. The accessor is there, but cannot be implemented. Perhaps
            //                                                the message should be adjusted. Should also check diagnostics for an attempt
            //                                                to implement other sealed members.
            compilation1.VerifyDiagnostics(
                // (21,9): error CS0550: 'Test1.I1.P3.get' adds an accessor not found in interface member 'I1.P3'
                //         get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("Test1.I1.P3.get", "I1.P3").WithLocation(21, 9),
                // (28,9): error CS0550: 'Test1.I1.P4.set' adds an accessor not found in interface member 'I1.P4'
                //         set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("Test1.I1.P4.set", "I1.P4").WithLocation(28, 9),
                // (36,9): error CS0550: 'ITest1.I1.P3.get' adds an accessor not found in interface member 'I1.P3'
                //         get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("ITest1.I1.P3.get", "I1.P3").WithLocation(36, 9),
                // (43,9): error CS0550: 'ITest1.I1.P4.set' adds an accessor not found in interface member 'I1.P4'
                //         set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("ITest1.I1.P4.set", "I1.P4").WithLocation(43, 9),
                // (65,13): error CS0550: 'I2.Test3.I2.P5.get' adds an accessor not found in interface member 'I2.P5'
                //             get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I2.Test3.I2.P5.get", "I2.P5").WithLocation(65, 13),
                // (72,13): error CS0550: 'I2.Test3.I2.P6.set' adds an accessor not found in interface member 'I2.P6'
                //             set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I2.Test3.I2.P6.set", "I2.P6").WithLocation(72, 13),
                // (80,13): error CS0550: 'I2.ITest3.I2.P5.get' adds an accessor not found in interface member 'I2.P5'
                //             get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I2.ITest3.I2.P5.get", "I2.P5").WithLocation(80, 13),
                // (87,13): error CS0550: 'I2.ITest3.I2.P6.set' adds an accessor not found in interface member 'I2.P6'
                //             set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I2.ITest3.I2.P6.set", "I2.P6").WithLocation(87, 13)
                );
        }

        [Fact]
        public void PropertyModifiers_28()
        {
            var source0 =
@"
public interface I1
{
    protected static int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 1;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    protected internal static int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 2;
        }
        set
        {
            System.Console.WriteLine(""set_P2"");
        }
    }

    private protected static int P3 
    {
        get
        {
            System.Console.WriteLine(""get_P3"");
            return 3;
        }
        set
        {
            System.Console.WriteLine(""set_P3"");
        }
    }

    static int P4 
    {
        protected get
        {
            System.Console.WriteLine(""get_P4"");
            return 4;
        }
        set 
        {
            System.Console.WriteLine(""set_P4"");
        }
    }

    static int P5 
    {
        get
        {
            System.Console.WriteLine(""get_P5"");
            return 5;
        }
        protected internal set 
        {
            System.Console.WriteLine(""set_P5"");
        }
    }

    static int P6 
    {
        private protected get
        {
            System.Console.WriteLine(""get_P6"");
            return 6;
        }
        set 
        {
            System.Console.WriteLine(""set_P6"");
        }
    }
}
";
            var source1 =
@"
class Test1 : I1
{
    static void Main()
    {
        _ = I1.P1;
        I1.P1 = 11;
        _ = I1.P2;
        I1.P2 = 11;
        _ = I1.P3;
        I1.P3 = 11;
        _ = I1.P4;
        I1.P4 = 11;
        _ = I1.P5;
        I1.P5 = 11;
        _ = I1.P6;
        I1.P6 = 11;
    }
}
";
            var compilation1 = CreateCompilation(source0 + source1,
                                                 options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2
get_P3
set_P3
get_P4
set_P4
get_P5
set_P5
get_P6
set_P6",
                symbolValidator: validate,
                verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

            validate(compilation1.SourceModule);

            var source2 =
@"
class Test1
{
    static void Main()
    {
        _ = I1.P2;
        I1.P2 = 11;
        I1.P4 = 11;
        _ = I1.P5;
        I1.P5 = 11;
        I1.P6 = 11;
    }
}
";

            var compilation2 = CreateCompilation(source0 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P2
set_P2
set_P4
get_P5
set_P5
set_P6
",
                verify: VerifyOnMonoOrCoreClr);

            var compilation3 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            var source3 =
@"
class Test1 : I1
{
    static void Main()
    {
        _ = I1.P1;
        I1.P1 = 11;
        _ = I1.P2;
        I1.P2 = 11;
        _ = I1.P4;
        I1.P4 = 11;
        _ = I1.P5;
        I1.P5 = 11;
        I1.P6 = 11;
    }
}
";

            var source4 =
@"
class Test1
{
    static void Main()
    {
        I1.P4 = 11;
        _ = I1.P5;
        I1.P6 = 11;
    }
}
";

            var source5 =
@"
class Test1
{
    static void Main()
    {
        _ = I1.P1;
        I1.P1 = 11;
        _ = I1.P2;
        I1.P2 = 11;
        _ = I1.P3;
        I1.P3 = 11;
        _ = I1.P4;
        I1.P5 = 11;
        _ = I1.P6;
    }
}
";

            foreach (var reference in new[] { compilation3.ToMetadataReference(), compilation3.EmitToImageReference() })
            {
                var compilation4 = CreateCompilation(source3, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                CompileAndVerify(compilation4, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2
get_P4
set_P4
get_P5
set_P5
set_P6
",
                    verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

                var compilation5 = CreateCompilation(source4, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                CompileAndVerify(compilation5, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"set_P4
get_P5
set_P6
",
                    verify: VerifyOnMonoOrCoreClr);

                var compilation6 = CreateCompilation(source5, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation6.VerifyDiagnostics(
                    // (6,16): error CS0122: 'I1.P1' is inaccessible due to its protection level
                    //         _ = I1.P1;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(6, 16),
                    // (7,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                    //         I1.P1 = 11;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(7, 12),
                    // (8,16): error CS0122: 'I1.P2' is inaccessible due to its protection level
                    //         _ = I1.P2;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(8, 16),
                    // (9,12): error CS0122: 'I1.P2' is inaccessible due to its protection level
                    //         I1.P2 = 11;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(9, 12),
                    // (10,16): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         _ = I1.P3;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(10, 16),
                    // (11,12): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         I1.P3 = 11;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(11, 12),
                    // (12,13): error CS0271: The property or indexer 'I1.P4' cannot be used in this context because the get accessor is inaccessible
                    //         _ = I1.P4;
                    Diagnostic(ErrorCode.ERR_InaccessibleGetter, "I1.P4").WithArguments("I1.P4").WithLocation(12, 13),
                    // (13,9): error CS0272: The property or indexer 'I1.P5' cannot be used in this context because the set accessor is inaccessible
                    //         I1.P5 = 11;
                    Diagnostic(ErrorCode.ERR_InaccessibleSetter, "I1.P5").WithArguments("I1.P5").WithLocation(13, 9),
                    // (14,13): error CS0271: The property or indexer 'I1.P6' cannot be used in this context because the get accessor is inaccessible
                    //         _ = I1.P6;
                    Diagnostic(ErrorCode.ERR_InaccessibleGetter, "I1.P6").WithArguments("I1.P6").WithLocation(14, 13)
                    );

                var compilation7 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation7.VerifyDiagnostics(
                    // (10,16): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         _ = I1.P3;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(10, 16),
                    // (11,12): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         I1.P3 = 11;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(11, 12),
                    // (16,13): error CS0271: The property or indexer 'I1.P6' cannot be used in this context because the get accessor is inaccessible
                    //         _ = I1.P6;
                    Diagnostic(ErrorCode.ERR_InaccessibleGetter, "I1.P6").WithArguments("I1.P6").WithLocation(16, 13)
                    );
            }

            static void validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var tuple in new[] { (name: "P1", access: Accessibility.Protected, getAccess: Accessibility.Protected, setAccess: Accessibility.Protected),
                                              (name: "P2", access: Accessibility.ProtectedOrInternal, getAccess: Accessibility.ProtectedOrInternal, setAccess: Accessibility.ProtectedOrInternal),
                                              (name: "P3", access: Accessibility.ProtectedAndInternal, getAccess: Accessibility.ProtectedAndInternal, setAccess: Accessibility.ProtectedAndInternal),
                                              (name: "P4", access: Accessibility.Public, getAccess: Accessibility.Protected, setAccess: Accessibility.Public),
                                              (name: "P5", access: Accessibility.Public, getAccess: Accessibility.Public, setAccess: Accessibility.ProtectedOrInternal),
                                              (name: "P6", access: Accessibility.Public, getAccess: Accessibility.ProtectedAndInternal, setAccess: Accessibility.Public)})
                {
                    var p1 = i1.GetMember<PropertySymbol>(tuple.name);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.True(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(tuple.access, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    validateAccessor(p1.GetMethod, tuple.getAccess);
                    validateAccessor(p1.SetMethod, tuple.setAccess);
                }

                void validateAccessor(MethodSymbol accessor, Accessibility accessibility)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.False(accessor.IsVirtual);
                    Assert.False(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.True(accessor.IsStatic);
                    Assert.False(accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(accessibility, accessor.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                }
            }
        }

        [Fact]
        public void PropertyModifiers_29()
        {
            var source1 =
@"
public interface I1
{
    protected abstract int P1 {get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_01(source1, source2, Accessibility.Protected,
                new DiagnosticDescription[] {
                // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9),
                // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         set
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(17, 9)
                },
                // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                );
        }

        [Fact]
        public void PropertyModifiers_30()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract int P1 {get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_01(source1, source2, Accessibility.ProtectedOrInternal,
                new DiagnosticDescription[] {
                // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9),
                // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         set
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(17, 9)
                },
                // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                );
        }

        [Fact]
        public void PropertyModifiers_31()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract int P1 {get; set;} 

    sealed void Test()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_01(source1, source2, Accessibility.ProtectedAndInternal,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9),
                    // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(17, 9)
                    ),
                expectedNoImplementation: ExpectedDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get").WithLocation(12, 9),
                    // (17,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(17, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_32()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {protected get; set;} 

    void M2()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.Protected, Accessibility.Public,
                // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9)
                );
        }

        [Fact]
        public void PropertyModifiers_33()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {protected internal get; set;} 

    void M2()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.ProtectedOrInternal, Accessibility.Public,
                // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.get'. 'Test1.P1.get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.P1.get", "Test1.P1.get", "9.0", "10.0").WithLocation(12, 9)
                );
        }

        [Fact]
        public void PropertyModifiers_34()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; private protected set;} 

    void M2()
    {
        P1 = P1;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.Public, Accessibility.ProtectedAndInternal,
                expectedIn9: ExpectedDiagnostics(
                    // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set", "9.0", "10.0").WithLocation(17, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (17,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.set'. 'Test1.P1.set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.P1.set", "Test1.P1.set").WithLocation(17, 9)
                    )
                );
        }

        [Fact]
        public void PropertyModifiers_35()
        {
            var source1 =
@"
public interface I1
{
    protected abstract int P1 {get; set;} 

    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.NetCoreApp);
        }

        [Fact]
        public void PropertyModifiers_36()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract int P1 {get; set;} 

    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.NetCoreApp);
        }

        [Fact]
        public void PropertyModifiers_37()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract int P1 {get; set;} 

    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.NetCoreApp,
                // (9,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //     int I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(9, 12)
                );
        }

        [Fact]
        public void PropertyModifiers_38()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {protected get; set;} 

    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.NetCoreApp);
        }

        [Fact]
        public void PropertyModifiers_39()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 {get; protected internal set;} 

    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.NetCoreApp);
        }

        [Fact]
        public void PropertyModifiers_40()
        {
            var source1 =
@"
public interface I1
{
    abstract int P1 { private protected get; set;} 

    public static void CallP1(I1 x) {x.P1 = x.P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    int I1.P1 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.NetCoreApp,
                // (9,12): error CS0122: 'I1.P1.get' is inaccessible due to its protection level
                //     int I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1.get").WithLocation(9, 12)
                );
        }

        [Fact]
        public void PropertyModifiers_41()
        {
            var source1 =
@"
public interface I1
{
    protected int P1
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {P1 = P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidatePropertyModifiers_20(source1, source2, Accessibility.Protected);
        }

        [Fact]
        public void PropertyModifiers_42()
        {
            var source1 =
@"
public interface I1
{
    protected internal int P1
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {P1 = P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidatePropertyModifiers_20(source1, source2, Accessibility.ProtectedOrInternal);
        }

        [Fact]
        public void PropertyModifiers_43()
        {
            var source1 =
@"
public interface I1
{
    private protected int P1
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {P1 = P1;}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidatePropertyModifiers_20(source1, source2, Accessibility.ProtectedAndInternal);
        }

        [Fact]
        public void PropertyModifiers_44()
        {
            var source1 =
@"
public interface I1
{
    public int P1 
    {
        protected get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    static void Test(I1 i1)
    {
        i1.P1 = i1.P1;
    }
}
public interface I2
{
    int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        protected set
        {
            System.Console.WriteLine(""set_P2"");
        }
    }

    static void Test(I2 i2)
    {
        i2.P2 = i2.P2;
    }
}
public interface I3
{
    int P3 
    {
        protected get => Test1.GetP3();
        set => System.Console.WriteLine(""set_P3"");
    }

    static void Test(I3 i3)
    {
        i3.P3 = i3.P3;
    }
}
public interface I4
{
    int P4
    {
        get => Test1.GetP4();
        protected set => System.Console.WriteLine(""set_P4"");
    }

    static void Test(I4 i4)
    {
        i4.P4 = i4.P4;
    }
}

class Test1 : I1, I2, I3, I4
{
    static void Main()
    {
        I1.Test(new Test1());
        I2.Test(new Test1());
        I3.Test(new Test1());
        I4.Test(new Test1());
    }

    public static int GetP3()
    {
        System.Console.WriteLine(""get_P3"");
        return 0;
    }

    public static int GetP4()
    {
        System.Console.WriteLine(""get_P4"");
        return 0;
    }
}
";

            ValidatePropertyModifiers_22(source1, Accessibility.Protected);
        }

        [Fact]
        public void PropertyModifiers_45()
        {
            var source1 =
@"
public interface I1
{
    public int P1 
    {
        protected internal get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
public interface I2
{
    int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        protected internal set
        {
            System.Console.WriteLine(""set_P2"");
        }
    }
}
public interface I3
{
    int P3 
    {
        protected internal get => Test1.GetP3();
        set => System.Console.WriteLine(""set_P3"");
    }
}
public interface I4
{
    int P4
    {
        get => Test1.GetP4();
        protected internal set => System.Console.WriteLine(""set_P4"");
    }
}

class Test1 : I1, I2, I3, I4
{
    static void Main()
    {
        I1 i1 = new Test1();
        I2 i2 = new Test1();
        I3 i3 = new Test1();
        I4 i4 = new Test1();

        i1.P1 = i1.P1;
        i2.P2 = i2.P2;
        i3.P3 = i3.P3;
        i4.P4 = i4.P4;
    }

    public static int GetP3()
    {
        System.Console.WriteLine(""get_P3"");
        return 0;
    }

    public static int GetP4()
    {
        System.Console.WriteLine(""get_P4"");
        return 0;
    }
}
";

            ValidatePropertyModifiers_22(source1, Accessibility.ProtectedOrInternal);
        }

        [Fact]
        public void PropertyModifiers_46()
        {
            var source1 =
@"
public interface I1
{
    public int P1 
    {
        private protected get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    static void Test(I1 i1)
    {
        i1.P1 = i1.P1;
    }
}
public interface I2
{
    int P2 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        private protected set
        {
            System.Console.WriteLine(""set_P2"");
        }
    }

    static void Test(I2 i2)
    {
        i2.P2 = i2.P2;
    }
}
public interface I3
{
    int P3 
    {
        private protected get => Test1.GetP3();
        set => System.Console.WriteLine(""set_P3"");
    }

    static void Test(I3 i3)
    {
        i3.P3 = i3.P3;
    }
}
public interface I4
{
    int P4
    {
        get => Test1.GetP4();
        private protected set => System.Console.WriteLine(""set_P4"");
    }

    static void Test(I4 i4)
    {
        i4.P4 = i4.P4;
    }
}

class Test1 : I1, I2, I3, I4
{
    static void Main()
    {
        I1.Test(new Test1());
        I2.Test(new Test1());
        I3.Test(new Test1());
        I4.Test(new Test1());
    }

    public static int GetP3()
    {
        System.Console.WriteLine(""get_P3"");
        return 0;
    }

    public static int GetP4()
    {
        System.Console.WriteLine(""get_P4"");
        return 0;
    }
}
";

            ValidatePropertyModifiers_22(source1, Accessibility.ProtectedAndInternal);
        }

        [Fact]
        public void IndexerModifiers_01()
        {
            var source1 =
@"
public interface I01{ public int this[int x] {get; set;} }
public interface I02{ protected int this[int x] {get;} }
public interface I03{ protected internal int this[int x] {set;} }
public interface I04{ internal int this[int x] {get;} }
public interface I05{ private int this[int x] {set;} }
public interface I06{ static int this[int x] {get;} }
public interface I07{ virtual int this[int x] {set;} }
public interface I08{ sealed int this[int x] {get;} }
public interface I09{ override int this[int x] {set;} }
public interface I10{ abstract int this[int x] {get;} }
public interface I11{ extern int this[int x] {get; set;} }

public interface I12{ int this[int x] { public get; set;} }
public interface I13{ int this[int x] { get; protected set;} }
public interface I14{ int this[int x] { protected internal get; set;} }
public interface I15{ int this[int x] { get; internal set;} }
public interface I16{ int this[int x] { private get; set;} }
public interface I17{ int this[int x] { private get;} }

public interface I18{ private protected int this[int x] { get; } }
public interface I19{ int this[int x] { get; private protected set;} }
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (6,48): error CS0501: 'I05.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                // public interface I05{ private int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I05.this[int].set").WithLocation(6, 48),
                // (7,34): error CS0106: The modifier 'static' is not valid for this item
                // public interface I06{ static int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(7, 34),
                // (8,48): error CS0501: 'I07.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                // public interface I07{ virtual int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I07.this[int].set").WithLocation(8, 48),
                // (9,47): error CS0501: 'I08.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                // public interface I08{ sealed int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I08.this[int].get").WithLocation(9, 47),
                // (10,36): error CS0106: The modifier 'override' is not valid for this item
                // public interface I09{ override int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("override").WithLocation(10, 36),
                // (14,48): error CS0273: The accessibility modifier of the 'I12.this[int].get' accessor must be more restrictive than the property or indexer 'I12.this[int]'
                // public interface I12{ int this[int x] { public get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I12.this[int].get", "I12.this[int]").WithLocation(14, 48),
                // (18,49): error CS0442: 'I16.this[int].get': abstract properties cannot have private accessors
                // public interface I16{ int this[int x] { private get; set;} }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I16.this[int].get").WithLocation(18, 49),
                // (19,27): error CS0276: 'I17.this[int]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                // public interface I17{ int this[int x] { private get;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I17.this[int]").WithLocation(19, 27),
                // (12,47): warning CS0626: Method, operator, or accessor 'I11.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I11.this[int].get").WithLocation(12, 47),
                // (12,52): warning CS0626: Method, operator, or accessor 'I11.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I11.this[int].set").WithLocation(12, 52)
                );

            ValidateSymbolsIndexerModifiers_01(compilation1);
        }

        private static void ValidateSymbolsIndexerModifiers_01(CSharpCompilation compilation1)
        {
            var p01 = compilation1.GetMember<PropertySymbol>("I01.this[]");

            Assert.True(p01.IsAbstract);
            Assert.False(p01.IsVirtual);
            Assert.False(p01.IsSealed);
            Assert.False(p01.IsStatic);
            Assert.False(p01.IsExtern);
            Assert.False(p01.IsOverride);
            Assert.Equal(Accessibility.Public, p01.DeclaredAccessibility);

            ValidateP01Accessor(p01.GetMethod);
            ValidateP01Accessor(p01.SetMethod);
            void ValidateP01Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p02 = compilation1.GetMember<PropertySymbol>("I02.this[]");
            var p02get = p02.GetMethod;

            Assert.True(p02.IsAbstract);
            Assert.False(p02.IsVirtual);
            Assert.False(p02.IsSealed);
            Assert.False(p02.IsStatic);
            Assert.False(p02.IsExtern);
            Assert.False(p02.IsOverride);
            Assert.Equal(Accessibility.Protected, p02.DeclaredAccessibility);

            Assert.True(p02get.IsAbstract);
            Assert.False(p02get.IsVirtual);
            Assert.True(p02get.IsMetadataVirtual());
            Assert.False(p02get.IsSealed);
            Assert.False(p02get.IsStatic);
            Assert.False(p02get.IsExtern);
            Assert.False(p02get.IsAsync);
            Assert.False(p02get.IsOverride);
            Assert.Equal(Accessibility.Protected, p02get.DeclaredAccessibility);

            var p03 = compilation1.GetMember<PropertySymbol>("I03.this[]");
            var p03set = p03.SetMethod;

            Assert.True(p03.IsAbstract);
            Assert.False(p03.IsVirtual);
            Assert.False(p03.IsSealed);
            Assert.False(p03.IsStatic);
            Assert.False(p03.IsExtern);
            Assert.False(p03.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, p03.DeclaredAccessibility);

            Assert.True(p03set.IsAbstract);
            Assert.False(p03set.IsVirtual);
            Assert.True(p03set.IsMetadataVirtual());
            Assert.False(p03set.IsSealed);
            Assert.False(p03set.IsStatic);
            Assert.False(p03set.IsExtern);
            Assert.False(p03set.IsAsync);
            Assert.False(p03set.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, p03set.DeclaredAccessibility);

            var p04 = compilation1.GetMember<PropertySymbol>("I04.this[]");
            var p04get = p04.GetMethod;

            Assert.True(p04.IsAbstract);
            Assert.False(p04.IsVirtual);
            Assert.False(p04.IsSealed);
            Assert.False(p04.IsStatic);
            Assert.False(p04.IsExtern);
            Assert.False(p04.IsOverride);
            Assert.Equal(Accessibility.Internal, p04.DeclaredAccessibility);

            Assert.True(p04get.IsAbstract);
            Assert.False(p04get.IsVirtual);
            Assert.True(p04get.IsMetadataVirtual());
            Assert.False(p04get.IsSealed);
            Assert.False(p04get.IsStatic);
            Assert.False(p04get.IsExtern);
            Assert.False(p04get.IsAsync);
            Assert.False(p04get.IsOverride);
            Assert.Equal(Accessibility.Internal, p04get.DeclaredAccessibility);

            var p05 = compilation1.GetMember<PropertySymbol>("I05.this[]");
            var p05set = p05.SetMethod;

            Assert.False(p05.IsAbstract);
            Assert.False(p05.IsVirtual);
            Assert.False(p05.IsSealed);
            Assert.False(p05.IsStatic);
            Assert.False(p05.IsExtern);
            Assert.False(p05.IsOverride);
            Assert.Equal(Accessibility.Private, p05.DeclaredAccessibility);

            Assert.False(p05set.IsAbstract);
            Assert.False(p05set.IsVirtual);
            Assert.False(p05set.IsMetadataVirtual());
            Assert.False(p05set.IsSealed);
            Assert.False(p05set.IsStatic);
            Assert.False(p05set.IsExtern);
            Assert.False(p05set.IsAsync);
            Assert.False(p05set.IsOverride);
            Assert.Equal(Accessibility.Private, p05set.DeclaredAccessibility);

            var p06 = compilation1.GetMember<PropertySymbol>("I06.this[]");
            var p06get = p06.GetMethod;

            Assert.True(p06.IsAbstract);
            Assert.False(p06.IsVirtual);
            Assert.False(p06.IsSealed);
            Assert.False(p06.IsStatic);
            Assert.False(p06.IsExtern);
            Assert.False(p06.IsOverride);
            Assert.Equal(Accessibility.Public, p06.DeclaredAccessibility);

            Assert.True(p06get.IsAbstract);
            Assert.False(p06get.IsVirtual);
            Assert.True(p06get.IsMetadataVirtual());
            Assert.False(p06get.IsSealed);
            Assert.False(p06get.IsStatic);
            Assert.False(p06get.IsExtern);
            Assert.False(p06get.IsAsync);
            Assert.False(p06get.IsOverride);
            Assert.Equal(Accessibility.Public, p06get.DeclaredAccessibility);

            var p07 = compilation1.GetMember<PropertySymbol>("I07.this[]");
            var p07set = p07.SetMethod;

            Assert.False(p07.IsAbstract);
            Assert.True(p07.IsVirtual);
            Assert.False(p07.IsSealed);
            Assert.False(p07.IsStatic);
            Assert.False(p07.IsExtern);
            Assert.False(p07.IsOverride);
            Assert.Equal(Accessibility.Public, p07.DeclaredAccessibility);

            Assert.False(p07set.IsAbstract);
            Assert.True(p07set.IsVirtual);
            Assert.True(p07set.IsMetadataVirtual());
            Assert.False(p07set.IsSealed);
            Assert.False(p07set.IsStatic);
            Assert.False(p07set.IsExtern);
            Assert.False(p07set.IsAsync);
            Assert.False(p07set.IsOverride);
            Assert.Equal(Accessibility.Public, p07set.DeclaredAccessibility);

            var p08 = compilation1.GetMember<PropertySymbol>("I08.this[]");
            var p08get = p08.GetMethod;

            Assert.False(p08.IsAbstract);
            Assert.False(p08.IsVirtual);
            Assert.False(p08.IsSealed);
            Assert.False(p08.IsStatic);
            Assert.False(p08.IsExtern);
            Assert.False(p08.IsOverride);
            Assert.Equal(Accessibility.Public, p08.DeclaredAccessibility);

            Assert.False(p08get.IsAbstract);
            Assert.False(p08get.IsVirtual);
            Assert.False(p08get.IsMetadataVirtual());
            Assert.False(p08get.IsSealed);
            Assert.False(p08get.IsStatic);
            Assert.False(p08get.IsExtern);
            Assert.False(p08get.IsAsync);
            Assert.False(p08get.IsOverride);
            Assert.Equal(Accessibility.Public, p08get.DeclaredAccessibility);

            var p09 = compilation1.GetMember<PropertySymbol>("I09.this[]");
            var p09set = p09.SetMethod;

            Assert.True(p09.IsAbstract);
            Assert.False(p09.IsVirtual);
            Assert.False(p09.IsSealed);
            Assert.False(p09.IsStatic);
            Assert.False(p09.IsExtern);
            Assert.False(p09.IsOverride);
            Assert.Equal(Accessibility.Public, p09.DeclaredAccessibility);

            Assert.True(p09set.IsAbstract);
            Assert.False(p09set.IsVirtual);
            Assert.True(p09set.IsMetadataVirtual());
            Assert.False(p09set.IsSealed);
            Assert.False(p09set.IsStatic);
            Assert.False(p09set.IsExtern);
            Assert.False(p09set.IsAsync);
            Assert.False(p09set.IsOverride);
            Assert.Equal(Accessibility.Public, p09set.DeclaredAccessibility);

            var p10 = compilation1.GetMember<PropertySymbol>("I10.this[]");
            var p10get = p10.GetMethod;

            Assert.True(p10.IsAbstract);
            Assert.False(p10.IsVirtual);
            Assert.False(p10.IsSealed);
            Assert.False(p10.IsStatic);
            Assert.False(p10.IsExtern);
            Assert.False(p10.IsOverride);
            Assert.Equal(Accessibility.Public, p10.DeclaredAccessibility);

            Assert.True(p10get.IsAbstract);
            Assert.False(p10get.IsVirtual);
            Assert.True(p10get.IsMetadataVirtual());
            Assert.False(p10get.IsSealed);
            Assert.False(p10get.IsStatic);
            Assert.False(p10get.IsExtern);
            Assert.False(p10get.IsAsync);
            Assert.False(p10get.IsOverride);
            Assert.Equal(Accessibility.Public, p10get.DeclaredAccessibility);

            var p11 = compilation1.GetMember<PropertySymbol>("I11.this[]");

            Assert.False(p11.IsAbstract);
            Assert.True(p11.IsVirtual);
            Assert.False(p11.IsSealed);
            Assert.False(p11.IsStatic);
            Assert.True(p11.IsExtern);
            Assert.False(p11.IsOverride);
            Assert.Equal(Accessibility.Public, p11.DeclaredAccessibility);

            ValidateP11Accessor(p11.GetMethod);
            ValidateP11Accessor(p11.SetMethod);
            void ValidateP11Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p12 = compilation1.GetMember<PropertySymbol>("I12.this[]");

            Assert.True(p12.IsAbstract);
            Assert.False(p12.IsVirtual);
            Assert.False(p12.IsSealed);
            Assert.False(p12.IsStatic);
            Assert.False(p12.IsExtern);
            Assert.False(p12.IsOverride);
            Assert.Equal(Accessibility.Public, p12.DeclaredAccessibility);

            ValidateP12Accessor(p12.GetMethod);
            ValidateP12Accessor(p12.SetMethod);
            void ValidateP12Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p13 = compilation1.GetMember<PropertySymbol>("I13.this[]");

            Assert.True(p13.IsAbstract);
            Assert.False(p13.IsVirtual);
            Assert.False(p13.IsSealed);
            Assert.False(p13.IsStatic);
            Assert.False(p13.IsExtern);
            Assert.False(p13.IsOverride);
            Assert.Equal(Accessibility.Public, p13.DeclaredAccessibility);

            ValidateP13Accessor(p13.GetMethod, Accessibility.Public);
            ValidateP13Accessor(p13.SetMethod, Accessibility.Protected);
            void ValidateP13Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p14 = compilation1.GetMember<PropertySymbol>("I14.this[]");

            Assert.True(p14.IsAbstract);
            Assert.False(p14.IsVirtual);
            Assert.False(p14.IsSealed);
            Assert.False(p14.IsStatic);
            Assert.False(p14.IsExtern);
            Assert.False(p14.IsOverride);
            Assert.Equal(Accessibility.Public, p14.DeclaredAccessibility);

            ValidateP14Accessor(p14.GetMethod, Accessibility.ProtectedOrInternal);
            ValidateP14Accessor(p14.SetMethod, Accessibility.Public);
            void ValidateP14Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p15 = compilation1.GetMember<PropertySymbol>("I15.this[]");

            Assert.True(p15.IsAbstract);
            Assert.False(p15.IsVirtual);
            Assert.False(p15.IsSealed);
            Assert.False(p15.IsStatic);
            Assert.False(p15.IsExtern);
            Assert.False(p15.IsOverride);
            Assert.Equal(Accessibility.Public, p15.DeclaredAccessibility);

            ValidateP15Accessor(p15.GetMethod, Accessibility.Public);
            ValidateP15Accessor(p15.SetMethod, Accessibility.Internal);
            void ValidateP15Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p16 = compilation1.GetMember<PropertySymbol>("I16.this[]");

            Assert.True(p16.IsAbstract);
            Assert.False(p16.IsVirtual);
            Assert.False(p16.IsSealed);
            Assert.False(p16.IsStatic);
            Assert.False(p16.IsExtern);
            Assert.False(p16.IsOverride);
            Assert.Equal(Accessibility.Public, p16.DeclaredAccessibility);

            ValidateP16Accessor(p16.GetMethod, Accessibility.Private);
            ValidateP16Accessor(p16.SetMethod, Accessibility.Public);
            void ValidateP16Accessor(MethodSymbol accessor, Accessibility accessibility)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(accessibility, accessor.DeclaredAccessibility);
            }

            var p17 = compilation1.GetMember<PropertySymbol>("I17.this[]");
            var p17get = p17.GetMethod;

            Assert.True(p17.IsAbstract);
            Assert.False(p17.IsVirtual);
            Assert.False(p17.IsSealed);
            Assert.False(p17.IsStatic);
            Assert.False(p17.IsExtern);
            Assert.False(p17.IsOverride);
            Assert.Equal(Accessibility.Public, p17.DeclaredAccessibility);

            Assert.True(p17get.IsAbstract);
            Assert.False(p17get.IsVirtual);
            Assert.True(p17get.IsMetadataVirtual());
            Assert.False(p17get.IsSealed);
            Assert.False(p17get.IsStatic);
            Assert.False(p17get.IsExtern);
            Assert.False(p17get.IsAsync);
            Assert.False(p17get.IsOverride);
            Assert.Equal(Accessibility.Private, p17get.DeclaredAccessibility);

            var p18 = compilation1.GetMember<PropertySymbol>("I18.this[]");
            var p18get = p18.GetMethod;

            Assert.True(p18.IsAbstract);
            Assert.False(p18.IsVirtual);
            Assert.False(p18.IsSealed);
            Assert.False(p18.IsStatic);
            Assert.False(p18.IsExtern);
            Assert.False(p18.IsOverride);
            Assert.Equal(Accessibility.ProtectedAndInternal, p18.DeclaredAccessibility);

            Assert.True(p18get.IsAbstract);
            Assert.False(p18get.IsVirtual);
            Assert.True(p18get.IsMetadataVirtual());
            Assert.False(p18get.IsSealed);
            Assert.False(p18get.IsStatic);
            Assert.False(p18get.IsExtern);
            Assert.False(p18get.IsAsync);
            Assert.False(p18get.IsOverride);
            Assert.Equal(Accessibility.ProtectedAndInternal, p18get.DeclaredAccessibility);

            var p19 = compilation1.GetMember<PropertySymbol>("I19.this[]");

            Assert.True(p19.IsAbstract);
            Assert.False(p19.IsVirtual);
            Assert.False(p19.IsSealed);
            Assert.False(p19.IsStatic);
            Assert.False(p19.IsExtern);
            Assert.False(p19.IsOverride);
            Assert.Equal(Accessibility.Public, p19.DeclaredAccessibility);

            ValidateP13Accessor(p19.GetMethod, Accessibility.Public);
            ValidateP13Accessor(p19.SetMethod, Accessibility.ProtectedAndInternal);
        }

        [Fact]
        public void IndexerModifiers_02()
        {
            var source1 =
@"
public interface I01{ public int this[int x] {get; set;} }
public interface I02{ protected int this[int x] {get;} }
public interface I03{ protected internal int this[int x] {set;} }
public interface I04{ internal int this[int x] {get;} }
public interface I05{ private int this[int x] {set;} }
public interface I06{ static int this[int x] {get;} }
public interface I07{ virtual int this[int x] {set;} }
public interface I08{ sealed int this[int x] {get;} }
public interface I09{ override int this[int x] {set;} }
public interface I10{ abstract int this[int x] {get;} }
public interface I11{ extern int this[int x] {get; set;} }

public interface I12{ int this[int x] { public get; set;} }
public interface I13{ int this[int x] { get; protected set;} }
public interface I14{ int this[int x] { protected internal get; set;} }
public interface I15{ int this[int x] { get; internal set;} }
public interface I16{ int this[int x] { private get; set;} }
public interface I17{ int this[int x] { private get;} }

public interface I18{ private protected int this[int x] { get; } }
public interface I19{ int this[int x] { get; private protected set;} }
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (2,34): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I01{ public int this[int x] {get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("public", "7.3", "8.0").WithLocation(2, 34),
                // (3,37): error CS8703: The modifier 'protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I02{ protected int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("protected", "7.3", "8.0").WithLocation(3, 37),
                // (4,46): error CS8703: The modifier 'protected internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I03{ protected internal int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("protected internal", "7.3", "8.0").WithLocation(4, 46),
                // (5,36): error CS8503: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I04{ internal int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("internal", "7.3", "8.0").WithLocation(5, 36),
                // (6,35): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I05{ private int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("private", "7.3", "8.0").WithLocation(6, 35),
                // (6,48): error CS0501: 'I05.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                // public interface I05{ private int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I05.this[int].set").WithLocation(6, 48),
                // (7,34): error CS0106: The modifier 'static' is not valid for this item
                // public interface I06{ static int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(7, 34),
                // (8,35): error CS8503: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I07{ virtual int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("virtual", "7.3", "8.0").WithLocation(8, 35),
                // (8,48): error CS0501: 'I07.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                // public interface I07{ virtual int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I07.this[int].set").WithLocation(8, 48),
                // (9,34): error CS8503: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I08{ sealed int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("sealed", "7.3", "8.0").WithLocation(9, 34),
                // (9,47): error CS0501: 'I08.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                // public interface I08{ sealed int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I08.this[int].get").WithLocation(9, 47),
                // (10,36): error CS0106: The modifier 'override' is not valid for this item
                // public interface I09{ override int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("override").WithLocation(10, 36),
                // (11,36): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I10{ abstract int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("abstract", "7.3", "8.0").WithLocation(11, 36),
                // (12,34): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("extern", "7.3", "8.0").WithLocation(12, 34),
                // (14,48): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I12{ int this[int x] { public get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("public", "7.3", "8.0").WithLocation(14, 48),
                // (14,48): error CS0273: The accessibility modifier of the 'I12.this[int].get' accessor must be more restrictive than the property or indexer 'I12.this[int]'
                // public interface I12{ int this[int x] { public get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I12.this[int].get", "I12.this[int]").WithLocation(14, 48),
                // (15,56): error CS8703: The modifier 'protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I13{ int this[int x] { get; protected set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("protected", "7.3", "8.0").WithLocation(15, 56),
                // (16,60): error CS8703: The modifier 'protected internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I14{ int this[int x] { protected internal get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("protected internal", "7.3", "8.0").WithLocation(16, 60),
                // (17,55): error CS8503: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I15{ int this[int x] { get; internal set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("internal", "7.3", "8.0").WithLocation(17, 55),
                // (18,49): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I16{ int this[int x] { private get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("private", "7.3", "8.0").WithLocation(18, 49),
                // (18,49): error CS0442: 'I16.this[int].get': abstract properties cannot have private accessors
                // public interface I16{ int this[int x] { private get; set;} }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I16.this[int].get").WithLocation(18, 49),
                // (19,49): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I17{ int this[int x] { private get;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "get").WithArguments("private", "7.3", "8.0").WithLocation(19, 49),
                // (19,27): error CS0276: 'I17.this[int]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                // public interface I17{ int this[int x] { private get;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I17.this[int]").WithLocation(19, 27),
                // (21,45): error CS8703: The modifier 'private protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I18{ private protected int this[int x] { get; } }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("private protected", "7.3", "8.0").WithLocation(21, 45),
                // (22,64): error CS8703: The modifier 'private protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                // public interface I19{ int this[int x] { get; private protected set;} }
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("private protected", "7.3", "8.0").WithLocation(22, 64),
                // (12,47): warning CS0626: Method, operator, or accessor 'I11.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I11.this[int].get").WithLocation(12, 47),
                // (12,52): warning CS0626: Method, operator, or accessor 'I11.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I11.this[int].set").WithLocation(12, 52)
                );

            ValidateSymbolsIndexerModifiers_01(compilation1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);
            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (3,37): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                // public interface I02{ protected int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "this").WithLocation(3, 37),
                // (4,46): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                // public interface I03{ protected internal int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "this").WithLocation(4, 46),
                // (6,48): error CS0501: 'I05.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                // public interface I05{ private int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I05.this[int].set").WithLocation(6, 48),
                // (7,34): error CS0106: The modifier 'static' is not valid for this item
                // public interface I06{ static int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(7, 34),
                // (8,48): error CS0501: 'I07.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                // public interface I07{ virtual int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I07.this[int].set").WithLocation(8, 48),
                // (9,47): error CS0501: 'I08.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                // public interface I08{ sealed int this[int x] {get;} }
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I08.this[int].get").WithLocation(9, 47),
                // (10,36): error CS0106: The modifier 'override' is not valid for this item
                // public interface I09{ override int this[int x] {set;} }
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("override").WithLocation(10, 36),
                // (12,47): error CS8701: Target runtime doesn't support default interface implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(12, 47),
                // (12,47): warning CS0626: Method, operator, or accessor 'I11.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I11.this[int].get").WithLocation(12, 47),
                // (12,52): error CS8701: Target runtime doesn't support default interface implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(12, 52),
                // (12,52): warning CS0626: Method, operator, or accessor 'I11.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                // public interface I11{ extern int this[int x] {get; set;} }
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I11.this[int].set").WithLocation(12, 52),
                // (14,48): error CS0273: The accessibility modifier of the 'I12.this[int].get' accessor must be more restrictive than the property or indexer 'I12.this[int]'
                // public interface I12{ int this[int x] { public get; set;} }
                Diagnostic(ErrorCode.ERR_InvalidPropertyAccessMod, "get").WithArguments("I12.this[int].get", "I12.this[int]").WithLocation(14, 48),
                // (15,56): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                // public interface I13{ int this[int x] { get; protected set;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(15, 56),
                // (16,60): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                // public interface I14{ int this[int x] { protected internal get; set;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "get").WithLocation(16, 60),
                // (18,49): error CS0442: 'I16.this[int].get': abstract properties cannot have private accessors
                // public interface I16{ int this[int x] { private get; set;} }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I16.this[int].get").WithLocation(18, 49),
                // (19,27): error CS0276: 'I17.this[int]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                // public interface I17{ int this[int x] { private get;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I17.this[int]").WithLocation(19, 27),
                // (21,45): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                // public interface I18{ private protected int this[int x] { get; } }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "this").WithLocation(21, 45),
                // (22,64): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                // public interface I19{ int this[int x] { get; private protected set;} }
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(22, 64)
                );

            ValidateSymbolsIndexerModifiers_01(compilation2);
        }

        [Fact]
        public void IndexerModifiers_03()
        {
            ValidateIndexerImplementation_101(@"
public interface I1
{
    public virtual int this[int i] 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1[0];
    }
}
");

            ValidateIndexerImplementation_101(@"
public interface I1
{
    public virtual int this[int i] 
    {
        get => Test1.GetP1();
    }
}

class Test1 : I1
{
    public static int GetP1()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1[0];
    }
}
");

            ValidateIndexerImplementation_101(@"
public interface I1
{
    public virtual int this[int i] => Test1.GetP1(); 
}

class Test1 : I1
{
    public static int GetP1()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        _ = i1[0];
    }
}
");

            ValidateIndexerImplementation_102(@"
public interface I1
{
    public virtual int this[int i] 
    {
        get
        {
            System.Console.WriteLine(""get P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1[0] = i1[0];
    }
}
");

            ValidateIndexerImplementation_102(@"
public interface I1
{
    public virtual int this[int i] 
    {
        get => Test1.GetP1();
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{
    public static int GetP1()
    {
        System.Console.WriteLine(""get P1"");
        return 0;
    }
    static void Main()
    {
        I1 i1 = new Test1();
        i1[0] = i1[0];
    }
}
");

            ValidateIndexerImplementation_103(@"
public interface I1
{
    public virtual int this[int i] 
    {
        set
        {
            System.Console.WriteLine(""set P1"");
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1[0] = 1;
    }
}
");

            ValidateIndexerImplementation_103(@"
public interface I1
{
    public virtual int this[int i] 
    {
        set => System.Console.WriteLine(""set P1"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1[0] = 1;
    }
}
");
        }

        [Fact]
        public void IndexerModifiers_04()
        {
            var source1 =
@"
public interface I1
{
    public virtual int this[int x] { get; } = 0; 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (4,45): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     public virtual int this[int x] { get; } = 0; 
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(4, 45),
                // (4,38): error CS0501: 'I1.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                //     public virtual int this[int x] { get; } = 0; 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.this[int].get").WithLocation(4, 38)
                );

            ValidatePropertyModifiers_04(compilation1, "this[]");
        }

        [Fact]
        public void IndexerModifiers_05()
        {
            var source1 =
@"
public interface I1
{
    public abstract int this[int x] {get; set;} 
}
public interface I2
{
    int this[int x] {get; set;} 
}

class Test1 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set => System.Console.WriteLine(""set_P1"");
    }
}
class Test2 : I2
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        set => System.Console.WriteLine(""set_P2"");
    }

    static void Main()
    {
        I1 x = new Test1();
        x[0] = x[0];
        I2 y = new Test2();
        y[0] = y[0];
    }
}
";

            ValidatePropertyModifiers_05(source1);
        }

        [Fact]
        public void IndexerModifiers_06()
        {
            var source1 =
@"
public interface I1
{
    public abstract int this[int x] {get; set;} 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation1.VerifyDiagnostics(
                // (4,25): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract int this[int x] {get; set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("abstract", "7.3", "8.0").WithLocation(4, 25),
                // (4,25): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract int this[int x] {get; set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("public", "7.3", "8.0").WithLocation(4, 25)
                );

            ValidatePropertyModifiers_06(compilation1, "this[]");
        }

        [Fact]
        public void IndexerModifiers_09()
        {
            var source1 =
@"
public interface I1
{
    private int this[int x]
    {
        get
        { 
            System.Console.WriteLine(""get_P1"");
            return 0;
        }           
    }
    sealed void M()
    {
        var x = this[0];
    }
}
public interface I2
{
    private int this[int x] 
    {
        get
        { 
            System.Console.WriteLine(""get_P2"");
            return 0;
        }           
        set
        { 
            System.Console.WriteLine(""set_P2"");
        }           
    }
    sealed void M()
    {
        this[0] = this[0];
    }
}
public interface I3
{
    private int this[int x] 
    {
        set
        { 
            System.Console.WriteLine(""set_P3"");
        }           
    }
    sealed void M()
    {
        this[0] = 0;
    }
}
public interface I4
{
    private int this[int x] 
    {
        get => GetP4();
    }

    private int GetP4()
    { 
        System.Console.WriteLine(""get_P4"");
        return 0;
    }           
    sealed void M()
    {
        var x = this[0];
    }
}
public interface I5
{
    private int this[int x] 
    {
        get => GetP5();
        set => System.Console.WriteLine(""set_P5"");
    }

    private int GetP5()
    { 
        System.Console.WriteLine(""get_P5"");
        return 0;
    }           
    sealed void M()
    {
        this[0] = this[0];
    }
}
public interface I6
{
    private int this[int x] 
    {
        set => System.Console.WriteLine(""set_P6"");
    }
    sealed void M()
    {
        this[0] = 0;
    }
}
public interface I7
{
    private int this[int x] => GetP7();

    private int GetP7()
    { 
        System.Console.WriteLine(""get_P7"");
        return 0;
    }           
    sealed void M()
    {
        var x = this[0];
    }
}

class Test1 : I1, I2, I3, I4, I5, I6, I7
{
    static void Main()
    {
        I1 x1 = new Test1();
        x1.M();
        I2 x2 = new Test1();
        x2.M();
        I3 x3 = new Test1();
        x3.M();
        I4 x4 = new Test1();
        x4.M();
        I5 x5 = new Test1();
        x5.M();
        I6 x6 = new Test1();
        x6.M();
        I7 x7 = new Test1();
        x7.M();
    }
}
";

            ValidatePropertyModifiers_09(source1);
        }

        [Fact]
        public void IndexerModifiers_10()
        {
            var source1 =
@"
public interface I1
{
    abstract private int this[byte x] { get; } 

    virtual private int this[int x] => 0;

    sealed private int this[short x] 
    {
        get => 0;
        set {}
    }

    private int this[long x] {get;} = 0;
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,37): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     private int this[long x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(14, 37),
                // (4,26): error CS0621: 'I1.this[byte]': virtual or abstract members cannot be private
                //     abstract private int this[byte x] { get; } 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "this").WithArguments("I1.this[byte]").WithLocation(4, 26),
                // (6,25): error CS0621: 'I1.this[int]': virtual or abstract members cannot be private
                //     virtual private int this[int x] => 0;
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "this").WithArguments("I1.this[int]").WithLocation(6, 25),
                // (8,24): error CS0238: 'I1.this[short]' cannot be sealed because it is not an override
                //     sealed private int this[short x] 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "this").WithArguments("I1.this[short]").WithLocation(8, 24),
                // (14,31): error CS0501: 'I1.this[long].get' must declare a body because it is not marked abstract, extern, or partial
                //     private int this[long x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.this[long].get").WithLocation(14, 31),
                // (17,15): error CS0535: 'Test1' does not implement interface member 'I1.this[byte]'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.this[byte]")
                );

            ValidatePropertyModifiers_10(compilation1);
        }

        [Fact]
        public void IndexerModifiers_11_01()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_01(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(12, 9),
                    // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(17, 9)
                    ),
                expectedNoImplementation: ExpectedDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.this[int]'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.this[int]")
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(12, 9),
                    // (17,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(17, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_02()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_03()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,12): error CS0122: 'I1.this[int]' is inaccessible due to its protection level
                //     int I1.this[int x] 
                Diagnostic(ErrorCode.ERR_BadAccess, "this").WithArguments("I1.this[int]").WithLocation(9, 12)
                );
        }

        [Fact]
        public void IndexerModifiers_11_04()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int this[int x]
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_05()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_06()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract int this[int x] {get; set;} 
}

class Test3 : Test1
{
    public override int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,38): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(9, 38),
                    // (9,43): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(9, 43)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,38): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(9, 38),
                    // (9,43): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(9, 43)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_07()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    int this[int x] {get; set;} 
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_08()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_08(source1, source2);
        }

        [Fact]
        public void IndexerModifiers_11_09()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.this[int]'. 'Test1.this[int]' cannot implement 'I1.this[int]' because it does not have the matching return type of 'int'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.this[int]", "Test1.this[int]", "int").WithLocation(2, 15)
                );
        }

        [Fact]
        public void IndexerModifiers_11_10()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (6,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].get'. 'Test2.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.this[int].get", "Test2.this[int].get", "9.0", "10.0").WithLocation(6, 9),
                    // (11,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].set'. 'Test2.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.this[int].set", "Test2.this[int].set", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (6,9): error CS9044: 'Test2' does not implement interface member 'I1.this[int].get'. 'Test2.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test2", "I1.this[int].get", "Test2.this[int].get").WithLocation(6, 9),
                    // (11,9): error CS9044: 'Test2' does not implement interface member 'I1.this[int].set'. 'Test2.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test2", "I1.this[int].set", "Test2.this[int].set").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_11_11()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
public class Test2 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_11(source1, source2,
                // (15,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].get'. 'Test2.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.this[int].get", "Test2.this[int].get", "9.0", "10.0").WithLocation(15, 9),
                // (20,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].set'. 'Test2.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         set
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.this[int].set", "Test2.this[int].set", "9.0", "10.0").WithLocation(20, 9)
                );
        }

        [Fact]
        public void IndexerModifiers_12()
        {
            var source1 =
@"
public interface I1
{
    internal abstract int this[int x] {get; set;} 
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(
                // (7,15): error CS0535: 'Test1' does not implement interface member 'I1.this[int]'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.this[int]")
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<PropertySymbol>("this[]");
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test1.FindImplementationForInterfaceMember(p1.GetMethod));
            Assert.Null(test1.FindImplementationForInterfaceMember(p1.SetMethod));
        }

        [Fact]
        public void IndexerModifiers_13()
        {
            var source1 =
@"
public interface I1
{
    public sealed int this[int x]
    {
        get
        { 
            System.Console.WriteLine(""get_P1"");
            return 0;
        }           
    }
}
public interface I2
{
    public sealed int this[int x] 
    {
        get
        { 
            System.Console.WriteLine(""get_P2"");
            return 0;
        }           
        set
        { 
            System.Console.WriteLine(""set_P2"");
        }           
    }
}
public interface I3
{
    public sealed int this[int x] 
    {
        set
        { 
            System.Console.WriteLine(""set_P3"");
        }           
    }
}
public interface I4
{
    public sealed int this[int x] 
    {
        get => GetP4();
    }

    private int GetP4()
    { 
        System.Console.WriteLine(""get_P4"");
        return 0;
    }           
}
public interface I5
{
    public sealed int this[int x] 
    {
        get => GetP5();
        set => System.Console.WriteLine(""set_P5"");
    }

    private int GetP5()
    { 
        System.Console.WriteLine(""get_P5"");
        return 0;
    }           
}
public interface I6
{
    public sealed int this[int x] 
    {
        set => System.Console.WriteLine(""set_P6"");
    }
}
public interface I7
{
    public sealed int this[int x] => GetP7();

    private int GetP7()
    { 
        System.Console.WriteLine(""get_P7"");
        return 0;
    }           
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        var x = i1[0];
        I2 i2 = new Test2();
        i2[0] = i2[0];
        I3 i3 = new Test3();
        i3[0] = x;
        I4 i4 = new Test4();
        x = i4[0];
        I5 i5 = new Test5();
        i5[0] = i5[0];
        I6 i6 = new Test6();
        i6[0] = x;
        I7 i7 = new Test7();
        x = i7[0];
    }

    public int this[int x] => throw null;
}
class Test2 : I2
{
    public int this[int x] 
    {
        get => throw null;          
        set => throw null;         
    }
}
class Test3 : I3
{
    public int this[int x] 
    {
        set => throw null;      
    }
}
class Test4 : I4
{
    public int this[int x] 
    {
        get => throw null;
    }
}
class Test5 : I5
{
    public int this[int x] 
    {
        get => throw null;
        set => throw null;
    }
}
class Test6 : I6
{
    public int this[int x] 
    {
        set => throw null;
    }
}
class Test7 : I7
{
    public int this[int x] => throw null;
}
";

            ValidatePropertyModifiers_13(source1);
        }

        [Fact]
        public void AccessModifiers_14()
        {
            var source1 =
@"
public interface I1
{
    public sealed int this[int x] {get;} = 0; 
}
public interface I2
{
    abstract sealed int this[int x] {get;} 
}
public interface I3
{
    virtual sealed int this[int x]
    {
        set {}
    }
}

class Test1 : I1, I2, I3
{
    int I1.this[int x] { get => throw null; }
    int I2.this[int x] { get => throw null; }
    int I3.this[int x] { set => throw null; }
}

class Test2 : I1, I2, I3
{}
";
            ValidatePropertyModifiers_14(source1,
                // (4,42): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     public sealed int this[int x] {get;} = 0; 
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(4, 42),
                // (4,36): error CS0501: 'I1.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                //     public sealed int this[int x] {get;} = 0; 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.this[int].get").WithLocation(4, 36),
                // (8,25): error CS0238: 'I2.this[int]' cannot be sealed because it is not an override
                //     abstract sealed int this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "this").WithArguments("I2.this[int]").WithLocation(8, 25),
                // (12,24): error CS0238: 'I3.this[int]' cannot be sealed because it is not an override
                //     virtual sealed int this[int x]
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "this").WithArguments("I3.this[int]").WithLocation(12, 24),
                // (20,12): error CS0539: 'Test1.this[int]' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I1.this[int x] { get => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "this").WithArguments("Test1.this[int]").WithLocation(20, 12),
                // (21,12): error CS0539: 'Test1.this[int]' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I2.this[int x] { get => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "this").WithArguments("Test1.this[int]").WithLocation(21, 12),
                // (22,12): error CS0539: 'Test1.this[int]' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I3.this[int x] { set => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "this").WithArguments("Test1.this[int]").WithLocation(22, 12)
                );
        }

        [Fact]
        [WorkItem(38398, "https://github.com/dotnet/roslyn/issues/38398")]
        public void InconsistentAccessibility_01()
        {
            var source1 =
@"
    interface I1
    {
        protected interface I2
        {
        }
    }

    class C1
    {
        protected interface I2
        {
        }
    }

    interface I3 : I1
    {
        protected I1.I2 M1();

        protected interface I5
        {
            I1.I2 M5();
        }
    }

    class CI3 : I3
    {
        I1.I2 I3.M1() => null;

        class CI5 : I3.I5
        {
            I1.I2 I3.I5.M5() => null;
        }
    }

    class C3 : I1
    {
        protected virtual void M1(I1.I2 x) { }

        protected interface I7
        {
            I1.I2 M7();
        }
    }

    class CC3 : C3
    {
        protected override void M1(I1.I2 x) { }

        class CI7 : C3.I7
        {
            I1.I2 C3.I7.M7() => null;
        }
    }

    class C33 : C1
    {
        protected virtual void M1(C1.I2 x) { }

        protected class C55
        {
            public virtual C1.I2 M55() => null;
        }
    }

    class CC33 : C33
    {
        protected override void M1(C1.I2 x) { }

        class CC55 : C33.C55
        {
            public override C1.I2 M55() => null;
        }
    }
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr).VerifyDiagnostics();
        }

        [Fact]
        [WorkItem(38398, "https://github.com/dotnet/roslyn/issues/38398")]
        public void InconsistentAccessibility_02()
        {
            var source1 =
@"
    interface I1
    {
        protected interface I2
        {
        }
    }

    class C1
    {
        protected interface I2
        {
        }
    }

    interface I3 : I1
    {
        interface I4
        {
            protected I1.I2 M4();
        }
    }

    class CI4 : I3.I4
    {
        I1.I2 I3.I4.M4() => null;
    }

    class C3 : I1
    {
        public interface I6
        {
            protected I1.I2 M6();
        }
    }

    class CI6 : C3.I6
    {
        I1.I2 C3.I6.M6() => null;
    }

    class C33 : C1
    {
        public class C44
        {
            protected virtual C1.I2 M44() => null;
        }
    }

    class CC44 : C33.C44
    {
        protected override C1.I2 M44() => null;
    }
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics(
                // (20,29): error CS0050: Inconsistent accessibility: return type 'I1.I2' is less accessible than method 'I3.I4.M4()'
                //             protected I1.I2 M4();
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "M4").WithArguments("I3.I4.M4()", "I1.I2").WithLocation(20, 29),
                // (24,17): error CS0535: 'CI4' does not implement interface member 'I3.I4.M4()'
                //     class CI4 : I3.I4
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3.I4").WithArguments("CI4", "I3.I4.M4()").WithLocation(24, 17),
                // (26,12): error CS0122: 'I1.I2' is inaccessible due to its protection level
                //         I1.I2 I3.I4.M4() => null;
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1.I2").WithLocation(26, 12),
                // (26,21): error CS9334: 'CI4.M4()' return type must be 'I1.I2' to match implemented member 'I3.I4.M4()'
                //         I1.I2 I3.I4.M4() => null;
                Diagnostic(ErrorCode.ERR_ExplicitInterfaceMemberReturnTypeMismatch, "M4").WithArguments("CI4.M4()", "I1.I2", "I3.I4.M4()").WithLocation(26, 21),
                // (33,29): error CS0050: Inconsistent accessibility: return type 'I1.I2' is less accessible than method 'C3.I6.M6()'
                //             protected I1.I2 M6();
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "M6").WithArguments("C3.I6.M6()", "I1.I2").WithLocation(33, 29),
                // (37,17): error CS0535: 'CI6' does not implement interface member 'C3.I6.M6()'
                //     class CI6 : C3.I6
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "C3.I6").WithArguments("CI6", "C3.I6.M6()").WithLocation(37, 17),
                // (39,12): error CS0122: 'I1.I2' is inaccessible due to its protection level
                //         I1.I2 C3.I6.M6() => null;
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1.I2").WithLocation(39, 12),
                // (39,21): error CS9334: 'CI6.M6()' return type must be 'I1.I2' to match implemented member 'C3.I6.M6()'
                //         I1.I2 C3.I6.M6() => null;
                Diagnostic(ErrorCode.ERR_ExplicitInterfaceMemberReturnTypeMismatch, "M6").WithArguments("CI6.M6()", "I1.I2", "C3.I6.M6()").WithLocation(39, 21),
                // (46,37): error CS0050: Inconsistent accessibility: return type 'C1.I2' is less accessible than method 'C33.C44.M44()'
                //             protected virtual C1.I2 M44() => null;
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "M44").WithArguments("C33.C44.M44()", "C1.I2").WithLocation(46, 37),
                // (52,31): error CS0122: 'C1.I2' is inaccessible due to its protection level
                //         protected override C1.I2 M44() => null;
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("C1.I2").WithLocation(52, 31)
                );
        }

        [Fact]
        [WorkItem(38398, "https://github.com/dotnet/roslyn/issues/38398")]
        public void InconsistentAccessibility_03()
        {
            var source1 =
@"
    interface I1<T>
    {
        protected interface I2
        {
        }
    }

    class C1<T>
    {
        protected interface I2
        {
        }
    }

    interface I3 : I1<int>
    {
        protected I1<string>.I2 M1();

        protected interface I5
        {
            I1<string>.I2 M5();
        }
    }

    class CI3 : I3
    {
        I1<string>.I2 I3.M1() => null;

        class CI5 : I3.I5
        {
            I1<string>.I2 I3.I5.M5() => null;
        }
    }

    class C3 : I1<int>
    {
        protected virtual void M1(I1<string>.I2 x) { }

        protected interface I7
        {
            I1<string>.I2 M7();
        }
    }

    class CC3 : C3
    {
        protected override void M1(I1<string>.I2 x) { }

        class CI7 : C3.I7
        {
            I1<string>.I2 C3.I7.M7() => null;
        }
    }

    class C33 : C1<int>
    {
        protected virtual void M1(C1<string>.I2 x) { }

        protected class C55
        {
            public virtual C1<string>.I2 M55() => null;
        }
    }

    class CC33 : C33
    {
        protected override void M1(C1<string>.I2 x) { }

        class CC55 : C33.C55
        {
            public override C1<string>.I2 M55() => null;
        }
    }
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr).VerifyDiagnostics();
        }

        [Fact]
        [WorkItem(38398, "https://github.com/dotnet/roslyn/issues/38398")]
        public void InconsistentAccessibility_04()
        {
            var source1 =
@"
    interface I1<T>
    {
        protected interface I2
        {
        }
    }

    class C1<T>
    {
        protected interface I2
        {
        }
    }

    interface I3 : I1<int>
    {
        interface I4
        {
            protected I1<string>.I2 M4();
        }
    }

    class CI4 : I3.I4
    {
        I1<string>.I2 I3.I4.M4() => null;
    }

    class C3 : I1<int>
    {
        public interface I6
        {
            protected I1<string>.I2 M6();
        }
    }

    class CI6 : C3.I6
    {
        I1<string>.I2 C3.I6.M6() => null;
    }

    class C33 : C1<int>
    {
        public class C44
        {
            protected virtual C1<string>.I2 M44() => null;
        }
    }

    class CC44 : C33.C44
    {
        protected override C1<string>.I2 M44() => null;
    }
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics(
                // (20,37): error CS0050: Inconsistent accessibility: return type 'I1<string>.I2' is less accessible than method 'I3.I4.M4()'
                //             protected I1<string>.I2 M4();
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "M4").WithArguments("I3.I4.M4()", "I1<string>.I2").WithLocation(20, 37),
                // (24,17): error CS0535: 'CI4' does not implement interface member 'I3.I4.M4()'
                //     class CI4 : I3.I4
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3.I4").WithArguments("CI4", "I3.I4.M4()").WithLocation(24, 17),
                // (26,20): error CS0122: 'I1<string>.I2' is inaccessible due to its protection level
                //         I1<string>.I2 I3.I4.M4() => null;
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1<string>.I2").WithLocation(26, 20),
                // (26,29): error CS9334: 'CI4.M4()' type must be 'I1<string>.I2' to match implemented member 'I3.I4.M4()'
                //         I1<string>.I2 I3.I4.M4() => null;
                Diagnostic(ErrorCode.ERR_ExplicitInterfaceMemberReturnTypeMismatch, "M4").WithArguments("CI4.M4()", "I1<string>.I2", "I3.I4.M4()").WithLocation(26, 29),
                // (33,37): error CS0050: Inconsistent accessibility: return type 'I1<string>.I2' is less accessible than method 'C3.I6.M6()'
                //             protected I1<string>.I2 M6();
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "M6").WithArguments("C3.I6.M6()", "I1<string>.I2").WithLocation(33, 37),
                // (37,17): error CS0535: 'CI6' does not implement interface member 'C3.I6.M6()'
                //     class CI6 : C3.I6
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "C3.I6").WithArguments("CI6", "C3.I6.M6()").WithLocation(37, 17),
                // (39,20): error CS0122: 'I1<string>.I2' is inaccessible due to its protection level
                //         I1<string>.I2 C3.I6.M6() => null;
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1<string>.I2").WithLocation(39, 20),
                // (39,29): error CS9334: 'CI6.M6()' type must be 'I1<string>.I2' to match implemented member 'C3.I6.M6()'
                //         I1<string>.I2 C3.I6.M6() => null;
                Diagnostic(ErrorCode.ERR_ExplicitInterfaceMemberReturnTypeMismatch, "M6").WithArguments("CI6.M6()", "I1<string>.I2", "C3.I6.M6()").WithLocation(39, 29),
                // (46,45): error CS0050: Inconsistent accessibility: return type 'C1<string>.I2' is less accessible than method 'C33.C44.M44()'
                //             protected virtual C1<string>.I2 M44() => null;
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "M44").WithArguments("C33.C44.M44()", "C1<string>.I2").WithLocation(46, 45),
                // (52,39): error CS0122: 'C1<string>.I2' is inaccessible due to its protection level
                //         protected override C1<string>.I2 M44() => null;
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("C1<string>.I2").WithLocation(52, 39)
                );
        }

        [Fact]
        public void IndexerModifiers_15()
        {
            var source1 =
@"
public interface I0
{
    abstract virtual int this[int x] { get; set; }
}
public interface I1
{
    abstract virtual int this[int x] { get { throw null; } }
}
public interface I2
{
    virtual abstract int this[int x] 
    {
        get { throw null; }
        set { throw null; }
    }
}
public interface I3
{
    abstract virtual int this[int x] { set { throw null; } }
}
public interface I4
{
    abstract virtual int this[int x] { get => throw null; }
}
public interface I5
{
    abstract virtual int this[int x] 
    {
        get => throw null;
        set => throw null;
    }
}
public interface I6
{
    abstract virtual int this[int x] { set => throw null; }
}
public interface I7
{
    abstract virtual int this[int x] => throw null;
}
public interface I8
{
    abstract virtual int this[int x] {get;} = 0;
}

class Test1 : I0, I1, I2, I3, I4, I5, I6, I7, I8
{
    int I0.this[int x] 
    {
        get { throw null; }
        set { throw null; }
    }
    int I1.this[int x] 
    {
        get { throw null; }
    }
    int I2.this[int x] 
    {
        get { throw null; }
        set { throw null; }
    }
    int I3.this[int x] 
    {
        set { throw null; }
    }
    int I4.this[int x] 
    {
        get { throw null; }
    }
    int I5.this[int x] 
    {
        get { throw null; }
        set { throw null; }
    }
    int I6.this[int x] 
    {
        set { throw null; }
    }
    int I7.this[int x] 
    {
        get { throw null; }
    }
    int I8.this[int x] 
    {
        get { throw null; }
    }
}

class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
{}
";
            ValidatePropertyModifiers_15(source1,
                // (44,45): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     abstract virtual int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(44, 45),
                // (4,26): error CS0503: The abstract property 'I0.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] { get; set; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I0.this[int]").WithLocation(4, 26),
                // (8,26): error CS0503: The abstract property 'I1.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] { get { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I1.this[int]").WithLocation(8, 26),
                // (8,40): error CS0500: 'I1.this[int].get' cannot declare a body because it is marked abstract
                //     abstract virtual int this[int x] { get { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I1.this[int].get").WithLocation(8, 40),
                // (12,26): error CS0503: The abstract property 'I2.this[int]' cannot be marked virtual
                //     virtual abstract int this[int x] 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I2.this[int]").WithLocation(12, 26),
                // (14,9): error CS0500: 'I2.this[int].get' cannot declare a body because it is marked abstract
                //         get { throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I2.this[int].get").WithLocation(14, 9),
                // (15,9): error CS0500: 'I2.this[int].set' cannot declare a body because it is marked abstract
                //         set { throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I2.this[int].set").WithLocation(15, 9),
                // (20,26): error CS0503: The abstract property 'I3.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] { set { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I3.this[int]").WithLocation(20, 26),
                // (20,40): error CS0500: 'I3.this[int].set' cannot declare a body because it is marked abstract
                //     abstract virtual int this[int x] { set { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I3.this[int].set").WithLocation(20, 40),
                // (24,26): error CS0503: The abstract property 'I4.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] { get => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I4.this[int]").WithLocation(24, 26),
                // (24,40): error CS0500: 'I4.this[int].get' cannot declare a body because it is marked abstract
                //     abstract virtual int this[int x] { get => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I4.this[int].get").WithLocation(24, 40),
                // (28,26): error CS0503: The abstract property 'I5.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I5.this[int]").WithLocation(28, 26),
                // (30,9): error CS0500: 'I5.this[int].get' cannot declare a body because it is marked abstract
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I5.this[int].get").WithLocation(30, 9),
                // (31,9): error CS0500: 'I5.this[int].set' cannot declare a body because it is marked abstract
                //         set => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I5.this[int].set").WithLocation(31, 9),
                // (36,26): error CS0503: The abstract property 'I6.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] { set => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I6.this[int]").WithLocation(36, 26),
                // (36,40): error CS0500: 'I6.this[int].set' cannot declare a body because it is marked abstract
                //     abstract virtual int this[int x] { set => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I6.this[int].set").WithLocation(36, 40),
                // (40,26): error CS0503: The abstract property 'I7.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] => throw null;
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I7.this[int]").WithLocation(40, 26),
                // (40,41): error CS0500: 'I7.this[int].get' cannot declare a body because it is marked abstract
                //     abstract virtual int this[int x] => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "throw null").WithArguments("I7.this[int].get").WithLocation(40, 41),
                // (44,26): error CS0503: The abstract property 'I8.this[int]' cannot be marked virtual
                //     abstract virtual int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "this").WithArguments("property", "I8.this[int]").WithLocation(44, 26),
                // (90,15): error CS0535: 'Test2' does not implement interface member 'I0.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I0").WithArguments("Test2", "I0.this[int]").WithLocation(90, 15),
                // (90,19): error CS0535: 'Test2' does not implement interface member 'I1.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.this[int]").WithLocation(90, 19),
                // (90,23): error CS0535: 'Test2' does not implement interface member 'I2.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test2", "I2.this[int]").WithLocation(90, 23),
                // (90,27): error CS0535: 'Test2' does not implement interface member 'I3.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test2", "I3.this[int]").WithLocation(90, 27),
                // (90,31): error CS0535: 'Test2' does not implement interface member 'I4.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test2", "I4.this[int]").WithLocation(90, 31),
                // (90,35): error CS0535: 'Test2' does not implement interface member 'I5.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I5").WithArguments("Test2", "I5.this[int]").WithLocation(90, 35),
                // (90,39): error CS0535: 'Test2' does not implement interface member 'I6.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I6").WithArguments("Test2", "I6.this[int]").WithLocation(90, 39),
                // (90,43): error CS0535: 'Test2' does not implement interface member 'I7.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I7").WithArguments("Test2", "I7.this[int]").WithLocation(90, 43),
                // (90,47): error CS0535: 'Test2' does not implement interface member 'I8.this[int]'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I8").WithArguments("Test2", "I8.this[int]").WithLocation(90, 47)
                );
        }

        [Fact]
        public void IndexerModifiers_16()
        {
            var source1 =
@"
public interface I1
{
    extern int this[int x] {get;} 
}
public interface I2
{
    virtual extern int this[int x] {set;}
}
public interface I4
{
    private extern int this[int x] {get;}
}
public interface I5
{
    extern sealed int this[int x] {set;}
}

class Test1 : I1, I2, I4, I5
{
}

class Test2 : I1, I2, I4, I5
{
    int I1.this[int x] => 0;
    int I2.this[int x] { set {} }
}
";
            ValidatePropertyModifiers_16(source1,
                new DiagnosticDescription[]
                {
                // (4,16): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern int this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("extern", "7.3", "8.0").WithLocation(4, 16),
                // (8,24): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern int this[int x] {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("extern", "7.3", "8.0").WithLocation(8, 24),
                // (8,24): error CS8503: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern int this[int x] {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("virtual", "7.3", "8.0").WithLocation(8, 24),
                // (12,24): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern int this[int x] {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("private", "7.3", "8.0").WithLocation(12, 24),
                // (12,24): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern int this[int x] {get;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("extern", "7.3", "8.0").WithLocation(12, 24),
                // (16,23): error CS8503: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed int this[int x] {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("sealed", "7.3", "8.0").WithLocation(16, 23),
                // (16,23): error CS8503: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed int this[int x] {set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "this").WithArguments("extern", "7.3", "8.0").WithLocation(16, 23),
                // (8,37): warning CS0626: Method, operator, or accessor 'I2.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern int this[int x] {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I2.this[int].set").WithLocation(8, 37),
                // (12,37): warning CS0626: Method, operator, or accessor 'I4.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern int this[int x] {get;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I4.this[int].get").WithLocation(12, 37),
                // (16,36): warning CS0626: Method, operator, or accessor 'I5.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed int this[int x] {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I5.this[int].set").WithLocation(16, 36),
                // (4,29): warning CS0626: Method, operator, or accessor 'I1.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int this[int x] {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.this[int].get").WithLocation(4, 29)
                },
                // (4,29): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern int this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(4, 29),
                // (8,37): error CS8501: Target runtime doesn't support default interface implementation.
                //     virtual extern int this[int x] {set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(8, 37),
                // (12,37): error CS8501: Target runtime doesn't support default interface implementation.
                //     private extern int this[int x] {get;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(12, 37),
                // (16,36): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern sealed int this[int x] {set;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(16, 36),
                // (8,37): warning CS0626: Method, operator, or accessor 'I2.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern int this[int x] {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I2.this[int].set").WithLocation(8, 37),
                // (12,37): warning CS0626: Method, operator, or accessor 'I4.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern int this[int x] {get;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I4.this[int].get").WithLocation(12, 37),
                // (16,36): warning CS0626: Method, operator, or accessor 'I5.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed int this[int x] {set;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I5.this[int].set").WithLocation(16, 36),
                // (4,29): warning CS0626: Method, operator, or accessor 'I1.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int this[int x] {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.this[int].get").WithLocation(4, 29)
                );
        }

        [Fact]
        public void IndexerModifiers_17()
        {
            var source1 =
@"
public interface I1
{
    abstract extern int this[int x] {get;} 
}
public interface I2
{
    extern int this[int x] => 0; 
}
public interface I3
{
    static extern int this[int x] {get => 0; set => throw null;} 
}
public interface I4
{
    private extern int this[int x] { get {throw null;} set {throw null;}}
}
public interface I5
{
    extern sealed int this[int x] {get;} = 0;
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    int I1.this[int x] => 0;
    int I2.this[int x] => 0;
    int I3.this[int x] { get => 0; set => throw null;}
    int I4.this[int x] { get => 0; set => throw null;}
    int I5.this[int x] => 0;
}
";
            ValidatePropertyModifiers_17(source1,
                // (20,42): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     extern sealed int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(20, 42),
                // (4,25): error CS0180: 'I1.this[int]' cannot be both extern and abstract
                //     abstract extern int this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_AbstractAndExtern, "this").WithArguments("I1.this[int]").WithLocation(4, 25),
                // (8,31): error CS0179: 'I2.this[int].get' cannot be extern and declare a body
                //     extern int this[int x] => 0; 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "0").WithArguments("I2.this[int].get").WithLocation(8, 31),
                // (12,23): error CS0106: The modifier 'static' is not valid for this item
                //     static extern int this[int x] {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(12, 23),
                // (12,36): error CS0179: 'I3.this[int].get' cannot be extern and declare a body
                //     static extern int this[int x] {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "get").WithArguments("I3.this[int].get").WithLocation(12, 36),
                // (12,46): error CS0179: 'I3.this[int].set' cannot be extern and declare a body
                //     static extern int this[int x] {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I3.this[int].set").WithLocation(12, 46),
                // (16,38): error CS0179: 'I4.this[int].get' cannot be extern and declare a body
                //     private extern int this[int x] { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "get").WithArguments("I4.this[int].get").WithLocation(16, 38),
                // (16,56): error CS0179: 'I4.this[int].set' cannot be extern and declare a body
                //     private extern int this[int x] { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I4.this[int].set").WithLocation(16, 56),
                // (23,15): error CS0535: 'Test1' does not implement interface member 'I1.this[int]'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.this[int]"),
                // (32,12): error CS0539: 'Test2.this[int]' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I4.this[int x] { get => 0; set => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "this").WithArguments("Test2.this[int]").WithLocation(32, 12),
                // (33,12): error CS0539: 'Test2.this[int]' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I5.this[int x] => 0;
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "this").WithArguments("Test2.this[int]").WithLocation(33, 12),
                // (20,36): warning CS0626: Method, operator, or accessor 'I5.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I5.this[int].get").WithLocation(20, 36)
                );
        }

        [Fact]
        public void IndexerModifiers_18()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get => 0; set => throw null;} 
}
public interface I2
{
    abstract private int this[int x] => 0; 
}
public interface I3
{
    static extern int this[int x] {get; set;} 
}
public interface I4
{
    abstract static int this[int x] { get {throw null;} set {throw null;}}
}
public interface I5
{
    override sealed int this[int x] {get;} = 0;
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    int I1.this[int x] { get => 0; set => throw null;}
    int I2.this[int x] => 0;
    int I3.this[int x] { get => 0; set => throw null;}
    int I4.this[int x] { get => 0; set => throw null;}
    int I5.this[int x] => 0;
}
";
            ValidatePropertyModifiers_18(source1,
                // (20,44): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     override sealed int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(20, 44),
                // (4,31): error CS0500: 'I1.this[int].get' cannot declare a body because it is marked abstract
                //     abstract int this[int x] {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I1.this[int].get").WithLocation(4, 31),
                // (4,41): error CS0500: 'I1.this[int].set' cannot declare a body because it is marked abstract
                //     abstract int this[int x] {get => 0; set => throw null;} 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I1.this[int].set").WithLocation(4, 41),
                // (8,26): error CS0621: 'I2.this[int]': virtual or abstract members cannot be private
                //     abstract private int this[int x] => 0; 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "this").WithArguments("I2.this[int]").WithLocation(8, 26),
                // (8,41): error CS0500: 'I2.this[int].get' cannot declare a body because it is marked abstract
                //     abstract private int this[int x] => 0; 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "0").WithArguments("I2.this[int].get").WithLocation(8, 41),
                // (12,23): error CS0106: The modifier 'static' is not valid for this item
                //     static extern int this[int x] {get; set;} 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(12, 23),
                // (16,25): error CS0106: The modifier 'static' is not valid for this item
                //     abstract static int this[int x] { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(16, 25),
                // (16,39): error CS0500: 'I4.this[int].get' cannot declare a body because it is marked abstract
                //     abstract static int this[int x] { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I4.this[int].get").WithLocation(16, 39),
                // (16,57): error CS0500: 'I4.this[int].set' cannot declare a body because it is marked abstract
                //     abstract static int this[int x] { get {throw null;} set {throw null;}}
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I4.this[int].set").WithLocation(16, 57),
                // (20,25): error CS0106: The modifier 'override' is not valid for this item
                //     override sealed int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("override").WithLocation(20, 25),
                // (20,38): error CS0501: 'I5.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                //     override sealed int this[int x] {get;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I5.this[int].get").WithLocation(20, 38),
                // (23,15): error CS0535: 'Test1' does not implement interface member 'I1.this[int]'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.this[int]"),
                // (23,19): error CS0535: 'Test1' does not implement interface member 'I2.this[int]'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I2.this[int]"),
                // (23,27): error CS0535: 'Test1' does not implement interface member 'I4.this[int]'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test1", "I4.this[int]").WithLocation(23, 27),
                // (30,12): error CS0122: 'I2.this[int]' is inaccessible due to its protection level
                //     int I2.this[int x] => 0;
                Diagnostic(ErrorCode.ERR_BadAccess, "this").WithArguments("I2.this[int]").WithLocation(30, 12),
                // (33,12): error CS0539: 'Test2.this[int]' in explicit interface declaration is not found among members of the interface that can be implemented
                //     int I5.this[int x] => 0;
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "this").WithArguments("Test2.this[int]").WithLocation(33, 12),
                // (12,36): warning CS0626: Method, operator, or accessor 'I3.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int this[int x] {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I3.this[int].get").WithLocation(12, 36),
                // (12,41): warning CS0626: Method, operator, or accessor 'I3.this[int].set' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int this[int x] {get; set;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "set").WithArguments("I3.this[int].set").WithLocation(12, 41)
                );
        }

        [Fact]
        public void IndexerModifiers_20()
        {
            var source1 =
@"
public interface I1
{
    internal int this[int x]
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {this[0] = this[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidatePropertyModifiers_20(source1, source2, Accessibility.Internal);
        }

        [Fact]
        public void IndexerModifiers_21()
        {
            var source1 =
@"
public interface I1
{
    private int this[int x] { get => throw null; set => throw null; }
}
public interface I2
{
    internal int this[int x] { get => throw null; set => throw null; }
}
public interface I3
{
    public int this[int x] { get => throw null; set => throw null; }
}
public interface I4
{
    int this[int x] { get => throw null; set => throw null; }
}

class Test1
{
    static void Test(I1 i1, I2 i2, I3 i3, I4 i4)
    {
        int x;
        x = i1[0];
        i1[0] = x;
        x = i2[0];
        i2[0] = x;
        x = i3[0];
        i3[0] = x;
        x = i4[0];
        i4[0] = x;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (24,13): error CS0122: 'I1.this[int]' is inaccessible due to its protection level
                //         x = i1[0];
                Diagnostic(ErrorCode.ERR_BadAccess, "i1[0]").WithArguments("I1.this[int]").WithLocation(24, 13),
                // (25,9): error CS0122: 'I1.this[int]' is inaccessible due to its protection level
                //         i1[0] = x;
                Diagnostic(ErrorCode.ERR_BadAccess, "i1[0]").WithArguments("I1.this[int]").WithLocation(25, 9)
                );

            var source2 =
@"
class Test2
{
    static void Test(I1 i1, I2 i2, I3 i3, I4 i4)
    {
        int x;
        x = i1[0];
        i1[0] = x;
        x = i2[0];
        i2[0] = x;
        x = i3[0];
        i3[0] = x;
        x = i4[0];
        i4[0] = x;
    }
}
";
            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (7,13): error CS0122: 'I1.this[int]' is inaccessible due to its protection level
                //         x = i1[0];
                Diagnostic(ErrorCode.ERR_BadAccess, "i1[0]").WithArguments("I1.this[int]").WithLocation(7, 13),
                // (8,9): error CS0122: 'I1.this[int]' is inaccessible due to its protection level
                //         i1[0] = x;
                Diagnostic(ErrorCode.ERR_BadAccess, "i1[0]").WithArguments("I1.this[int]").WithLocation(8, 9),
                // (9,13): error CS0122: 'I2.this[int]' is inaccessible due to its protection level
                //         x = i2[0];
                Diagnostic(ErrorCode.ERR_BadAccess, "i2[0]").WithArguments("I2.this[int]").WithLocation(9, 13),
                // (10,9): error CS0122: 'I2.this[int]' is inaccessible due to its protection level
                //         i2[0] = x;
                Diagnostic(ErrorCode.ERR_BadAccess, "i2[0]").WithArguments("I2.this[int]").WithLocation(10, 9)
                );
        }

        [Fact]
        public void IndexerModifiers_22()
        {
            var source1 =
@"
public interface I1
{
    public int this[int x] 
    {
        internal get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
public interface I2
{
    int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P2"");
            return 0;
        }
        internal set
        {
            System.Console.WriteLine(""set_P2"");
        }
    }
}
public interface I3
{
    int this[int x] 
    {
        internal get => Test1.GetP3();
        set => System.Console.WriteLine(""set_P3"");
    }
}
public interface I4
{
    int this[int x]
    {
        get => Test1.GetP4();
        internal set => System.Console.WriteLine(""set_P4"");
    }
}

class Test1 : I1, I2, I3, I4
{
    static void Main()
    {
        I1 i1 = new Test1();
        I2 i2 = new Test1();
        I3 i3 = new Test1();
        I4 i4 = new Test1();

        i1[0] = i1[0];
        i2[0] = i2[0];
        i3[0] = i3[0];
        i4[0] = i4[0];
    }

    public static int GetP3()
    {
        System.Console.WriteLine(""get_P3"");
        return 0;
    }

    public static int GetP4()
    {
        System.Console.WriteLine(""get_P4"");
        return 0;
    }
}
";

            ValidatePropertyModifiers_22(source1, Accessibility.Internal);
        }

        [Fact]
        public void IndexerModifiers_23_00()
        {
            var source1 =
@"
public interface I1
{
}
public interface I3
{
    int this[int x]
    {
        private get 
        {
            System.Console.WriteLine(""get_P3"");
            return 0;
        } 
        set {System.Console.WriteLine(""set_P3"");}
    }

    void M2()
    {
        this[0] = this[1];
    }
}
public interface I4
{
    int this[int x]
    {
        get {System.Console.WriteLine(""get_P4""); return 0;} 
        private set {System.Console.WriteLine(""set_P4"");}
    }

    void M2()
    {
        this[0] = this[1];
    }
}
public interface I5
{
    int this[int x]
    {
        private get => GetP5();
        set => System.Console.WriteLine(""set_P5"");
    }

    private int GetP5()
    {
        System.Console.WriteLine(""get_P5"");
        return 0;
    }

    void M2()
    {
        this[0] = this[1];
    }
}
public interface I6
{
    int this[int x]
    {
        get => GetP6();
        private set => System.Console.WriteLine(""set_P6"");
    }

    private int GetP6()
    {
        System.Console.WriteLine(""get_P6"");
        return 0;
    }

    void M2()
    {
        this[0] = this[1];
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public int this[int x] 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
class Test5 : I5
{
    public int this[int x] 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2);

            var source3 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public int this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test5 : I5
{
    public int this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source3);

            var source4 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public virtual int this[int x] 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
class Test5 : I5
{
    public virtual int this[int x] 
    {
        get
        {
            throw null;
        }
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
        set
        {
            throw null;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source4);

            var source5 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    public virtual int this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test5 : I5
{
    public virtual int this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source5);

            var source6 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test3();
        I4 i4 = new Test4();
        I5 i5 = new Test5();
        I6 i6 = new Test6();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
class Test3 : I3
{
    int I3.this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test4 : I4
{
    int I4.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test5 : I5
{
    int I5.this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test6 : I6
{
    int I6.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source6);

            var source7 =
@"
class Test1 : I1
{
    static void Main()
    {
        I3 i3 = new Test33();
        I4 i4 = new Test44();
        I5 i5 = new Test55();
        I6 i6 = new Test66();
        i3.M2();
        i4.M2();
        i5.M2();
        i6.M2();
    }
}
interface Test3 : I3
{
    int I3.this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test3.set_P3"");
        }
    }
}
class Test33 : Test3 {}
interface Test4 : I4
{
    int I4.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test4.get_P4"");
            return 0;
        }
    }
}
class Test44 : Test4 {}
interface Test5 : I5
{
    int I5.this[int x] 
    {
        set
        {
            System.Console.WriteLine(""Test5.set_P5"");
        }
    }
}
class Test55 : Test5 {}
interface Test6 : I6
{
    int I6.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test6.get_P6"");
            return 0;
        }
    }
}
class Test66 : Test6 {}
";
            ValidatePropertyModifiers_23(source1, source7);
        }

        [Fact]
        public void IndexerModifiers_23_01()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 

    void M2()
    {
        this[0] = this[1];
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.Internal, Accessibility.Public,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(12, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(12, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_02()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_03()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,12): error CS0122: 'I1.this[int].get' is inaccessible due to its protection level
                //     int I1.this[int x] 
                Diagnostic(ErrorCode.ERR_BadAccess, "this").WithArguments("I1.this[int].get").WithLocation(9, 12)
                );
        }

        [Fact]
        public void IndexerModifiers_23_04()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_05()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_06()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract int this[int x] {get; set;} 
}

class Test3 : Test1
{
    public override int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,38): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(9, 38)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,38): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(9, 38)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_07()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    int this[int x] {get; set;} 
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].get'. 'Test1.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test1", "I1.this[int].get", "Test1.this[int].get").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_08()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_08(source1, source2);
        }

        [Fact]
        public void IndexerModifiers_23_09()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.this[int]'. 'Test1.this[int]' cannot implement 'I1.this[int]' because it does not have the matching return type of 'int'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.this[int]", "Test1.this[int]", "int").WithLocation(2, 15)
                );
        }

        [Fact]
        public void IndexerModifiers_23_10()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (6,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].get'. 'Test2.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.this[int].get", "Test2.this[int].get", "9.0", "10.0").WithLocation(6, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (6,9): error CS9044: 'Test2' does not implement interface member 'I1.this[int].get'. 'Test2.this[int].get' cannot implicitly implement an inaccessible member.
                    //         get
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "get").WithArguments("Test2", "I1.this[int].get", "Test2.this[int].get").WithLocation(6, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_11()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {internal get; set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
public class Test2 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_11(source1, source2,
                // (15,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].get'. 'Test2.this[int].get' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         get
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "get").WithArguments("Test2", "I1.this[int].get", "Test2.this[int].get", "9.0", "10.0").WithLocation(15, 9)
                );
        }

        [Fact]
        public void IndexerModifiers_23_51()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 

    void M2()
    {
        this[0] = this[0];
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";
            ValidatePropertyModifiers_23(source1, source2, Accessibility.Public, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (17,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(17, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (17,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(17, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_52()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_53()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,12): error CS0122: 'I1.this[int].set' is inaccessible due to its protection level
                //     int I1.this[int x] 
                Diagnostic(ErrorCode.ERR_BadAccess, "this").WithArguments("I1.this[int].set").WithLocation(9, 12)
                );
        }

        [Fact]
        public void IndexerModifiers_23_54()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_55()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_56()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract int this[int x] {get; set;} 
}

class Test3 : Test1
{
    public override int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,43): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(9, 43)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,43): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //     public abstract int this[int x] {get; set;} 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(9, 43)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_57()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    int this[int x] {get; set;} 
}
";

            ValidatePropertyModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.this[int].set'. 'Test1.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test1", "I1.this[int].set", "Test1.this[int].set").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_58()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}

public class Test2 : I1
{
    int I1.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test2.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}

";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_08(source1, source2);
        }

        [Fact]
        public void IndexerModifiers_23_59()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 
}

public class TestHelper
{
    public static void CallP1(I1 x) {x[0] = x[0];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual long this[int x] 
    {
        get
        {
            System.Console.WriteLine(""Test1.get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidatePropertyModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.this[int]'. 'Test1.this[int]' cannot implement 'I1.this[int]' because it does not have the matching return type of 'int'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.this[int]", "Test1.this[int]", "int").WithLocation(2, 15)
                );
        }

        [Fact]
        public void IndexerModifiers_23_60()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].set'. 'Test2.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.this[int].set", "Test2.this[int].set", "9.0", "10.0").WithLocation(11, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test2' does not implement interface member 'I1.this[int].set'. 'Test2.this[int].set' cannot implicitly implement an inaccessible member.
                    //         set
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "set").WithArguments("Test2", "I1.this[int].set", "Test2.this[int].set").WithLocation(11, 9)
                    )
                );
        }

        [Fact]
        public void IndexerModifiers_23_61()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[int x] {get; internal set;} 

    sealed void Test()
    {
        this[0] = this[0];
    }
}
public class Test2 : I1
{
    public int this[int x] 
    {
        get
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        set
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidatePropertyModifiers_11_11(source1, source2,
                // (20,9): error CS8704: 'Test2' does not implement interface member 'I1.this[int].set'. 'Test2.this[int].set' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         set
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "set").WithArguments("Test2", "I1.this[int].set", "Test2.this[int].set", "9.0", "10.0").WithLocation(20, 9)
                );
        }

        [Fact]
        public void IndexerModifiers_24()
        {
            var source1 =
@"
public interface I1
{
    int this[int x]
    {
        get 
        {
            System.Console.WriteLine(""get_P1"");
            return 0;
        }
        internal set 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() {this[0] = this[1];}
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";
            ValidatePropertyModifiers_24(source1, source2);
        }

        [Fact]
        public void IndexerModifiers_25()
        {
            var source1 =
@"
public interface I1
{
    int this[int x] { private get => throw null; set => throw null; }
}
public interface I2
{
    int this[int x] { internal get => throw null; set => throw null; }
}
public interface I3
{
    public int this[int x] { get => throw null; private set => throw null; }
}
public interface I4
{
    int this[int x] { get => throw null; internal set => throw null; }
}

public class Test1 : I1, I2, I3, I4
{
    static void Main()
    {
        int x;
        I1 i1 = new Test1();
        I2 i2 = new Test1();
        I3 i3 = new Test1();
        I4 i4 = new Test1();

        x = i1[0];
        i1[0] = x;
        x = i2[0];
        i2[0] = x;
        x = i3[0];
        i3[0] = x;
        x = i4[0];
        i4[0] = x;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (29,13): error CS0271: The property or indexer 'I1.this[int]' cannot be used in this context because the get accessor is inaccessible
                //         x = i1[0];
                Diagnostic(ErrorCode.ERR_InaccessibleGetter, "i1[0]").WithArguments("I1.this[int]").WithLocation(29, 13),
                // (34,9): error CS0272: The property or indexer 'I3.this[int]' cannot be used in this context because the set accessor is inaccessible
                //         i3[0] = x;
                Diagnostic(ErrorCode.ERR_InaccessibleSetter, "i3[0]").WithArguments("I3.this[int]").WithLocation(34, 9)
                );

            var source2 =
@"
class Test2
{
    static void Main()
    {
        int x;
        I1 i1 = new Test1();
        I2 i2 = new Test1();
        I3 i3 = new Test1();
        I4 i4 = new Test1();

        x = i1[0];
        i1[0] = x;
        x = i2[0];
        i2[0] = x;
        x = i3[0];
        i3[0] = x;
        x = i4[0];
        i4[0] = x;
    }
}
";
            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (12,13): error CS0271: The property or indexer 'I1.this[int]' cannot be used in this context because the get accessor is inaccessible
                //         x = i1[0];
                Diagnostic(ErrorCode.ERR_InaccessibleGetter, "i1[0]").WithArguments("I1.this[int]").WithLocation(12, 13),
                // (14,13): error CS0271: The property or indexer 'I2.this[int]' cannot be used in this context because the get accessor is inaccessible
                //         x = i2[0];
                Diagnostic(ErrorCode.ERR_InaccessibleGetter, "i2[0]").WithArguments("I2.this[int]").WithLocation(14, 13),
                // (17,9): error CS0272: The property or indexer 'I3.this[int]' cannot be used in this context because the set accessor is inaccessible
                //         i3[0] = x;
                Diagnostic(ErrorCode.ERR_InaccessibleSetter, "i3[0]").WithArguments("I3.this[int]").WithLocation(17, 9),
                // (19,9): error CS0272: The property or indexer 'I4.this[int]' cannot be used in this context because the set accessor is inaccessible
                //         i4[0] = x;
                Diagnostic(ErrorCode.ERR_InaccessibleSetter, "i4[0]").WithArguments("I4.this[int]").WithLocation(19, 9)
                );
        }

        [Fact]
        public void IndexerModifiers_26()
        {
            var source1 =
@"
public interface I1
{
    abstract int this[sbyte x] { private get; set; }
    abstract int this[byte x] { get; private set; }
    abstract int this[short x] { internal get; }
    int this[ushort x] {internal get;} = 0;
    int this[int x] { internal get {throw null;} }
    int this[uint x] { internal set {throw null;} }
    int this[long x] { internal get => throw null; }
    int this[ulong x] { internal set => throw null; }
    int this[float x] { internal get {throw null;} private set {throw null;}}
    int this[double x] { internal get => throw null; private set => throw null;}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (7,40): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     int this[ushort x] {internal get;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(7, 40),
                // (4,42): error CS0442: 'I1.this[sbyte].get': abstract properties cannot have private accessors
                //     abstract int this[sbyte x] { private get; set; }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "get").WithArguments("I1.this[sbyte].get").WithLocation(4, 42),
                // (5,46): error CS0442: 'I1.this[byte].set': abstract properties cannot have private accessors
                //     abstract int this[byte x] { get; private set; }
                Diagnostic(ErrorCode.ERR_PrivateAbstractAccessor, "set").WithArguments("I1.this[byte].set").WithLocation(5, 46),
                // (6,18): error CS0276: 'I1.this[short]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     abstract int this[short x] { internal get; }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I1.this[short]").WithLocation(6, 18),
                // (7,9): error CS0276: 'I1.this[ushort]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int this[ushort x] {internal get;} = 0;
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I1.this[ushort]").WithLocation(7, 9),
                // (8,9): error CS0276: 'I1.this[int]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int this[int x] { internal get {throw null;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I1.this[int]").WithLocation(8, 9),
                // (9,9): error CS0276: 'I1.this[uint]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int this[uint x] { internal set {throw null;} }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I1.this[uint]").WithLocation(9, 9),
                // (10,9): error CS0276: 'I1.this[long]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int this[long x] { internal get => throw null; }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I1.this[long]").WithLocation(10, 9),
                // (11,9): error CS0276: 'I1.this[ulong]': accessibility modifiers on accessors may only be used if the property or indexer has both a get and a set accessor
                //     int this[ulong x] { internal set => throw null; }
                Diagnostic(ErrorCode.ERR_AccessModMissingAccessor, "this").WithArguments("I1.this[ulong]").WithLocation(11, 9),
                // (12,9): error CS0274: Cannot specify accessibility modifiers for both accessors of the property or indexer 'I1.this[float]'
                //     int this[float x] { internal get {throw null;} private set {throw null;}}
                Diagnostic(ErrorCode.ERR_DuplicatePropertyAccessMods, "this").WithArguments("I1.this[float]").WithLocation(12, 9),
                // (13,9): error CS0274: Cannot specify accessibility modifiers for both accessors of the property or indexer 'I1.this[double]'
                //     int this[double x] { internal get => throw null; private set => throw null;}
                Diagnostic(ErrorCode.ERR_DuplicatePropertyAccessMods, "this").WithArguments("I1.this[double]").WithLocation(13, 9)
                );
        }

        [Fact]
        public void IndexerModifiers_27()
        {
            var source1 =
@"
public interface I1
{
    int this[short x]
    {
        private get {throw null;} 
        set {}
    }

    int this[int x]
    {
        get {throw null;} 
        private set {}
    }
}

class Test1 : I1
{
    int I1.this[short x]
    {
        get {throw null;} 
        set {}
    }

    int I1.this[int x]
    {
        get {throw null;} 
        set {}
    }
}

interface ITest1 : I1
{
    int I1.this[short x]
    {
        get {throw null;} 
        set {}
    }

    int I1.this[int x]
    {
        get {throw null;} 
        set {}
    }
}

public interface I2
{
    int this[short x]
    {
        private get {throw null;} 
        set {}
    }

    int this[int x]
    {
        get {throw null;} 
        private set {}
    }

    class Test3 : I2
    {
        int I2.this[short x]
        {
            get {throw null;} 
            set {}
        }

        int I2.this[int x]
        {
            get {throw null;} 
            set {}
        }
    }

    interface ITest3 : I2
    {
        int I2.this[short x]
        {
            get {throw null;} 
            set {}
        }

        int I2.this[int x]
        {
            get {throw null;} 
            set {}
        }
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (21,9): error CS0550: 'Test1.I1.this[short].get' adds an accessor not found in interface member 'I1.this[short]'
                //         get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("Test1.I1.this[short].get", "I1.this[short]").WithLocation(21, 9),
                // (28,9): error CS0550: 'Test1.I1.this[int].set' adds an accessor not found in interface member 'I1.this[int]'
                //         set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("Test1.I1.this[int].set", "I1.this[int]").WithLocation(28, 9),
                // (36,9): error CS0550: 'ITest1.I1.this[short].get' adds an accessor not found in interface member 'I1.this[short]'
                //         get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("ITest1.I1.this[short].get", "I1.this[short]").WithLocation(36, 9),
                // (43,9): error CS0550: 'ITest1.I1.this[int].set' adds an accessor not found in interface member 'I1.this[int]'
                //         set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("ITest1.I1.this[int].set", "I1.this[int]").WithLocation(43, 9),
                // (65,13): error CS0550: 'I2.Test3.I2.this[short].get' adds an accessor not found in interface member 'I2.this[short]'
                //             get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I2.Test3.I2.this[short].get", "I2.this[short]").WithLocation(65, 13),
                // (72,13): error CS0550: 'I2.Test3.I2.this[int].set' adds an accessor not found in interface member 'I2.this[int]'
                //             set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I2.Test3.I2.this[int].set", "I2.this[int]").WithLocation(72, 13),
                // (80,13): error CS0550: 'I2.ITest3.I2.this[short].get' adds an accessor not found in interface member 'I2.this[short]'
                //             get {throw null;} 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I2.ITest3.I2.this[short].get", "I2.this[short]").WithLocation(80, 13),
                // (87,13): error CS0550: 'I2.ITest3.I2.this[int].set' adds an accessor not found in interface member 'I2.this[int]'
                //             set {}
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I2.ITest3.I2.this[int].set", "I2.this[int]").WithLocation(87, 13)
                );
        }

        [Fact]
        public void EventModifiers_01()
        {
            var source1 =
@"
public interface I1
{
    public event System.Action P01;
    protected event System.Action P02 {add{}}
    protected internal event System.Action P03 {remove{}}
    internal event System.Action P04 {add{}}
    private event System.Action P05 {remove{}}
    static event System.Action P06 {add{}}
    virtual event System.Action P07 {remove{}}
    sealed event System.Action P08 {add{}}
    override event System.Action P09 {remove{}}
    abstract event System.Action P10 {add{}}
    extern event System.Action P11 {add{} remove{}}
    extern event System.Action P12 {add; remove;}
    extern event System.Action P13;
    private protected event System.Action P14;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.GetDiagnostics().Where(d => d.Code != (int)ErrorCode.ERR_EventNeedsBothAccessors).Verify(
                // (12,34): error CS0106: The modifier 'override' is not valid for this item
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 34),
                // (13,38): error CS8712: 'I1.P10': abstract event cannot use event accessor syntax
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.P10").WithLocation(13, 38),
                // (14,37): error CS0179: 'I1.P11.add' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "add").WithArguments("I1.P11.add").WithLocation(14, 37),
                // (14,43): error CS0179: 'I1.P11.remove' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "remove").WithArguments("I1.P11.remove").WithLocation(14, 43),
                // (15,37): warning CS0626: Method, operator, or accessor 'I1.P12.add' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "add").WithArguments("I1.P12.add").WithLocation(15, 37),
                // (15,40): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 40),
                // (15,42): warning CS0626: Method, operator, or accessor 'I1.P12.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "remove").WithArguments("I1.P12.remove").WithLocation(15, 42),
                // (15,48): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 48),
                // (16,32): warning CS0626: Method, operator, or accessor 'I1.P13.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P13").WithArguments("I1.P13.remove").WithLocation(16, 32)
                );

            ValidateSymbolsEventModifiers_01(compilation1);
        }

        private static void ValidateSymbolsEventModifiers_01(CSharpCompilation compilation1)
        {
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p01 = i1.GetMember<EventSymbol>("P01");

            Assert.True(p01.IsAbstract);
            Assert.False(p01.IsVirtual);
            Assert.False(p01.IsSealed);
            Assert.False(p01.IsStatic);
            Assert.False(p01.IsExtern);
            Assert.False(p01.IsOverride);
            Assert.Equal(Accessibility.Public, p01.DeclaredAccessibility);

            ValidateP01Accessor(p01.AddMethod);
            ValidateP01Accessor(p01.RemoveMethod);
            void ValidateP01Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }

            var p02 = i1.GetMember<EventSymbol>("P02");
            var p02get = p02.AddMethod;

            Assert.False(p02.IsAbstract);
            Assert.True(p02.IsVirtual);
            Assert.False(p02.IsSealed);
            Assert.False(p02.IsStatic);
            Assert.False(p02.IsExtern);
            Assert.False(p02.IsOverride);
            Assert.Equal(Accessibility.Protected, p02.DeclaredAccessibility);

            Assert.False(p02get.IsAbstract);
            Assert.True(p02get.IsVirtual);
            Assert.True(p02get.IsMetadataVirtual());
            Assert.False(p02get.IsSealed);
            Assert.False(p02get.IsStatic);
            Assert.False(p02get.IsExtern);
            Assert.False(p02get.IsAsync);
            Assert.False(p02get.IsOverride);
            Assert.Equal(Accessibility.Protected, p02get.DeclaredAccessibility);

            var p03 = i1.GetMember<EventSymbol>("P03");
            var p03set = p03.RemoveMethod;

            Assert.False(p03.IsAbstract);
            Assert.True(p03.IsVirtual);
            Assert.False(p03.IsSealed);
            Assert.False(p03.IsStatic);
            Assert.False(p03.IsExtern);
            Assert.False(p03.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, p03.DeclaredAccessibility);

            Assert.False(p03set.IsAbstract);
            Assert.True(p03set.IsVirtual);
            Assert.True(p03set.IsMetadataVirtual());
            Assert.False(p03set.IsSealed);
            Assert.False(p03set.IsStatic);
            Assert.False(p03set.IsExtern);
            Assert.False(p03set.IsAsync);
            Assert.False(p03set.IsOverride);
            Assert.Equal(Accessibility.ProtectedOrInternal, p03set.DeclaredAccessibility);

            var p04 = i1.GetMember<EventSymbol>("P04");
            var p04get = p04.AddMethod;

            Assert.False(p04.IsAbstract);
            Assert.True(p04.IsVirtual);
            Assert.False(p04.IsSealed);
            Assert.False(p04.IsStatic);
            Assert.False(p04.IsExtern);
            Assert.False(p04.IsOverride);
            Assert.Equal(Accessibility.Internal, p04.DeclaredAccessibility);

            Assert.False(p04get.IsAbstract);
            Assert.True(p04get.IsVirtual);
            Assert.True(p04get.IsMetadataVirtual());
            Assert.False(p04get.IsSealed);
            Assert.False(p04get.IsStatic);
            Assert.False(p04get.IsExtern);
            Assert.False(p04get.IsAsync);
            Assert.False(p04get.IsOverride);
            Assert.Equal(Accessibility.Internal, p04get.DeclaredAccessibility);

            var p05 = i1.GetMember<EventSymbol>("P05");
            var p05set = p05.RemoveMethod;

            Assert.False(p05.IsAbstract);
            Assert.False(p05.IsVirtual);
            Assert.False(p05.IsSealed);
            Assert.False(p05.IsStatic);
            Assert.False(p05.IsExtern);
            Assert.False(p05.IsOverride);
            Assert.Equal(Accessibility.Private, p05.DeclaredAccessibility);

            Assert.False(p05set.IsAbstract);
            Assert.False(p05set.IsVirtual);
            Assert.False(p05set.IsMetadataVirtual());
            Assert.False(p05set.IsSealed);
            Assert.False(p05set.IsStatic);
            Assert.False(p05set.IsExtern);
            Assert.False(p05set.IsAsync);
            Assert.False(p05set.IsOverride);
            Assert.Equal(Accessibility.Private, p05set.DeclaredAccessibility);

            var p06 = i1.GetMember<EventSymbol>("P06");
            var p06get = p06.AddMethod;

            Assert.False(p06.IsAbstract);
            Assert.False(p06.IsVirtual);
            Assert.False(p06.IsSealed);
            Assert.True(p06.IsStatic);
            Assert.False(p06.IsExtern);
            Assert.False(p06.IsOverride);
            Assert.Equal(Accessibility.Public, p06.DeclaredAccessibility);

            Assert.False(p06get.IsAbstract);
            Assert.False(p06get.IsVirtual);
            Assert.False(p06get.IsMetadataVirtual());
            Assert.False(p06get.IsSealed);
            Assert.True(p06get.IsStatic);
            Assert.False(p06get.IsExtern);
            Assert.False(p06get.IsAsync);
            Assert.False(p06get.IsOverride);
            Assert.Equal(Accessibility.Public, p06get.DeclaredAccessibility);

            var p07 = i1.GetMember<EventSymbol>("P07");
            var p07set = p07.RemoveMethod;

            Assert.False(p07.IsAbstract);
            Assert.True(p07.IsVirtual);
            Assert.False(p07.IsSealed);
            Assert.False(p07.IsStatic);
            Assert.False(p07.IsExtern);
            Assert.False(p07.IsOverride);
            Assert.Equal(Accessibility.Public, p07.DeclaredAccessibility);

            Assert.False(p07set.IsAbstract);
            Assert.True(p07set.IsVirtual);
            Assert.True(p07set.IsMetadataVirtual());
            Assert.False(p07set.IsSealed);
            Assert.False(p07set.IsStatic);
            Assert.False(p07set.IsExtern);
            Assert.False(p07set.IsAsync);
            Assert.False(p07set.IsOverride);
            Assert.Equal(Accessibility.Public, p07set.DeclaredAccessibility);

            var p08 = i1.GetMember<EventSymbol>("P08");
            var p08get = p08.AddMethod;

            Assert.False(p08.IsAbstract);
            Assert.False(p08.IsVirtual);
            Assert.False(p08.IsSealed);
            Assert.False(p08.IsStatic);
            Assert.False(p08.IsExtern);
            Assert.False(p08.IsOverride);
            Assert.Equal(Accessibility.Public, p08.DeclaredAccessibility);

            Assert.False(p08get.IsAbstract);
            Assert.False(p08get.IsVirtual);
            Assert.False(p08get.IsMetadataVirtual());
            Assert.False(p08get.IsSealed);
            Assert.False(p08get.IsStatic);
            Assert.False(p08get.IsExtern);
            Assert.False(p08get.IsAsync);
            Assert.False(p08get.IsOverride);
            Assert.Equal(Accessibility.Public, p08get.DeclaredAccessibility);

            var p09 = i1.GetMember<EventSymbol>("P09");
            var p09set = p09.RemoveMethod;

            Assert.False(p09.IsAbstract);
            Assert.True(p09.IsVirtual);
            Assert.False(p09.IsSealed);
            Assert.False(p09.IsStatic);
            Assert.False(p09.IsExtern);
            Assert.False(p09.IsOverride);
            Assert.Equal(Accessibility.Public, p09.DeclaredAccessibility);

            Assert.False(p09set.IsAbstract);
            Assert.True(p09set.IsVirtual);
            Assert.True(p09set.IsMetadataVirtual());
            Assert.False(p09set.IsSealed);
            Assert.False(p09set.IsStatic);
            Assert.False(p09set.IsExtern);
            Assert.False(p09set.IsAsync);
            Assert.False(p09set.IsOverride);
            Assert.Equal(Accessibility.Public, p09set.DeclaredAccessibility);

            var p10 = i1.GetMember<EventSymbol>("P10");
            var p10get = p10.AddMethod;

            Assert.True(p10.IsAbstract);
            Assert.False(p10.IsVirtual);
            Assert.False(p10.IsSealed);
            Assert.False(p10.IsStatic);
            Assert.False(p10.IsExtern);
            Assert.False(p10.IsOverride);
            Assert.Equal(Accessibility.Public, p10.DeclaredAccessibility);

            Assert.True(p10get.IsAbstract);
            Assert.False(p10get.IsVirtual);
            Assert.True(p10get.IsMetadataVirtual());
            Assert.False(p10get.IsSealed);
            Assert.False(p10get.IsStatic);
            Assert.False(p10get.IsExtern);
            Assert.False(p10get.IsAsync);
            Assert.False(p10get.IsOverride);
            Assert.Equal(Accessibility.Public, p10get.DeclaredAccessibility);

            foreach (var name in new[] { "P11", "P12", "P13" })
            {
                var p11 = i1.GetMember<EventSymbol>(name);

                Assert.False(p11.IsAbstract);
                Assert.True(p11.IsVirtual);
                Assert.False(p11.IsSealed);
                Assert.False(p11.IsStatic);
                Assert.True(p11.IsExtern);
                Assert.False(p11.IsOverride);
                Assert.Equal(Accessibility.Public, p11.DeclaredAccessibility);

                ValidateP11Accessor(p11.AddMethod);
                ValidateP11Accessor(p11.RemoveMethod);
                void ValidateP11Accessor(MethodSymbol accessor)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.True(accessor.IsVirtual);
                    Assert.True(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.True(accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                }
            }

            var p14 = i1.GetMember<EventSymbol>("P14");

            Assert.True(p14.IsAbstract);
            Assert.False(p14.IsVirtual);
            Assert.False(p14.IsSealed);
            Assert.False(p14.IsStatic);
            Assert.False(p14.IsExtern);
            Assert.False(p14.IsOverride);
            Assert.Equal(Accessibility.ProtectedAndInternal, p14.DeclaredAccessibility);

            ValidateP14Accessor(p14.AddMethod);
            ValidateP14Accessor(p14.RemoveMethod);
            void ValidateP14Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.ProtectedAndInternal, accessor.DeclaredAccessibility);
            }

        }

        [Fact]
        public void EventModifiers_02()
        {
            var source1 =
@"
public interface I1
{
    public event System.Action P01;
    protected event System.Action P02 {add{}}
    protected internal event System.Action P03 {remove{}}
    internal event System.Action P04 {add{}}
    private event System.Action P05 {remove{}}
    static event System.Action P06 {add{}}
    virtual event System.Action P07 {remove{}}
    sealed event System.Action P08 {add{}}
    override event System.Action P09 {remove{}}
    abstract event System.Action P10 {add{}}
    extern event System.Action P11 {add{} remove{}}
    extern event System.Action P12 {add; remove;}
    extern event System.Action P13;
    private protected event System.Action P14;
    protected event System.Action P15;
    protected internal event System.Action P16;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.GetDiagnostics().Where(d => d.Code != (int)ErrorCode.ERR_EventNeedsBothAccessors).Verify(
                // (4,32): error CS8703: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public event System.Action P01;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P01").WithArguments("public", "7.3", "8.0").WithLocation(4, 32),
                // (5,40): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     protected event System.Action P02 {add{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(5, 40),
                // (6,49): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     protected internal event System.Action P03 {remove{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(6, 49),
                // (7,39): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     internal event System.Action P04 {add{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(7, 39),
                // (8,38): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     private event System.Action P05 {remove{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(8, 38),
                // (9,32): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static event System.Action P06 {add{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "P06").WithArguments("default interface implementation", "8.0").WithLocation(9, 32),
                // (10,38): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     virtual event System.Action P07 {remove{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(10, 38),
                // (11,37): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     sealed event System.Action P08 {add{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(11, 37),
                // (12,34): error CS0106: The modifier 'override' is not valid for this item
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 34),
                // (12,39): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(12, 39),
                // (13,39): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(13, 39),
                // (13,38): error CS8712: 'I1.P10': abstract event cannot use event accessor syntax
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.P10").WithLocation(13, 38),
                // (14,37): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(14, 37),
                // (14,37): error CS0179: 'I1.P11.add' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "add").WithArguments("I1.P11.add").WithLocation(14, 37),
                // (14,43): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(14, 43),
                // (14,43): error CS0179: 'I1.P11.remove' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "remove").WithArguments("I1.P11.remove").WithLocation(14, 43),
                // (15,37): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(15, 37),
                // (15,37): warning CS0626: Method, operator, or accessor 'I1.P12.add' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "add").WithArguments("I1.P12.add").WithLocation(15, 37),
                // (15,40): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 40),
                // (15,42): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(15, 42),
                // (15,42): warning CS0626: Method, operator, or accessor 'I1.P12.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "remove").WithArguments("I1.P12.remove").WithLocation(15, 42),
                // (15,48): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 48),
                // (16,32): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P13").WithArguments("extern", "7.3", "8.0").WithLocation(16, 32),
                // (16,32): warning CS0626: Method, operator, or accessor 'I1.P13.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P13").WithArguments("I1.P13.remove").WithLocation(16, 32),
                // (17,43): error CS8703: The modifier 'private protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private protected event System.Action P14;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P14").WithArguments("private protected", "7.3", "8.0").WithLocation(17, 43),
                // (18,35): error CS8703: The modifier 'protected' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     protected event System.Action P15;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P15").WithArguments("protected", "7.3", "8.0").WithLocation(18, 35),
                // (19,44): error CS8703: The modifier 'protected internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     protected internal event System.Action P16;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P16").WithArguments("protected internal", "7.3", "8.0").WithLocation(19, 44)
                );

            ValidateSymbolsEventModifiers_01(compilation1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);
            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.GetDiagnostics().Where(d => d.Code != (int)ErrorCode.ERR_EventNeedsBothAccessors).Verify(
                // (5,35): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected event System.Action P02 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P02").WithLocation(5, 35),
                // (5,40): error CS8701: Target runtime doesn't support default interface implementation.
                //     protected event System.Action P02 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(5, 40),
                // (6,44): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal event System.Action P03 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P03").WithLocation(6, 44),
                // (6,49): error CS8701: Target runtime doesn't support default interface implementation.
                //     protected internal event System.Action P03 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(6, 49),
                // (7,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     internal event System.Action P04 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(7, 39),
                // (8,38): error CS8701: Target runtime doesn't support default interface implementation.
                //     private event System.Action P05 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(8, 38),
                // (9,37): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static event System.Action P06 {add{}}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "add").WithArguments("static members in interfaces", "15.0").WithLocation(9, 37),
                // (10,38): error CS8701: Target runtime doesn't support default interface implementation.
                //     virtual event System.Action P07 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(10, 38),
                // (11,37): error CS8701: Target runtime doesn't support default interface implementation.
                //     sealed event System.Action P08 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(11, 37),
                // (12,34): error CS0106: The modifier 'override' is not valid for this item
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 34),
                // (12,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(12, 39),
                // (13,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(13, 39),
                // (13,38): error CS8712: 'I1.P10': abstract event cannot use event accessor syntax
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.P10").WithLocation(13, 38),
                // (14,37): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(14, 37),
                // (14,37): error CS0179: 'I1.P11.add' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "add").WithArguments("I1.P11.add").WithLocation(14, 37),
                // (14,43): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(14, 43),
                // (14,43): error CS0179: 'I1.P11.remove' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "remove").WithArguments("I1.P11.remove").WithLocation(14, 43),
                // (15,37): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(15, 37),
                // (15,37): warning CS0626: Method, operator, or accessor 'I1.P12.add' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "add").WithArguments("I1.P12.add").WithLocation(15, 37),
                // (15,40): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 40),
                // (15,42): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(15, 42),
                // (15,42): warning CS0626: Method, operator, or accessor 'I1.P12.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "remove").WithArguments("I1.P12.remove").WithLocation(15, 42),
                // (15,48): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 48),
                // (16,32): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "P13").WithLocation(16, 32),
                // (16,32): warning CS0626: Method, operator, or accessor 'I1.P13.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P13").WithArguments("I1.P13.remove").WithLocation(16, 32),
                // (17,43): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected event System.Action P14;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P14").WithLocation(17, 43),
                // (18,35): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected event System.Action P15;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P15").WithLocation(18, 35),
                // (19,44): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal event System.Action P16;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P16").WithLocation(19, 44)
                );

            ValidateSymbolsEventModifiers_01(compilation2);

            var expected = new[]
            {
                // (5,35): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected event System.Action P02 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P02").WithLocation(5, 35),
                // (5,40): error CS8701: Target runtime doesn't support default interface implementation.
                //     protected event System.Action P02 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(5, 40),
                // (6,44): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal event System.Action P03 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P03").WithLocation(6, 44),
                // (6,49): error CS8701: Target runtime doesn't support default interface implementation.
                //     protected internal event System.Action P03 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(6, 49),
                // (7,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     internal event System.Action P04 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(7, 39),
                // (8,38): error CS8701: Target runtime doesn't support default interface implementation.
                //     private event System.Action P05 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(8, 38),
                // (10,38): error CS8701: Target runtime doesn't support default interface implementation.
                //     virtual event System.Action P07 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(10, 38),
                // (11,37): error CS8701: Target runtime doesn't support default interface implementation.
                //     sealed event System.Action P08 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(11, 37),
                // (12,34): error CS0106: The modifier 'override' is not valid for this item
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P09").WithArguments("override").WithLocation(12, 34),
                // (12,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     override event System.Action P09 {remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(12, 39),
                // (13,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(13, 39),
                // (13,38): error CS8712: 'I1.P10': abstract event cannot use event accessor syntax
                //     abstract event System.Action P10 {add{}}
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.P10").WithLocation(13, 38),
                // (14,37): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(14, 37),
                // (14,37): error CS0179: 'I1.P11.add' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "add").WithArguments("I1.P11.add").WithLocation(14, 37),
                // (14,43): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(14, 43),
                // (14,43): error CS0179: 'I1.P11.remove' cannot be extern and declare a body
                //     extern event System.Action P11 {add{} remove{}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "remove").WithArguments("I1.P11.remove").WithLocation(14, 43),
                // (15,37): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(15, 37),
                // (15,37): warning CS0626: Method, operator, or accessor 'I1.P12.add' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "add").WithArguments("I1.P12.add").WithLocation(15, 37),
                // (15,40): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 40),
                // (15,42): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(15, 42),
                // (15,42): warning CS0626: Method, operator, or accessor 'I1.P12.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "remove").WithArguments("I1.P12.remove").WithLocation(15, 42),
                // (15,48): error CS0073: An add or remove accessor must have a body
                //     extern event System.Action P12 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(15, 48),
                // (16,32): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "P13").WithLocation(16, 32),
                // (16,32): warning CS0626: Method, operator, or accessor 'I1.P13.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P13;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P13").WithArguments("I1.P13.remove").WithLocation(16, 32),
                // (17,43): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected event System.Action P14;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P14").WithLocation(17, 43),
                // (18,35): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected event System.Action P15;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P15").WithLocation(18, 35),
                // (19,44): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal event System.Action P16;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "P16").WithLocation(19, 44)
            };

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);
            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.GetDiagnostics().Where(d => d.Code != (int)ErrorCode.ERR_EventNeedsBothAccessors).Verify(expected);

            ValidateSymbolsEventModifiers_01(compilation2);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);
            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.GetDiagnostics().Where(d => d.Code != (int)ErrorCode.ERR_EventNeedsBothAccessors).Verify(expected);

            ValidateSymbolsEventModifiers_01(compilation2);
        }

        [Fact]
        public void EventModifiers_03()
        {
            ValidateEventImplementation_102(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        add => System.Console.WriteLine(""add E1"");
        remove => System.Console.WriteLine(""remove E1"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1.E1 += null;
        i1.E1 -= null;
    }
}
");
            ValidateEventImplementation_102(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        add {System.Console.WriteLine(""add E1"");}
        remove {System.Console.WriteLine(""remove E1"");}
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1.E1 += null;
        i1.E1 -= null;
    }
}
");
        }

        [Fact]
        public void EventModifiers_04()
        {
            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 {}
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 {}
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: false, haveRemove: false);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        add;
    }
}

class Test1 : I1
{}
",
                new[] {
                // (6,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";"),
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: true, haveRemove: false);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        add {}
    }
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: true, haveRemove: false);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        add => throw null;
    }
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: true, haveRemove: false);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        remove;
    }
}

class Test1 : I1
{}
",
                new[] {
                // (6,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(6, 15),
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: false, haveRemove: true);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        remove {}
    }
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: false, haveRemove: true);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        remove => throw null;
    }
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: false, haveRemove: true);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 
    {
        add;
        remove;
    }
}

class Test1 : I1
{}
",
                new[] {
                // (6,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";"),
                // (7,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(7, 15)
                },
                haveAdd: true, haveRemove: true);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1;
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1;
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40),
                // (4,40): warning CS0067: The event 'I1.E1' is never used
                //     public virtual event System.Action E1;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: true, haveRemove: true);

            ValidateEventImplementation_101(@"
public interface I1
{
    public virtual event System.Action E1 = null;
}

class Test1 : I1
{}
",
                new[] {
                // (4,40): error CS0068: 'I1.E1': instance event in interface cannot have initializer
                //     public virtual event System.Action E1 = null;
                Diagnostic(ErrorCode.ERR_InterfaceEventInitializer, "E1").WithArguments("I1.E1").WithLocation(4, 40),
                // (4,40): error CS0065: 'I1.E1': event property must have both add and remove accessors
                //     public virtual event System.Action E1 = null;
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "E1").WithArguments("I1.E1").WithLocation(4, 40),
                // (4,40): warning CS0067: The event 'I1.E1' is never used
                //     public virtual event System.Action E1 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "E1").WithArguments("I1.E1").WithLocation(4, 40)
                },
                haveAdd: true, haveRemove: true);
        }

        [Fact]
        public void EventModifiers_05()
        {
            var source1 =
@"
public interface I1
{
    public abstract event System.Action P1; 
}
public interface I2
{
    event System.Action P2;
}

class Test1 : I1
{
    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove => System.Console.WriteLine(""set_P1"");
    }
}
class Test2 : I2
{
    public event System.Action P2 
    {
        add
        {
            System.Console.WriteLine(""get_P2"");
        }
        remove => System.Console.WriteLine(""set_P2"");
    }

    static void Main()
    {
        I1 x = new Test1();
        x.P1 += null;
        x.P1 -= null;
        I2 y = new Test2();
        y.P2 += null;
        y.P2 -= null;
    }
}
";

            ValidateEventModifiers_05(source1);
        }

        private void ValidateEventModifiers_05(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput:
@"get_P1
set_P1
get_P2
set_P2", symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                for (int i = 1; i <= 2; i++)
                {
                    var test1 = m.GlobalNamespace.GetTypeMember("Test" + i);
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleEvent(i1);
                    var test1P1 = GetSingleEvent(test1);

                    Assert.True(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                    Assert.Same(test1P1, test1.FindImplementationForInterfaceMember(p1));

                    ValidateAccessor(p1.AddMethod, test1P1.AddMethod);
                    ValidateAccessor(p1.RemoveMethod, test1P1.RemoveMethod);

                    void ValidateAccessor(MethodSymbol accessor, MethodSymbol implementation)
                    {
                        Assert.True(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.True(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                        Assert.Same(implementation, test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        private static EventSymbol GetSingleEvent(NamedTypeSymbol container)
        {
            return container.GetMembers().OfType<EventSymbol>().Single();
        }

        private static EventSymbol GetSingleEvent(CSharpCompilation compilation, string containerName)
        {
            return GetSingleEvent(compilation.GetTypeByMetadataName(containerName));
        }

        private static EventSymbol GetSingleEvent(ModuleSymbol m, string containerName)
        {
            return GetSingleEvent(m.GlobalNamespace.GetTypeMember(containerName));
        }

        [Fact]
        public void EventModifiers_06()
        {
            var source1 =
@"
public interface I1
{
    public abstract event System.Action P1; 
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation1.VerifyDiagnostics(
                // (4,41): error CS8503: The modifier 'abstract' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract event System.Action P1; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("abstract", "7.3", "8.0").WithLocation(4, 41),
                // (4,41): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public abstract event System.Action P1; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("public", "7.3", "8.0").WithLocation(4, 41)
                );

            ValidateEventModifiers_06(compilation1);
        }

        private static void ValidateEventModifiers_06(CSharpCompilation compilation1)
        {
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<EventSymbol>("P1");

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);

            ValidateAccessor(p1.AddMethod);
            ValidateAccessor(p1.RemoveMethod);

            void ValidateAccessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
            }
        }

        [Fact]
        public void EventModifiers_07()
        {
            var source1 =
@"
public interface I1
{
    public static event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    internal static event System.Action P2 
    {
        add
        {
            System.Console.WriteLine(""get_P2"");
            P3 += value;
        }
        remove
        {
            System.Console.WriteLine(""set_P2"");
            P3 -= value;
        }
    }

    private static event System.Action P3 
    {
        add => System.Console.WriteLine(""get_P3"");
        remove => System.Console.WriteLine(""set_P3"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.P1 += null; 
        I1.P1 -= null;
        I1.P2 += null;
        I1.P2 -= null;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            var expected =
@"get_P1
set_P1
get_P2
get_P3
set_P2
set_P3";

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expected, symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var tuple in new[] { (name: "P1", access: Accessibility.Public),
                                              (name: "P2", access: Accessibility.Internal),
                                              (name: "P3", access: Accessibility.Private)})
                {
                    var p1 = i1.GetMember<EventSymbol>(tuple.name);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.True(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(tuple.access, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    ValidateAccessor(p1.AddMethod);
                    ValidateAccessor(p1.RemoveMethod);

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.True(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(tuple.access, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }

            var source2 =
@"
public interface I1
{
    public static event System.Action P1; 

    internal static event System.Action P2 
    {
        add;
        remove;
    }

    private static event System.Action P3 = null;
}

class Test1 : I1
{
    static void Main()
    {
    }
}
";
            var compilation2 = CreateCompilation(source2, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyEmitDiagnostics(
                // (4,39): warning CS0067: The event 'I1.P1' is never used
                //     public static event System.Action P1;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P1").WithArguments("I1.P1").WithLocation(4, 39),
                // (8,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(8, 12),
                // (9,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(9, 15),
                // (12,40): warning CS0414: The field 'I1.P3' is assigned but its value is never used
                //     private static event System.Action P3 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedFieldAssg, "P3").WithArguments("I1.P3").WithLocation(12, 40)
                );

            Validate(compilation2.SourceModule);

            var compilation3 = CreateCompilation(source2, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular14, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation3.VerifyEmitDiagnostics(
                // (4,39): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static event System.Action P1; 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "P1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 39),
                // (4,39): warning CS0067: The event 'I1.P1' is never used
                //     public static event System.Action P1; 
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P1").WithArguments("I1.P1").WithLocation(4, 39),
                // (8,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         add;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "add").WithArguments("static members in interfaces", "15.0").WithLocation(8, 9),
                // (8,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(8, 12),
                // (9,9): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //         remove;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "remove").WithArguments("static members in interfaces", "15.0").WithLocation(9, 9),
                // (9,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(9, 15),
                // (12,40): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static event System.Action P3 = null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "P3").WithArguments("static members in interfaces", "15.0").WithLocation(12, 40),
                // (12,40): warning CS0414: The field 'I1.P3' is assigned but its value is never used
                //     private static event System.Action P3 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedFieldAssg, "P3").WithArguments("I1.P3").WithLocation(12, 40)
                );

            Validate(compilation3.SourceModule);

            var expected2 = new[]
            {
                // (4,39): warning CS0067: The event 'I1.P1' is never used
                //     public static event System.Action P1; 
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P1").WithArguments("I1.P1").WithLocation(4, 39),
                // (8,12): error CS0073: An add or remove accessor must have a body
                //         add;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(8, 12),
                // (9,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(9, 15),
                // (12,40): warning CS0414: The field 'I1.P3' is assigned but its value is never used
                //     private static event System.Action P3 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedFieldAssg, "P3").WithArguments("I1.P3").WithLocation(12, 40)
            };

            compilation3 = CreateCompilation(source2, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation3.VerifyEmitDiagnostics(expected2);

            Validate(compilation3.SourceModule);

            compilation3 = CreateCompilation(source2, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation3.VerifyEmitDiagnostics(expected2);

            Validate(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation4, expectedOutput: expected, symbolValidator: Validate);

            Validate(compilation4.SourceModule);

            compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation4, expectedOutput: expected, symbolValidator: Validate);

            Validate(compilation4.SourceModule);
        }

        [Fact]
        public void EventModifiers_08()
        {
            var source1 =
@"
public interface I1
{
    abstract static event System.Action P1; 

    virtual static event System.Action P2 {add {} remove{}} 
    
    sealed static event System.Action P3 {add; remove;}
}

class Test1 : I1
{
    event System.Action I1.P1 {add {} remove{}} 
    event System.Action I1.P2 {add {} remove{}} 
    event System.Action I1.P3 {add {} remove{}} 
}

class Test2 : I1
{}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (4,41): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     abstract static event System.Action P1; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("abstract", "9.0", "11.0").WithLocation(4, 41),
                // (6,40): error CS8703: The modifier 'virtual' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     virtual static event System.Action P2 {add {} remove{}} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P2").WithArguments("virtual", "9.0", "11.0").WithLocation(6, 40),
                // (8,39): error CS8703: The modifier 'sealed' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     sealed static event System.Action P3 {add; remove;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("sealed", "9.0", "11.0").WithLocation(8, 39),
                // (8,46): error CS0073: An add or remove accessor must have a body
                //     sealed static event System.Action P3 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(8, 46),
                // (8,54): error CS0073: An add or remove accessor must have a body
                //     sealed static event System.Action P3 {add; remove;}
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(8, 54),
                // (13,28): error CS0539: 'Test1.P1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I1.P1 {add {} remove{}} 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P1").WithArguments("Test1.P1").WithLocation(13, 28),
                // (14,28): error CS0539: 'Test1.P2' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I1.P2 {add {} remove{}} 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P2").WithArguments("Test1.P2").WithLocation(14, 28),
                // (15,28): error CS0539: 'Test1.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I1.P3 {add {} remove{}} 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test1.P3").WithLocation(15, 28),
                // (18,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(18, 15),
                // (11,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(11, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<EventSymbol>("P1");

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.True(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));

            ValidateAccessor1(p1.AddMethod);
            ValidateAccessor1(p1.RemoveMethod);
            void ValidateAccessor1(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.True(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
            }

            var p2 = i1.GetMember<EventSymbol>("P2");

            Assert.False(p2.IsAbstract);
            Assert.True(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.True(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
            Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));

            ValidateAccessor2(p2.AddMethod);
            ValidateAccessor2(p2.RemoveMethod);
            void ValidateAccessor2(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.True(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Same(accessor, test1.FindImplementationForInterfaceMember(accessor));
            }

            var p3 = i1.GetMember<EventSymbol>("P3");

            Assert.False(p3.IsAbstract);
            Assert.False(p3.IsVirtual);
            Assert.False(p3.IsSealed);
            Assert.True(p3.IsStatic);
            Assert.False(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));

            ValidateAccessor3(p3.AddMethod);
            ValidateAccessor3(p3.RemoveMethod);
            void ValidateAccessor3(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.True(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
            }
        }

        [Fact]
        public void EventModifiers_09()
        {
            var source1 =
@"
public interface I1
{
    private event System.Action P1 
    {
        add
        { 
            System.Console.WriteLine(""get_P1"");
        }           
        remove
        { 
            System.Console.WriteLine(""set_P1"");
        }           
    }
    sealed void M()
    {
        P1 += null;
        P1 -= null;
    }
}
public interface I2
{
    private event System.Action P2 
    {
        add => System.Console.WriteLine(""get_P2"");
        remove => System.Console.WriteLine(""set_P2"");
    }

    sealed void M()
    {
        P2 += null;
        P2 -= null;
    }
}

class Test1 : I1, I2
{
    static void Main()
    {
        I1 x1 = new Test1();
        x1.M();
        I2 x2 = new Test1();
        x2.M();
    }
}
";

            ValidateEventModifiers_09(source1);
        }

        private void ValidateEventModifiers_09(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");

                for (int i = 1; i <= 2; i++)
                {
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleEvent(i1);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Private, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    ValidateAccessor(p1.AddMethod);
                    ValidateAccessor(p1.RemoveMethod);

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        [Fact]
        public void EventModifiers_10()
        {
            var source1 =
@"
public interface I1
{
    abstract private event System.Action P1; 

    virtual private event System.Action P2;

    sealed private event System.Action P3 
    {
        add => throw null;
        remove {}
    }

    private event System.Action P4 = null;
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyEmitDiagnostics(
                // (4,42): error CS0621: 'I1.P1': virtual or abstract members cannot be private
                //     abstract private event System.Action P1; 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "P1").WithArguments("I1.P1").WithLocation(4, 42),
                // (6,41): error CS0065: 'I1.P2': event property must have both add and remove accessors
                //     virtual private event System.Action P2;
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P2").WithArguments("I1.P2").WithLocation(6, 41),
                // (6,41): error CS0621: 'I1.P2': virtual or abstract members cannot be private
                //     virtual private event System.Action P2;
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "P2").WithArguments("I1.P2").WithLocation(6, 41),
                // (6,41): warning CS0067: The event 'I1.P2' is never used
                //     virtual private event System.Action P2;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P2").WithArguments("I1.P2").WithLocation(6, 41),
                // (8,40): error CS0238: 'I1.P3' cannot be sealed because it is not an override
                //     sealed private event System.Action P3 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "P3").WithArguments("I1.P3").WithLocation(8, 40),
                // (14,33): error CS0068: 'I1.P4': instance event in interface cannot have initializer
                //     private event System.Action P4 = null;
                Diagnostic(ErrorCode.ERR_InterfaceEventInitializer, "P4").WithArguments("I1.P4").WithLocation(14, 33),
                // (14,33): error CS0065: 'I1.P4': event property must have both add and remove accessors
                //     private event System.Action P4 = null;
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P4").WithArguments("I1.P4").WithLocation(14, 33),
                // (17,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1"),
                // (14,33): warning CS0067: The event 'I1.P4' is never used
                //     private event System.Action P4 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P4").WithArguments("I1.P4").WithLocation(14, 33)
                );

            ValidateEventModifiers_10(compilation1);
        }

        private static void ValidateEventModifiers_10(CSharpCompilation compilation1)
        {
            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMembers().OfType<EventSymbol>().ElementAt(0);

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Private, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));

            ValidateP1Accessor(p1.AddMethod);
            ValidateP1Accessor(p1.RemoveMethod);
            void ValidateP1Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
            }

            var p2 = i1.GetMembers().OfType<EventSymbol>().ElementAt(1);

            Assert.False(p2.IsAbstract);
            Assert.True(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Private, p2.DeclaredAccessibility);
            Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));

            ValidateP2Accessor(p2.AddMethod);
            ValidateP2Accessor(p2.RemoveMethod);
            void ValidateP2Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Same(accessor, test1.FindImplementationForInterfaceMember(accessor));
            }

            var p3 = i1.GetMembers().OfType<EventSymbol>().ElementAt(2);

            Assert.False(p3.IsAbstract);
            Assert.False(p3.IsVirtual);
            Assert.True(p3.IsSealed);
            Assert.False(p3.IsStatic);
            Assert.False(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Private, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));

            ValidateP3Accessor(p3.AddMethod);
            ValidateP3Accessor(p3.RemoveMethod);
            void ValidateP3Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.True(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
            }

            var p4 = i1.GetMembers().OfType<EventSymbol>().ElementAt(3);

            Assert.False(p4.IsAbstract);
            Assert.False(p4.IsVirtual);
            Assert.False(p4.IsSealed);
            Assert.False(p4.IsStatic);
            Assert.False(p4.IsExtern);
            Assert.False(p4.IsOverride);
            Assert.Equal(Accessibility.Private, p4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4));

            ValidateP4Accessor(p4.AddMethod);
            ValidateP4Accessor(p4.RemoveMethod);
            void ValidateP4Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
            }
        }

        [Fact]
        public void EventModifiers_11_01()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 

    sealed void Test()
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11(source1, source2, Accessibility.Internal,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(12, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedNoImplementation: ExpectedDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(12, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(16, 9)
                    )
                );
        }

        private void ValidateEventModifiers_11(string source1, string source2, Accessibility accessibility, DiagnosticDescription[] expectedIn9, params DiagnosticDescription[] expectedNoImplementation)
        {
            ValidateEventModifiers_11(source1, source2, accessibility, expectedIn9, expectedAcrossAssemblyBoundaries: Array.Empty<DiagnosticDescription>(), expectedNoImplementation);
        }

        private void ValidateEventModifiers_11(string source1, string source2, Accessibility accessibility, DiagnosticDescription[] expectedIn9, DiagnosticDescription[] expectedAcrossAssemblyBoundaries, params DiagnosticDescription[] expectedNoImplementation)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expectedIn9);

            Validate1(compilation1.SourceModule);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => Validate1(m)).VerifyDiagnostics();

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var p1 = GetSingleEvent(i1);
                var test1P1 = GetSingleEvent(test1);
                var p1add = p1.AddMethod;
                var p1remove = p1.RemoveMethod;

                ValidateEvent(p1);
                ValidateMethod(p1add);
                ValidateMethod(p1remove);
                Assert.Same(test1P1, test1.FindImplementationForInterfaceMember(p1));
                Assert.Same(test1P1.AddMethod, test1.FindImplementationForInterfaceMember(p1add));
                Assert.Same(test1P1.RemoveMethod, test1.FindImplementationForInterfaceMember(p1remove));
            }

            void ValidateEvent(EventSymbol p1)
            {
                Assert.True(p1.IsAbstract);
                Assert.False(p1.IsVirtual);
                Assert.False(p1.IsSealed);
                Assert.False(p1.IsStatic);
                Assert.False(p1.IsExtern);
                Assert.False(p1.IsOverride);
                Assert.Equal(accessibility, p1.DeclaredAccessibility);
            }

            void ValidateMethod(MethodSymbol m1)
            {
                Assert.True(m1.IsAbstract);
                Assert.False(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(accessibility, m1.DeclaredAccessibility);
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var p1 = GetSingleEvent(i1);
                ValidateEvent(p1);
                ValidateMethod(p1.AddMethod);
                ValidateMethod(p1.RemoveMethod);
            }

            var source3 =
@"
class Test2 : I1
{
}
";

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                Validate1(compilation3.SourceModule);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => Validate1(m)).VerifyDiagnostics();
                }

                Validate1(compilation3.SourceModule);

                var compilation5 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation5.VerifyDiagnostics(expectedNoImplementation);

                ValidateEventNotImplemented_11(compilation5, "Test2");
            }
        }

        private static void ValidateEventNotImplemented_11(CSharpCompilation compilation, string className)
        {
            var test2 = compilation.GetTypeByMetadataName(className);
            var i1 = compilation.GetTypeByMetadataName("I1");
            var p1 = GetSingleEvent(i1);
            Assert.Null(test2.FindImplementationForInterfaceMember(p1));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1.AddMethod));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1.RemoveMethod));
        }

        [Fact]
        public void EventModifiers_11_02()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(11, 9),
                    // (15,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(15, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(11, 9),
                    // (15,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(15, 9)
                    )
                );
        }

        private void ValidateEventModifiers_11_02(string source1, string source2,
                                                  DiagnosticDescription[] expectedIn9,
                                                  params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(expectedIn9);

            ValidateEventImplementation_11(compilation1.SourceModule);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateEventImplementation_11(m)).VerifyDiagnostics();

            ValidateEventImplementation_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                ValidateEventImplementation_11(compilation3.SourceModule);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateEventImplementation_11(m)).VerifyDiagnostics();
                }

                ValidateEventImplementation_11(compilation3.SourceModule);
            }
        }

        private static void ValidateEventImplementation_11(ModuleSymbol m)
        {
            ValidateEventImplementation_11(m, implementedByBase: false);
        }

        private static void ValidateEventImplementationByBase_11(ModuleSymbol m)
        {
            ValidateEventImplementation_11(m, implementedByBase: true);
        }

        private static void ValidateEventImplementation_11(ModuleSymbol m, bool implementedByBase)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            var p1 = GetSingleEvent(i1);
            var test1P1 = GetSingleEvent(implementedByBase ? test1.BaseTypeNoUseSiteDiagnostics : test1);
            var p1Add = p1.AddMethod;
            var p1Remove = p1.RemoveMethod;

            Assert.Same(test1P1, test1.FindImplementationForInterfaceMember(p1));

            Assert.Same(test1P1.AddMethod, test1.FindImplementationForInterfaceMember(p1Add));
            Assert.Same(test1P1.RemoveMethod, test1.FindImplementationForInterfaceMember(p1Remove));

            Assert.True(test1P1.AddMethod.IsMetadataVirtual());
            Assert.True(test1P1.RemoveMethod.IsMetadataVirtual());
        }

        [Fact]
        public void EventModifiers_11_03()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_03(source1, source2, TargetFramework.Standard,
                // (9,28): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //     event System.Action I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(9, 28)
                );
        }

        private void ValidateEventModifiers_11_03(string source1, string source2, TargetFramework targetFramework,
                                                  params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe, targetFramework: targetFramework,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput: !(targetFramework == TargetFramework.Standard || ExecutionConditionUtil.IsMonoOrCoreClr) ? null :
@"get_P1
set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidateEventImplementation_11);

            ValidateEventImplementation_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: targetFramework,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular, targetFramework: targetFramework);

                compilation3.VerifyDiagnostics(expected);

                if (expected.Length == 0)
                {
                    CompileAndVerify(compilation3, expectedOutput: !(targetFramework == TargetFramework.Standard || ExecutionConditionUtil.IsMonoOrCoreClr) ? null :
@"get_P1
set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidateEventImplementation_11);
                }

                ValidateEventImplementation_11(compilation3.SourceModule);
            }
        }

        [Fact]
        public void EventModifiers_11_04()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}

public class Test2 : I1
{
    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""Test2.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(11, 9),
                    // (15,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(15, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(11, 9),
                    // (15,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(15, 9)
                    )
                );
        }

        [Fact]
        public void EventModifiers_11_05()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}

public class Test2 : I1
{
    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""Test2.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(11, 9),
                    // (15,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(15, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(11, 9),
                    // (15,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(15, 9)
                    )
                );
        }

        [Fact]
        public void EventModifiers_11_06()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}

public class Test2 : I1
{
    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""Test2.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}
";

            var source2 =
@"abstract
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test3());
    }

    public abstract event System.Action P1; 
}

class Test3 : Test1
{
    public override event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

";

            ValidateEventModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (9,41): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract event System.Action P1; 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "P1").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(9, 41),
                    // (9,41): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //     public abstract event System.Action P1; 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "P1").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(9, 41)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (9,41): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //     public abstract event System.Action P1; 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "P1").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(9, 41),
                    // (9,41): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //     public abstract event System.Action P1; 
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "P1").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(9, 41)
                    )
                );
        }

        [Fact]
        public void EventModifiers_11_07()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}

public class Test2 : I1
{
    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""Test2.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1, I2
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}

public interface I2
{
    event System.Action P1; 
}
";

            ValidateEventModifiers_11_02(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (11,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(11, 9),
                    // (15,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(15, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (11,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(11, 9),
                    // (15,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(15, 9)
                    )
                );
        }

        [Fact]
        public void EventModifiers_11_08()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}

public class Test2 : I1
{
    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""Test2.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test2.set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual event System.Action<object> P1 
    {
        add
        {
            System.Console.WriteLine(""Test1.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_08(source1, source2);
        }

        private void ValidateEventModifiers_11_08(string source1, string source2)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                  parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation1, expectedOutput:
@"Test2.get_P1
Test2.set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidateEventImplementationByBase_11);

            ValidateEventImplementationByBase_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation3, expectedOutput:
@"Test2.get_P1
Test2.set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidateEventImplementationByBase_11);

            ValidateEventImplementationByBase_11(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            CompileAndVerify(compilation4, expectedOutput:
@"Test2.get_P1
Test2.set_P1", verify: VerifyOnMonoOrCoreClr, symbolValidator: ValidateEventImplementationByBase_11);

            ValidateEventImplementationByBase_11(compilation4.SourceModule);
        }

        [Fact]
        public void EventModifiers_11_09()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

public class TestHelper
{
    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        TestHelper.CallP1(new Test1());
    }

    public virtual event System.Action<object> P1 
    {
        add
        {
            System.Console.WriteLine(""Test1.get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""Test1.set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_09(source1, source2,
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.P1'. 'Test1.P1' cannot implement 'I1.P1' because it does not have the matching return type of 'Action'.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I1").WithArguments("Test1", "I1.P1", "Test1.P1", "System.Action").WithLocation(2, 15)
                );
        }

        private void ValidateEventModifiers_11_09(string source1, string source2,
                                                  params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(expected);

            ValidateEventNotImplemented_11(compilation1, "Test1");

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation3.VerifyDiagnostics(expected);

            ValidateEventNotImplemented_11(compilation3, "Test1");

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation4.VerifyDiagnostics(expected);

            ValidateEventNotImplemented_11(compilation4, "Test1");
        }

        [Fact]
        public void EventModifiers_11_10()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 

    sealed void Test()
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
public class Test2 : I1
{
    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidateEventModifiers_11_10(source1, source2,
                expectedIn9: ExpectedDiagnostics(
                    // (6,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.add'. 'Test2.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test2", "I1.P1.add", "Test2.P1.add", "9.0", "10.0").WithLocation(6, 9),
                    // (10,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.remove'. 'Test2.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test2", "I1.P1.remove", "Test2.P1.remove", "9.0", "10.0").WithLocation(10, 9)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (6,9): error CS9044: 'Test2' does not implement interface member 'I1.P1.add'. 'Test2.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test2", "I1.P1.add", "Test2.P1.add").WithLocation(6, 9),
                    // (10,9): error CS9044: 'Test2' does not implement interface member 'I1.P1.remove'. 'Test2.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test2", "I1.P1.remove", "Test2.P1.remove").WithLocation(10, 9)
                    )
                );
        }

        private void ValidateEventModifiers_11_10(string source1, string source2,
                                                  DiagnosticDescription[] expectedIn9,
                                                  params DiagnosticDescription[] expectedAcrossAssemblyBoundaries)
        {
            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(expectedIn9);

            ValidateEventImplementationByBase_11(compilation1.SourceModule);

            compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateEventImplementationByBase_11(m)).VerifyDiagnostics();

            ValidateEventImplementationByBase_11(compilation1.SourceModule);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();

            foreach (var reference in new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.Regular9,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries.Length != 0 ? expectedAcrossAssemblyBoundaries : expectedIn9);

                ValidateEventImplementationByBase_11(compilation3.SourceModule);

                compilation3 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

                if (expectedAcrossAssemblyBoundaries.Length != 0)
                {
                    compilation3.VerifyDiagnostics(expectedAcrossAssemblyBoundaries);
                }
                else
                {
                    CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                                     verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateEventImplementationByBase_11(m)).VerifyDiagnostics();
                }

                ValidateEventImplementationByBase_11(compilation3.SourceModule);
            }
        }

        [Fact]
        public void EventModifiers_11_11()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 

    sealed void Test()
    {
        P1 += null;
        P1 -= null;
    }
}
public class Test2 : I1
{
    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }
}
";

            ValidateEventModifiers_11_11(source1, source2,
                // (16,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.add'. 'Test2.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         add
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test2", "I1.P1.add", "Test2.P1.add", "9.0", "10.0").WithLocation(16, 9),
                // (20,9): error CS8704: 'Test2' does not implement interface member 'I1.P1.remove'. 'Test2.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         remove
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test2", "I1.P1.remove", "Test2.P1.remove", "9.0", "10.0").WithLocation(20, 9)
                );
        }

        private void ValidateEventModifiers_11_11(string source1, string source2,
                                                     params DiagnosticDescription[] expected)
        {
            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 assemblyName: "EventModifiers_11_11");

            compilation2.VerifyDiagnostics(expected);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular,
                                             targetFramework: TargetFramework.NetCoreApp,
                                             assemblyName: "EventModifiers_11_11");

            compilation2.VerifyDiagnostics();

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1",
                             verify: VerifyOnMonoOrCoreClr, symbolValidator: (m) => ValidateEventImplementationByBase_11(m)).VerifyDiagnostics();

            ValidateEventImplementationByBase_11(compilation3.SourceModule);
        }

        [Fact]
        public void EventModifiers_12()
        {
            var source1 =
@"
public interface I1
{
    internal abstract event System.Action P1; 
}

class Test1 : I1
{
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(
                // (7,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(7, 15)
                );

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var i1 = compilation1.GetTypeByMetadataName("I1");
            var p1 = i1.GetMember<EventSymbol>("P1");
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test1.FindImplementationForInterfaceMember(p1.AddMethod));
            Assert.Null(test1.FindImplementationForInterfaceMember(p1.RemoveMethod));
        }

        [Fact]
        public void EventModifiers_13()
        {
            var source1 =
@"
public interface I1
{
    public sealed event System.Action P1 
    {
        add
        { 
            System.Console.WriteLine(""get_P1"");
        }           
        remove
        { 
            System.Console.WriteLine(""set_P1"");
        }           
    }
}
public interface I2
{
    public sealed event System.Action P2 
    {
        add => System.Console.WriteLine(""get_P2"");
        remove => System.Console.WriteLine(""set_P2"");
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1 i1 = new Test1();
        i1.P1 += null;
        i1.P1 -= null;
        I2 i2 = new Test2();
        i2.P2 += null;
        i2.P2 -= null;
    }

    public event System.Action P1 
    {
        add => throw null;          
        remove => throw null;         
    }
}
class Test2 : I2
{
    public event System.Action P2 
    {
        add => throw null;          
        remove => throw null;         
    }
}
";

            ValidateEventModifiers_13(source1);
        }

        private void ValidateEventModifiers_13(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            void Validate(ModuleSymbol m)
            {
                for (int i = 1; i <= 2; i++)
                {
                    var test1 = m.GlobalNamespace.GetTypeMember("Test" + i);
                    var i1 = m.GlobalNamespace.GetTypeMember("I" + i);
                    var p1 = GetSingleEvent(i1);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.False(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    ValidateAccessor(p1.AddMethod);
                    ValidateAccessor(p1.RemoveMethod);

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.False(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }

            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate);
            Validate(compilation1.SourceModule);
        }

        [Fact]
        public void EventModifiers_14()
        {
            var source1 =
@"
public interface I1
{
    public sealed event System.Action P1 = null; 
}
public interface I2
{
    abstract sealed event System.Action P2 {add; remove;} 
}
public interface I3
{
    virtual sealed event System.Action P3 
    {
        add {}
        remove {}
    }
}

class Test1 : I1, I2, I3
{
    event System.Action I1.P1 { add => throw null; remove => throw null; }
    event System.Action I2.P2 { add => throw null; remove => throw null; }
    event System.Action I3.P3 { add => throw null; remove => throw null; }
}

class Test2 : I1, I2, I3
{}
";
            ValidateEventModifiers_14(source1,
                // (4,39): error CS0068: 'I1.P1': instance event in interface cannot have initializer
                //     public sealed event System.Action P1 = null; 
                Diagnostic(ErrorCode.ERR_InterfaceEventInitializer, "P1").WithArguments("I1.P1").WithLocation(4, 39),
                // (4,39): error CS0065: 'I1.P1': event property must have both add and remove accessors
                //     public sealed event System.Action P1; 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P1").WithArguments("I1.P1").WithLocation(4, 39),
                // (8,41): error CS0238: 'I2.P2' cannot be sealed because it is not an override
                //     abstract sealed event System.Action P2 {add; remove;} 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "P2").WithArguments("I2.P2").WithLocation(8, 41),
                // (8,44): error CS8712: 'I2.P2': abstract event cannot use event accessor syntax
                //     abstract sealed event System.Action P2 {add; remove;} 
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I2.P2").WithLocation(8, 44),
                // (12,40): error CS0238: 'I3.P3' cannot be sealed because it is not an override
                //     virtual sealed event System.Action P3 
                Diagnostic(ErrorCode.ERR_SealedNonOverride, "P3").WithArguments("I3.P3").WithLocation(12, 40),
                // (21,28): error CS0539: 'Test1.P1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I1.P1 { add => throw null; remove => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P1").WithArguments("Test1.P1").WithLocation(21, 28),
                // (22,28): error CS0539: 'Test1.P2' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I2.P2 { add => throw null; remove => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P2").WithArguments("Test1.P2").WithLocation(22, 28),
                // (23,28): error CS0539: 'Test1.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I3.P3 { add => throw null; remove => throw null; }
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test1.P3").WithLocation(23, 28),
                // (4,39): warning CS0067: The event 'I1.P1' is never used
                //     public sealed event System.Action P1 = null; 
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P1").WithArguments("I1.P1").WithLocation(4, 39)
                );
        }

        private void ValidateEventModifiers_14(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var p1 = GetSingleEvent(compilation1, "I1");

            Assert.False(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Null(test2.FindImplementationForInterfaceMember(p1));

            Validate1(p1.AddMethod);
            Validate1(p1.RemoveMethod);
            void Validate1(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }

            var p2 = GetSingleEvent(compilation1, "I2");
            var test1P2 = test1.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I2.")).Single();

            Assert.True(p2.IsAbstract);
            Assert.False(p2.IsVirtual);
            Assert.True(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p2));
            Assert.Null(test2.FindImplementationForInterfaceMember(p2));

            Validate2(p2.AddMethod);
            Validate2(p2.RemoveMethod);
            void Validate2(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.True(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }

            var p3 = GetSingleEvent(compilation1, "I3");
            var test1P3 = test1.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I3.")).Single();

            Assert.False(p3.IsAbstract);
            Assert.True(p3.IsVirtual);
            Assert.True(p3.IsSealed);
            Assert.False(p3.IsStatic);
            Assert.False(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3));

            Validate3(p3.AddMethod);
            Validate3(p3.RemoveMethod);
            void Validate3(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.True(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }
        }

        [Fact]
        public void EventModifiers_15()
        {
            var source1 =
@"
public interface I0
{
    abstract virtual event System.Action P0;
}
public interface I1
{
    abstract virtual event System.Action P1 { add { throw null; } }
}
public interface I2
{
    virtual abstract event System.Action P2 
    {
        add { throw null; }
        remove { throw null; }
    }
}
public interface I3
{
    abstract virtual event System.Action P3 { remove { throw null; } }
}
public interface I4
{
    abstract virtual event System.Action P4 { add => throw null; }
}
public interface I5
{
    abstract virtual event System.Action P5 
    {
        add => throw null;
        remove => throw null;
    }
}
public interface I6
{
    abstract virtual event System.Action P6 { remove => throw null; }
}
public interface I7
{
    abstract virtual event System.Action P7 { add; }
}
public interface I8
{
    abstract virtual event System.Action P8 { remove; } 
}

class Test1 : I0, I1, I2, I3, I4, I5, I6, I7, I8
{
    event System.Action I0.P0 
    {
        add { throw null; }
        remove { throw null; }
    }
    event System.Action I1.P1 
    {
        add { throw null; }
    }
    event System.Action I2.P2 
    {
        add { throw null; }
        remove { throw null; }
    }
    event System.Action I3.P3 
    {
        remove { throw null; }
    }
    event System.Action I4.P4 
    {
        add { throw null; }
    }
    event System.Action I5.P5 
    {
        add { throw null; }
        remove { throw null; }
    }
    event System.Action I6.P6 
    {
        remove { throw null; }
    }
    event System.Action I7.P7 
    {
        add { throw null; }
    }
    event System.Action I8.P8 
    {
        remove { throw null; }
    }
}

class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
{}
";
            ValidateEventModifiers_15(source1,
                // (4,42): error CS0503: The abstract event 'I0.P0' cannot be marked virtual
                //     abstract virtual event System.Action P0;
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P0").WithArguments("event", "I0.P0").WithLocation(4, 42),
                // (8,42): error CS0503: The abstract event 'I1.P1' cannot be marked virtual
                //     abstract virtual event System.Action P1 { add { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P1").WithArguments("event", "I1.P1").WithLocation(8, 42),
                // (8,45): error CS8712: 'I1.P1': abstract event cannot use event accessor syntax
                //     abstract virtual event System.Action P1 { add { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.P1").WithLocation(8, 45),
                // (12,42): error CS0503: The abstract event 'I2.P2' cannot be marked virtual
                //     virtual abstract event System.Action P2 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P2").WithArguments("event", "I2.P2").WithLocation(12, 42),
                // (13,5): error CS8712: 'I2.P2': abstract event cannot use event accessor syntax
                //     {
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I2.P2").WithLocation(13, 5),
                // (20,42): error CS0503: The abstract event 'I3.P3' cannot be marked virtual
                //     abstract virtual event System.Action P3 { remove { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P3").WithArguments("event", "I3.P3").WithLocation(20, 42),
                // (20,45): error CS8712: 'I3.P3': abstract event cannot use event accessor syntax
                //     abstract virtual event System.Action P3 { remove { throw null; } }
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I3.P3").WithLocation(20, 45),
                // (24,42): error CS0503: The abstract event 'I4.P4' cannot be marked virtual
                //     abstract virtual event System.Action P4 { add => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P4").WithArguments("event", "I4.P4").WithLocation(24, 42),
                // (24,45): error CS8712: 'I4.P4': abstract event cannot use event accessor syntax
                //     abstract virtual event System.Action P4 { add => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I4.P4").WithLocation(24, 45),
                // (28,42): error CS0503: The abstract event 'I5.P5' cannot be marked virtual
                //     abstract virtual event System.Action P5 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P5").WithArguments("event", "I5.P5").WithLocation(28, 42),
                // (29,5): error CS8712: 'I5.P5': abstract event cannot use event accessor syntax
                //     {
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I5.P5").WithLocation(29, 5),
                // (36,42): error CS0503: The abstract event 'I6.P6' cannot be marked virtual
                //     abstract virtual event System.Action P6 { remove => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P6").WithArguments("event", "I6.P6").WithLocation(36, 42),
                // (36,45): error CS8712: 'I6.P6': abstract event cannot use event accessor syntax
                //     abstract virtual event System.Action P6 { remove => throw null; }
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I6.P6").WithLocation(36, 45),
                // (40,42): error CS0503: The abstract event 'I7.P7' cannot be marked virtual
                //     abstract virtual event System.Action P7 { add; }
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P7").WithArguments("event", "I7.P7").WithLocation(40, 42),
                // (40,45): error CS8712: 'I7.P7': abstract event cannot use event accessor syntax
                //     abstract virtual event System.Action P7 { add; }
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I7.P7").WithLocation(40, 45),
                // (44,42): error CS0503: The abstract event 'I8.P8' cannot be marked virtual
                //     abstract virtual event System.Action P8 { remove; } 
                Diagnostic(ErrorCode.ERR_AbstractNotVirtual, "P8").WithArguments("event", "I8.P8").WithLocation(44, 42),
                // (44,45): error CS8712: 'I8.P8': abstract event cannot use event accessor syntax
                //     abstract virtual event System.Action P8 { remove; } 
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I8.P8").WithLocation(44, 45),
                // (54,28): error CS0065: 'Test1.I1.P1': event property must have both add and remove accessors
                //     event System.Action I1.P1 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P1").WithArguments("Test1.I1.P1").WithLocation(54, 28),
                // (63,28): error CS0065: 'Test1.I3.P3': event property must have both add and remove accessors
                //     event System.Action I3.P3 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P3").WithArguments("Test1.I3.P3").WithLocation(63, 28),
                // (67,28): error CS0065: 'Test1.I4.P4': event property must have both add and remove accessors
                //     event System.Action I4.P4 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P4").WithArguments("Test1.I4.P4").WithLocation(67, 28),
                // (76,28): error CS0065: 'Test1.I6.P6': event property must have both add and remove accessors
                //     event System.Action I6.P6 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P6").WithArguments("Test1.I6.P6").WithLocation(76, 28),
                // (80,28): error CS0065: 'Test1.I7.P7': event property must have both add and remove accessors
                //     event System.Action I7.P7 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P7").WithArguments("Test1.I7.P7").WithLocation(80, 28),
                // (84,28): error CS0065: 'Test1.I8.P8': event property must have both add and remove accessors
                //     event System.Action I8.P8 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "P8").WithArguments("Test1.I8.P8").WithLocation(84, 28),
                // (90,15): error CS0535: 'Test2' does not implement interface member 'I0.P0'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I0").WithArguments("Test2", "I0.P0").WithLocation(90, 15),
                // (90,19): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(90, 19),
                // (90,23): error CS0535: 'Test2' does not implement interface member 'I2.P2'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test2", "I2.P2").WithLocation(90, 23),
                // (90,27): error CS0535: 'Test2' does not implement interface member 'I3.P3'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test2", "I3.P3").WithLocation(90, 27),
                // (90,31): error CS0535: 'Test2' does not implement interface member 'I4.P4'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test2", "I4.P4").WithLocation(90, 31),
                // (90,35): error CS0535: 'Test2' does not implement interface member 'I5.P5'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I5").WithArguments("Test2", "I5.P5").WithLocation(90, 35),
                // (90,39): error CS0535: 'Test2' does not implement interface member 'I6.P6'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I6").WithArguments("Test2", "I6.P6").WithLocation(90, 39),
                // (90,43): error CS0535: 'Test2' does not implement interface member 'I7.P7'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I7").WithArguments("Test2", "I7.P7").WithLocation(90, 43),
                // (90,47): error CS0535: 'Test2' does not implement interface member 'I8.P8'
                // class Test2 : I0, I1, I2, I3, I4, I5, I6, I7, I8
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I8").WithArguments("Test2", "I8.P8").WithLocation(90, 47)
                );
        }

        private void ValidateEventModifiers_15(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");

            for (int i = 0; i <= 8; i++)
            {
                var i1 = compilation1.GetTypeByMetadataName("I" + i);
                var p2 = GetSingleEvent(i1);
                var test1P2 = test1.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith(i1.Name)).Single();

                Assert.True(p2.IsAbstract);
                Assert.True(p2.IsVirtual);
                Assert.False(p2.IsSealed);
                Assert.False(p2.IsStatic);
                Assert.False(p2.IsExtern);
                Assert.False(p2.IsOverride);
                Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
                Assert.Same(test1P2, test1.FindImplementationForInterfaceMember(p2));
                Assert.Null(test2.FindImplementationForInterfaceMember(p2));

                switch (i)
                {
                    case 3:
                    case 6:
                    case 8:
                        Assert.Null(p2.AddMethod);
                        ValidateAccessor(p2.RemoveMethod, test1P2.RemoveMethod);
                        break;
                    case 1:
                    case 4:
                    case 7:
                        Assert.Null(p2.RemoveMethod);
                        ValidateAccessor(p2.AddMethod, test1P2.AddMethod);
                        break;
                    default:
                        ValidateAccessor(p2.AddMethod, test1P2.AddMethod);
                        ValidateAccessor(p2.RemoveMethod, test1P2.RemoveMethod);
                        break;
                }

                void ValidateAccessor(MethodSymbol accessor, MethodSymbol implementedBy)
                {
                    Assert.True(accessor.IsAbstract);
                    Assert.True(accessor.IsVirtual);
                    Assert.True(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.False(accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                    Assert.Same(implementedBy, test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
                }
            }
        }

        [Fact]
        public void EventModifiers_16()
        {
            var source1 =
@"
public interface I1
{
    extern event System.Action P1; 
}
public interface I2
{
    virtual extern event System.Action P2;
}
public interface I3
{
    static extern event System.Action P3; 
}
public interface I4
{
    private extern event System.Action P4;
}
public interface I5
{
    extern sealed event System.Action P5;
}
public interface I6
{
    static event System.Action P6
    {
        add => throw null;
        remove => throw null;
    }
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    event System.Action I1.P1 { add{} remove{} }
    event System.Action I2.P2 { add{} remove{} }
}
";
            ValidateEventModifiers_16(source1);
        }

        private void ValidateEventModifiers_16(string source1)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, symbolValidator: Validate);

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var test2 = m.GlobalNamespace.GetTypeMember("Test2");
                bool isSource = !(m is PEModuleSymbol);
                var p1 = GetSingleEvent(m, "I1");
                var test2P1 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I1.")).Single();

                Assert.False(p1.IsAbstract);
                Assert.True(p1.IsVirtual);
                Assert.False(p1.IsSealed);
                Assert.False(p1.IsStatic);
                Assert.Equal(isSource, p1.IsExtern);
                Assert.False(p1.IsOverride);
                Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
                Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
                Assert.Same(test2P1, test2.FindImplementationForInterfaceMember(p1));

                ValidateP1Accessor(p1.AddMethod, test2P1.AddMethod);
                ValidateP1Accessor(p1.RemoveMethod, test2P1.RemoveMethod);
                void ValidateP1Accessor(MethodSymbol accessor, MethodSymbol implementation)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.True(accessor.IsVirtual);
                    Assert.True(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.Equal(isSource, accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                    Assert.Same(accessor, test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
                }

                var p2 = GetSingleEvent(m, "I2");
                var test2P2 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I2.")).Single();

                Assert.False(p2.IsAbstract);
                Assert.True(p2.IsVirtual);
                Assert.False(p2.IsSealed);
                Assert.False(p2.IsStatic);
                Assert.Equal(isSource, p2.IsExtern);
                Assert.False(p2.IsOverride);
                Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
                Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));
                Assert.Same(test2P2, test2.FindImplementationForInterfaceMember(p2));

                ValidateP2Accessor(p2.AddMethod, test2P2.AddMethod);
                ValidateP2Accessor(p2.RemoveMethod, test2P2.RemoveMethod);
                void ValidateP2Accessor(MethodSymbol accessor, MethodSymbol implementation)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.True(accessor.IsVirtual);
                    Assert.True(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.Equal(isSource, accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                    Assert.Same(accessor, test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
                }

                var i3 = m.ContainingAssembly.GetTypeByMetadataName("I3");
                var p3 = GetSingleEvent(i3);

                Assert.False(p3.IsAbstract);
                Assert.False(p3.IsVirtual);
                Assert.False(p3.IsSealed);
                Assert.True(p3.IsStatic);
                Assert.Equal(isSource, p3.IsExtern);
                Assert.False(p3.IsOverride);
                Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p3));
                Assert.Null(test2.FindImplementationForInterfaceMember(p3));

                ValidateP3Accessor(p3.AddMethod);
                ValidateP3Accessor(p3.RemoveMethod);
                void ValidateP3Accessor(MethodSymbol accessor)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.False(accessor.IsVirtual);
                    Assert.False(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.True(accessor.IsStatic);
                    Assert.Equal(isSource, accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
                }

                var p4 = GetSingleEvent(m, "I4");

                Assert.False(p4.IsAbstract);
                Assert.False(p4.IsVirtual);
                Assert.False(p4.IsSealed);
                Assert.False(p4.IsStatic);
                Assert.Equal(isSource, p4.IsExtern);
                Assert.False(p4.IsOverride);
                Assert.Equal(Accessibility.Private, p4.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p4));
                Assert.Null(test2.FindImplementationForInterfaceMember(p4));

                ValidateP4Accessor(p4.AddMethod);
                ValidateP4Accessor(p4.RemoveMethod);
                void ValidateP4Accessor(MethodSymbol accessor)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.False(accessor.IsVirtual);
                    Assert.False(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.Equal(isSource, accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
                }

                var p5 = GetSingleEvent(m, "I5");

                Assert.False(p5.IsAbstract);
                Assert.False(p5.IsVirtual);
                Assert.False(p5.IsSealed);
                Assert.False(p5.IsStatic);
                Assert.Equal(isSource, p5.IsExtern);
                Assert.False(p5.IsOverride);
                Assert.Equal(Accessibility.Public, p5.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(p5));
                Assert.Null(test2.FindImplementationForInterfaceMember(p5));

                ValidateP5Accessor(p5.AddMethod);
                ValidateP5Accessor(p5.RemoveMethod);
                void ValidateP5Accessor(MethodSymbol accessor)
                {
                    Assert.False(accessor.IsAbstract);
                    Assert.False(accessor.IsVirtual);
                    Assert.False(accessor.IsMetadataVirtual());
                    Assert.False(accessor.IsSealed);
                    Assert.False(accessor.IsStatic);
                    Assert.Equal(isSource, accessor.IsExtern);
                    Assert.False(accessor.IsAsync);
                    Assert.False(accessor.IsOverride);
                    Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
                }
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(
                // (4,32): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern event System.Action P1; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P1").WithArguments("extern", "7.3", "8.0").WithLocation(4, 32),
                // (8,40): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern event System.Action P2;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P2").WithArguments("extern", "7.3", "8.0").WithLocation(8, 40),
                // (8,40): error CS8703: The modifier 'virtual' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     virtual extern event System.Action P2;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P2").WithArguments("virtual", "7.3", "8.0").WithLocation(8, 40),
                // (12,39): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static extern event System.Action P3; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("static", "7.3", "8.0").WithLocation(12, 39),
                // (12,39): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static extern event System.Action P3; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P3").WithArguments("extern", "7.3", "8.0").WithLocation(12, 39),
                // (16,40): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern event System.Action P4;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P4").WithArguments("private", "7.3", "8.0").WithLocation(16, 40),
                // (16,40): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private extern event System.Action P4;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P4").WithArguments("extern", "7.3", "8.0").WithLocation(16, 40),
                // (20,39): error CS8703: The modifier 'sealed' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed event System.Action P5;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P5").WithArguments("sealed", "7.3", "8.0").WithLocation(20, 39),
                // (20,39): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern sealed event System.Action P5;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P5").WithArguments("extern", "7.3", "8.0").WithLocation(20, 39),
                // (24,32): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static event System.Action P6
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "P6").WithArguments("default interface implementation", "8.0").WithLocation(24, 32),
                // (8,40): warning CS0626: Method, operator, or accessor 'I2.P2.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern event System.Action P2;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P2").WithArguments("I2.P2.remove").WithLocation(8, 40),
                // (12,39): warning CS0626: Method, operator, or accessor 'I3.P3.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern event System.Action P3; 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P3").WithArguments("I3.P3.remove").WithLocation(12, 39),
                // (16,40): warning CS0626: Method, operator, or accessor 'I4.P4.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern event System.Action P4;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P4").WithArguments("I4.P4.remove").WithLocation(16, 40),
                // (20,39): warning CS0626: Method, operator, or accessor 'I5.P5.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed event System.Action P5;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P5").WithArguments("I5.P5.remove").WithLocation(20, 39),
                // (4,32): warning CS0626: Method, operator, or accessor 'I1.P1.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P1; 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P1").WithArguments("I1.P1.remove").WithLocation(4, 32)
                );

            Validate(compilation2.SourceModule);

            var compilation3 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);
            Assert.False(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (4,32): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern event System.Action P1; 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "P1").WithLocation(4, 32),
                // (8,40): error CS8701: Target runtime doesn't support default interface implementation.
                //     virtual extern event System.Action P2;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "P2").WithLocation(8, 40),
                // (16,40): error CS8701: Target runtime doesn't support default interface implementation.
                //     private extern event System.Action P4;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "P4").WithLocation(16, 40),
                // (20,39): error CS8701: Target runtime doesn't support default interface implementation.
                //     extern sealed event System.Action P5;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "P5").WithLocation(20, 39),
                // (8,40): warning CS0626: Method, operator, or accessor 'I2.P2.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     virtual extern event System.Action P2;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P2").WithArguments("I2.P2.remove").WithLocation(8, 40),
                // (12,39): warning CS0626: Method, operator, or accessor 'I3.P3.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern event System.Action P3; 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P3").WithArguments("I3.P3.remove").WithLocation(12, 39),
                // (16,40): warning CS0626: Method, operator, or accessor 'I4.P4.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     private extern event System.Action P4;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P4").WithArguments("I4.P4.remove").WithLocation(16, 40),
                // (20,39): warning CS0626: Method, operator, or accessor 'I5.P5.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern sealed event System.Action P5;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P5").WithArguments("I5.P5.remove").WithLocation(20, 39),
                // (4,32): warning CS0626: Method, operator, or accessor 'I1.P1.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P1; 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P1").WithArguments("I1.P1.remove").WithLocation(4, 32)
                );

            Validate(compilation3.SourceModule);
        }

        [Fact]
        public void EventModifiers_17()
        {
            var source1 =
@"
public interface I1
{
    abstract extern event System.Action P1;
}
public interface I2
{
    extern event System.Action P2 = null;
}
public interface I3
{
    static extern event System.Action P3 {add => throw null; remove => throw null;}
}
public interface I4
{
    private extern event System.Action P4 { add {throw null;} remove {throw null;}}
}

class Test1 : I1, I2, I3, I4
{
}

class Test2 : I1, I2, I3, I4
{
    event System.Action I1.P1 { add => throw null; remove => throw null;}
    event System.Action I2.P2 { add => throw null; remove => throw null;}
    event System.Action I3.P3 { add => throw null; remove => throw null;}
    event System.Action I4.P4 { add => throw null; remove => throw null;}
}
";
            ValidateEventModifiers_17(source1,
                // (4,41): error CS0180: 'I1.P1' cannot be both extern and abstract
                //     abstract extern event System.Action P1;
                Diagnostic(ErrorCode.ERR_AbstractAndExtern, "P1").WithArguments("I1.P1").WithLocation(4, 41),
                // (8,32): error CS0068: 'I2.P2': instance event in interface cannot have initializer
                //     extern event System.Action P2 = null;
                Diagnostic(ErrorCode.ERR_InterfaceEventInitializer, "P2").WithArguments("I2.P2").WithLocation(8, 32),
                // (12,43): error CS0179: 'I3.P3.add' cannot be extern and declare a body
                //     static extern event System.Action P3 {add => throw null; remove => throw null;}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "add").WithArguments("I3.P3.add").WithLocation(12, 43),
                // (12,62): error CS0179: 'I3.P3.remove' cannot be extern and declare a body
                //     static extern event System.Action P3 {add => throw null; remove => throw null;}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "remove").WithArguments("I3.P3.remove").WithLocation(12, 62),
                // (16,45): error CS0179: 'I4.P4.add' cannot be extern and declare a body
                //     private extern event System.Action P4 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "add").WithArguments("I4.P4.add").WithLocation(16, 45),
                // (16,63): error CS0179: 'I4.P4.remove' cannot be extern and declare a body
                //     private extern event System.Action P4 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "remove").WithArguments("I4.P4.remove").WithLocation(16, 63),
                // (19,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1, I2, I3, I4
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(19, 15),
                // (27,28): error CS0539: 'Test2.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I3.P3 { add => throw null; remove => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test2.P3").WithLocation(27, 28),
                // (28,28): error CS0539: 'Test2.P4' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I4.P4 { add => throw null; remove => throw null;}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P4").WithArguments("Test2.P4").WithLocation(28, 28),
                // (8,32): warning CS0626: Method, operator, or accessor 'I2.P2.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern event System.Action P2 = null;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P2").WithArguments("I2.P2.remove").WithLocation(8, 32),
                // (8,32): warning CS0067: The event 'I2.P2' is never used
                //     extern event System.Action P2 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P2").WithArguments("I2.P2").WithLocation(8, 32)
                );
        }

        private void ValidateEventModifiers_17(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var p1 = GetSingleEvent(compilation1, "I1");
            var test2P1 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I1.")).Single();

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.True(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(test2P1, test2.FindImplementationForInterfaceMember(p1));

            ValidateP1Accessor(p1.AddMethod, test2P1.AddMethod);
            ValidateP1Accessor(p1.RemoveMethod, test2P1.RemoveMethod);
            void ValidateP1Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p2 = GetSingleEvent(compilation1, "I2");
            var test2P2 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I2.")).Single();

            Assert.False(p2.IsAbstract);
            Assert.True(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.True(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Public, p2.DeclaredAccessibility);
            Assert.Same(p2, test1.FindImplementationForInterfaceMember(p2));
            Assert.Same(test2P2, test2.FindImplementationForInterfaceMember(p2));

            ValidateP2Accessor(p2.AddMethod, test2P2.AddMethod);
            ValidateP2Accessor(p2.RemoveMethod, test2P2.RemoveMethod);
            void ValidateP2Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.False(accessor.IsAbstract);
                Assert.True(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Same(accessor, test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p3 = GetSingleEvent(compilation1, "I3");
            var test2P3 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I3.")).Single();

            Assert.False(p3.IsAbstract);
            Assert.False(p3.IsVirtual);
            Assert.False(p3.IsSealed);
            Assert.True(p3.IsStatic);
            Assert.True(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3));

            ValidateP3Accessor(p3.AddMethod);
            ValidateP3Accessor(p3.RemoveMethod);
            void ValidateP3Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.True(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }

            var p4 = GetSingleEvent(compilation1, "I4");

            Assert.False(p4.IsAbstract);
            Assert.False(p4.IsVirtual);
            Assert.False(p4.IsSealed);
            Assert.False(p4.IsStatic);
            Assert.True(p4.IsExtern);
            Assert.False(p4.IsOverride);
            Assert.Equal(Accessibility.Private, p4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4));
            Assert.Null(test2.FindImplementationForInterfaceMember(p4));

            ValidateP4Accessor(p4.AddMethod);
            ValidateP4Accessor(p4.RemoveMethod);
            void ValidateP4Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }
        }

        [Fact]
        public void EventModifiers_18()
        {
            var source1 =
@"
public interface I1
{
    abstract event System.Action P1 {add => throw null; remove => throw null;} 
}
public interface I2
{
    abstract private event System.Action P2 = null; 
}
public interface I3
{
    static extern event System.Action P3;
}
public interface I4
{
    abstract static event System.Action P4 { add {throw null;} remove {throw null;}}
}
public interface I5
{
    override sealed event System.Action P5 { add {throw null;} remove {throw null;}}
}

class Test1 : I1, I2, I3, I4, I5
{
}

class Test2 : I1, I2, I3, I4, I5
{
    event System.Action I1.P1 { add {throw null;} remove {throw null;}}
    event System.Action I2.P2 { add {throw null;} remove {throw null;}}
    event System.Action I3.P3 { add {throw null;} remove {throw null;}}
    event System.Action I4.P4 { add {throw null;} remove {throw null;}}
    event System.Action I5.P5 { add {throw null;} remove {throw null;}}
}
";
            ValidateEventModifiers_18(source1,
                // (4,37): error CS8712: 'I1.P1': abstract event cannot use event accessor syntax
                //     abstract event System.Action P1 {add => throw null; remove => throw null;} 
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.P1").WithLocation(4, 37),
                // (8,42): error CS0068: 'I2.P2': instance event in interface cannot have initializer
                //     abstract private event System.Action P2 = null; 
                Diagnostic(ErrorCode.ERR_InterfaceEventInitializer, "P2").WithArguments("I2.P2").WithLocation(8, 42),
                // (16,41): error CS8703: The modifier 'abstract' is not valid for this item in C# 9.0. Please use language version '11.0' or greater.
                //     abstract static event System.Action P4 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "P4").WithArguments("abstract", "9.0", "11.0").WithLocation(16, 41),
                // (16,44): error CS8712: 'I4.P4': abstract event cannot use event accessor syntax
                //     abstract static event System.Action P4 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I4.P4").WithLocation(16, 44),
                // (20,41): error CS0106: The modifier 'override' is not valid for this item
                //     override sealed event System.Action P5 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "P5").WithArguments("override").WithLocation(20, 41),
                // (12,39): warning CS0626: Method, operator, or accessor 'I3.P3.remove' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern event System.Action P3;
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "P3").WithArguments("I3.P3.remove").WithLocation(12, 39),
                // (8,42): error CS0621: 'I2.P2': virtual or abstract members cannot be private
                //     abstract private event System.Action P2 = null; 
                Diagnostic(ErrorCode.ERR_VirtualPrivate, "P2").WithArguments("I2.P2").WithLocation(8, 42),
                // (23,15): error CS0535: 'Test1' does not implement interface member 'I1.P1'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.P1").WithLocation(23, 15),
                // (23,19): error CS0535: 'Test1' does not implement interface member 'I2.P2'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I2.P2").WithLocation(23, 19),
                // (23,27): error CS0535: 'Test1' does not implement interface member 'I4.P4'
                // class Test1 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test1", "I4.P4").WithLocation(23, 27),
                // (30,28): error CS0122: 'I2.P2' is inaccessible due to its protection level
                //     event System.Action I2.P2 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I2.P2").WithLocation(30, 28),
                // (31,28): error CS0539: 'Test2.P3' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I3.P3 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P3").WithArguments("Test2.P3").WithLocation(31, 28),
                // (32,28): error CS0539: 'Test2.P4' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I4.P4 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P4").WithArguments("Test2.P4").WithLocation(32, 28),
                // (33,28): error CS0539: 'Test2.P5' in explicit interface declaration is not found among members of the interface that can be implemented
                //     event System.Action I5.P5 { add {throw null;} remove {throw null;}}
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "P5").WithArguments("Test2.P5").WithLocation(33, 28),
                // (27,27): error CS0535: 'Test2' does not implement interface member 'I4.P4'
                // class Test2 : I1, I2, I3, I4, I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test2", "I4.P4").WithLocation(27, 27),
                // (8,42): warning CS0067: The event 'I2.P2' is never used
                //     abstract private event System.Action P2 = null; 
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "P2").WithArguments("I2.P2").WithLocation(8, 42)
                );
        }

        private void ValidateEventModifiers_18(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular9,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            var test1 = compilation1.GetTypeByMetadataName("Test1");
            var test2 = compilation1.GetTypeByMetadataName("Test2");
            var p1 = GetSingleEvent(compilation1, "I1");
            var test2P1 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I1.")).Single();

            Assert.True(p1.IsAbstract);
            Assert.False(p1.IsVirtual);
            Assert.False(p1.IsSealed);
            Assert.False(p1.IsStatic);
            Assert.False(p1.IsExtern);
            Assert.False(p1.IsOverride);
            Assert.Equal(Accessibility.Public, p1.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p1));
            Assert.Same(test2P1, test2.FindImplementationForInterfaceMember(p1));

            ValidateP1Accessor(p1.AddMethod, test2P1.AddMethod);
            ValidateP1Accessor(p1.RemoveMethod, test2P1.RemoveMethod);
            void ValidateP1Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p2 = GetSingleEvent(compilation1, "I2");
            var test2P2 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I2.")).Single();

            Assert.True(p2.IsAbstract);
            Assert.False(p2.IsVirtual);
            Assert.False(p2.IsSealed);
            Assert.False(p2.IsStatic);
            Assert.False(p2.IsExtern);
            Assert.False(p2.IsOverride);
            Assert.Equal(Accessibility.Private, p2.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p2));
            Assert.Same(test2P2, test2.FindImplementationForInterfaceMember(p2));

            ValidateP2Accessor(p2.AddMethod, test2P2.AddMethod);
            ValidateP2Accessor(p2.RemoveMethod, test2P2.RemoveMethod);
            void ValidateP2Accessor(MethodSymbol accessor, MethodSymbol implementation)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Private, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Same(implementation, test2.FindImplementationForInterfaceMember(accessor));
            }

            var p3 = GetSingleEvent(compilation1, "I3");
            var test2P3 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I3.")).Single();

            Assert.False(p3.IsAbstract);
            Assert.False(p3.IsVirtual);
            Assert.False(p3.IsSealed);
            Assert.True(p3.IsStatic);
            Assert.True(p3.IsExtern);
            Assert.False(p3.IsOverride);
            Assert.Equal(Accessibility.Public, p3.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p3));
            Assert.Null(test2.FindImplementationForInterfaceMember(p3));

            ValidateP3Accessor(p3.AddMethod);
            ValidateP3Accessor(p3.RemoveMethod);
            void ValidateP3Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.True(accessor.IsStatic);
                Assert.True(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }

            var p4 = GetSingleEvent(compilation1, "I4");
            var test2P4 = test2.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I4.")).Single();

            Assert.True(p4.IsAbstract);
            Assert.False(p4.IsVirtual);
            Assert.False(p4.IsSealed);
            Assert.True(p4.IsStatic);
            Assert.False(p4.IsExtern);
            Assert.False(p4.IsOverride);
            Assert.Equal(Accessibility.Public, p4.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p4));
            Assert.Null(test2.FindImplementationForInterfaceMember(p4));

            ValidateP4Accessor(p4.AddMethod);
            ValidateP4Accessor(p4.RemoveMethod);
            void ValidateP4Accessor(MethodSymbol accessor)
            {
                Assert.True(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.True(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.True(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }

            var p5 = GetSingleEvent(compilation1, "I5");

            Assert.False(p5.IsAbstract);
            Assert.False(p5.IsVirtual);
            Assert.False(p5.IsSealed);
            Assert.False(p5.IsStatic);
            Assert.False(p5.IsExtern);
            Assert.False(p5.IsOverride);
            Assert.Equal(Accessibility.Public, p5.DeclaredAccessibility);
            Assert.Null(test1.FindImplementationForInterfaceMember(p5));
            Assert.Null(test2.FindImplementationForInterfaceMember(p5));

            ValidateP5Accessor(p5.AddMethod);
            ValidateP5Accessor(p5.RemoveMethod);
            void ValidateP5Accessor(MethodSymbol accessor)
            {
                Assert.False(accessor.IsAbstract);
                Assert.False(accessor.IsVirtual);
                Assert.False(accessor.IsMetadataVirtual());
                Assert.False(accessor.IsSealed);
                Assert.False(accessor.IsStatic);
                Assert.False(accessor.IsExtern);
                Assert.False(accessor.IsAsync);
                Assert.False(accessor.IsOverride);
                Assert.Equal(Accessibility.Public, accessor.DeclaredAccessibility);
                Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                Assert.Null(test2.FindImplementationForInterfaceMember(accessor));
            }
        }

        [Fact]
        public void EventModifiers_20()
        {
            var source1 =
@"
public interface I1
{
    internal event System.Action P1
    {
        add 
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() 
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidateEventModifiers_20(source1, source2, Accessibility.Internal);
        }

        private void ValidateEventModifiers_20(string source1, string source2, Accessibility accessibility)
        {
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation1,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Single();
                var p1 = GetSingleEvent(i1);
                var p1add = p1.AddMethod;
                var p1remove = p1.RemoveMethod;

                ValidateEvent(p1);
                ValidateMethod(p1add);
                ValidateMethod(p1remove);
                Assert.Same(p1, test1.FindImplementationForInterfaceMember(p1));
                Assert.Same(p1add, test1.FindImplementationForInterfaceMember(p1add));
                Assert.Same(p1remove, test1.FindImplementationForInterfaceMember(p1remove));
            }

            void ValidateEvent(EventSymbol p1)
            {
                Assert.False(p1.IsAbstract);
                Assert.True(p1.IsVirtual);
                Assert.False(p1.IsSealed);
                Assert.False(p1.IsStatic);
                Assert.False(p1.IsExtern);
                Assert.False(p1.IsOverride);
                Assert.Equal(accessibility, p1.DeclaredAccessibility);
            }

            void ValidateMethod(MethodSymbol m1)
            {
                Assert.False(m1.IsAbstract);
                Assert.True(m1.IsVirtual);
                Assert.True(m1.IsMetadataVirtual());
                Assert.False(m1.IsSealed);
                Assert.False(m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.False(m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(accessibility, m1.DeclaredAccessibility);
            }

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics();

            {
                var i1 = compilation2.GetTypeByMetadataName("I1");
                var p1 = GetSingleEvent(i1);

                ValidateEvent(p1);
                ValidateMethod(p1.AddMethod);
                ValidateMethod(p1.RemoveMethod);
            }

            var compilation3 = CreateCompilation(source2, new[] { compilation2.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation3,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, new[] { compilation2.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            CompileAndVerify(compilation4,
                expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
",
                verify: VerifyOnMonoOrCoreClr,
                symbolValidator: Validate1);

            Validate1(compilation4.SourceModule);
        }

        [Fact]
        public void EventModifiers_21()
        {
            var source1 =
@"
public interface I1
{
    private static event System.Action P1 { add => throw null; remove => throw null; }

    internal static event System.Action P2 { add => throw null; remove => throw null; }

    public static event System.Action P3 { add => throw null; remove => throw null; }

    static event System.Action P4 { add => throw null; remove => throw null; }
}

class Test1
{
    static void Main()
    {
        I1.P1 += null;
        I1.P1 -= null;
        I1.P2 += null;
        I1.P2 -= null;
        I1.P3 += null;
        I1.P3 -= null;
        I1.P4 += null;
        I1.P4 -= null;
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (17,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         I1.P1 += null;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(17, 12),
                // (18,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         I1.P1 -= null;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(18, 12)
                );

            var source2 =
@"
class Test2
{
    static void Main()
    {
        I1.P1 += null;
        I1.P1 -= null;
        I1.P2 += null;
        I1.P2 -= null;
        I1.P3 += null;
        I1.P3 -= null;
        I1.P4 += null;
        I1.P4 -= null;
    }
}
";
            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(
                // (6,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         I1.P1 += null;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(6, 12),
                // (7,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //         I1.P1 -= null;
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(7, 12),
                // (8,12): error CS0122: 'I1.P2' is inaccessible due to its protection level
                //         I1.P2 += null;
                Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(8, 12),
                // (9,12): error CS0122: 'I1.P2' is inaccessible due to its protection level
                //         I1.P2 -= null;
                Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(9, 12)
                );
        }

        [Fact]
        public void EventModifiers_22()
        {
            var source0 =
@"
public interface I1
{
    protected static event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    protected internal static event System.Action P2 
    {
        add
        {
            System.Console.WriteLine(""get_P2"");
        }
        remove
        {
            System.Console.WriteLine(""set_P2"");
        }
    }

    private protected static event System.Action P3 
    {
        add => System.Console.WriteLine(""get_P3"");
        remove => System.Console.WriteLine(""set_P3"");
    }
}
";
            var source1 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.P1 += null; 
        I1.P1 -= null;
        I1.P2 += null;
        I1.P2 -= null;
        I1.P3 += null;
        I1.P3 -= null;
    }
}
";
            var compilation1 = CreateCompilation(source0 + source1,
                                                 options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2
get_P3
set_P3", symbolValidator: validate, verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

            validate(compilation1.SourceModule);

            var source2 =
@"
class Test1
{
    static void Main()
    {
        I1.P2 += null;
        I1.P2 -= null;
    }
}
";

            var compilation2 = CreateCompilation(source0 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P2
set_P2",
                verify: VerifyOnMonoOrCoreClr);

            var compilation3 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            var source3 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.P1 += null; 
        I1.P1 -= null;
        I1.P2 += null;
        I1.P2 -= null;
    }
}
";

            var source4 =
@"
class Test1
{
    static void Main()
    {
        I1.P1 += null; 
        I1.P1 -= null;
        I1.P2 += null;
        I1.P2 -= null;
        I1.P3 += null;
        I1.P3 -= null;
    }
}
";

            foreach (var reference in new[] { compilation3.ToMetadataReference(), compilation3.EmitToImageReference() })
            {
                var compilation4 = CreateCompilation(source3, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                CompileAndVerify(compilation4, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"get_P1
set_P1
get_P2
set_P2",
                    verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

                var compilation5 = CreateCompilation(source4, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation5.VerifyDiagnostics(
                    // (6,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                    //         I1.P1 += null; 
                    Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(6, 12),
                    // (7,12): error CS0122: 'I1.P1' is inaccessible due to its protection level
                    //         I1.P1 -= null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(7, 12),
                    // (8,12): error CS0122: 'I1.P2' is inaccessible due to its protection level
                    //         I1.P2 += null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(8, 12),
                    // (9,12): error CS0122: 'I1.P2' is inaccessible due to its protection level
                    //         I1.P2 -= null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P2").WithArguments("I1.P2").WithLocation(9, 12),
                    // (10,12): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         I1.P3 += null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(10, 12),
                    // (11,12): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         I1.P3 -= null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(11, 12)
                    );

                var compilation6 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation6.VerifyDiagnostics(
                    // (10,12): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         I1.P3 += null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(10, 12),
                    // (11,12): error CS0122: 'I1.P3' is inaccessible due to its protection level
                    //         I1.P3 -= null;
                    Diagnostic(ErrorCode.ERR_BadAccess, "P3").WithArguments("I1.P3").WithLocation(11, 12)
                    );
            }

            void validate(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = m.GlobalNamespace.GetTypeMember("I1");

                foreach (var tuple in new[] { (name: "P1", access: Accessibility.Protected),
                                              (name: "P2", access: Accessibility.ProtectedOrInternal),
                                              (name: "P3", access: Accessibility.ProtectedAndInternal)})
                {
                    var p1 = i1.GetMember<EventSymbol>(tuple.name);

                    Assert.False(p1.IsAbstract);
                    Assert.False(p1.IsVirtual);
                    Assert.False(p1.IsSealed);
                    Assert.True(p1.IsStatic);
                    Assert.False(p1.IsExtern);
                    Assert.False(p1.IsOverride);
                    Assert.Equal(tuple.access, p1.DeclaredAccessibility);
                    Assert.Null(test1.FindImplementationForInterfaceMember(p1));

                    ValidateAccessor(p1.AddMethod);
                    ValidateAccessor(p1.RemoveMethod);

                    void ValidateAccessor(MethodSymbol accessor)
                    {
                        Assert.False(accessor.IsAbstract);
                        Assert.False(accessor.IsVirtual);
                        Assert.False(accessor.IsMetadataVirtual());
                        Assert.False(accessor.IsSealed);
                        Assert.True(accessor.IsStatic);
                        Assert.False(accessor.IsExtern);
                        Assert.False(accessor.IsAsync);
                        Assert.False(accessor.IsOverride);
                        Assert.Equal(tuple.access, accessor.DeclaredAccessibility);
                        Assert.Null(test1.FindImplementationForInterfaceMember(accessor));
                    }
                }
            }
        }

        [Fact]
        public void EventModifiers_23()
        {
            var source1 =
@"
public interface I1
{
    protected abstract event System.Action P1; 

    sealed void Test()
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11(source1, source2, Accessibility.Protected,
                new DiagnosticDescription[]
                {
                // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         add
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(12, 9),
                // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                //         remove
                Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(16, 9)
                },
                // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                // class Test2 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                );
        }

        [Fact]
        public void EventModifiers_24()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract event System.Action P1; 

    sealed void Test()
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11(source1, source2, Accessibility.ProtectedOrInternal,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(12, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedNoImplementation: ExpectedDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                    )
                );
        }

        [Fact]
        public void EventModifiers_25()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract event System.Action P1; 

    sealed void Test()
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.Test();
    }

    public event System.Action P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11(source1, source2, Accessibility.ProtectedAndInternal,
                expectedIn9: ExpectedDiagnostics(
                    // (12,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add", "9.0", "10.0").WithLocation(12, 9),
                    // (16,9): error CS8704: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement a non-public member in C# 9.0. Please use language version '10.0' or greater.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfNonPublicInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove", "9.0", "10.0").WithLocation(16, 9)
                    ),
                expectedNoImplementation: ExpectedDiagnostics(
                    // (2,15): error CS0535: 'Test2' does not implement interface member 'I1.P1'
                    // class Test2 : I1
                    Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test2", "I1.P1").WithLocation(2, 15)
                    ),
                expectedAcrossAssemblyBoundaries: ExpectedDiagnostics(
                    // (12,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.add'. 'Test1.P1.add' cannot implicitly implement an inaccessible member.
                    //         add
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "add").WithArguments("Test1", "I1.P1.add", "Test1.P1.add").WithLocation(12, 9),
                    // (16,9): error CS9044: 'Test1' does not implement interface member 'I1.P1.remove'. 'Test1.P1.remove' cannot implicitly implement an inaccessible member.
                    //         remove
                    Diagnostic(ErrorCode.ERR_ImplicitImplementationOfInaccessibleInterfaceMember, "remove").WithArguments("Test1", "I1.P1.remove", "Test1.P1.remove").WithLocation(16, 9)
                    )
                );
        }

        [Fact]
        public void EventModifiers_26()
        {
            var source1 =
@"
public interface I1
{
    protected abstract event System.Action P1; 

    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_03(source1, source2, TargetFramework.NetCoreApp);
        }

        [Fact]
        public void EventModifiers_27()
        {
            var source1 =
@"
public interface I1
{
    protected internal abstract event System.Action P1; 

    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_03(source1, source2, TargetFramework.NetCoreApp);
        }

        [Fact]
        public void EventModifiers_28()
        {
            var source1 =
@"
public interface I1
{
    private protected abstract event System.Action P1; 

    public static void CallP1(I1 x) 
    {
        x.P1 += null;
        x.P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.CallP1(new Test1());
    }

    event System.Action I1.P1 
    {
        add
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove
        {
            System.Console.WriteLine(""set_P1"");
        }
    }
}
";

            ValidateEventModifiers_11_03(source1, source2, TargetFramework.NetCoreApp,
                // (9,28): error CS0122: 'I1.P1' is inaccessible due to its protection level
                //     event System.Action I1.P1 
                Diagnostic(ErrorCode.ERR_BadAccess, "P1").WithArguments("I1.P1").WithLocation(9, 28)
                );
        }

        [Fact]
        public void EventModifiers_29()
        {
            var source1 =
@"
public interface I1
{
    protected event System.Action P1
    {
        add 
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() 
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidateEventModifiers_20(source1, source2, Accessibility.Protected);
        }

        [Fact]
        public void EventModifiers_30()
        {
            var source1 =
@"
public interface I1
{
    protected internal event System.Action P1
    {
        add 
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() 
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidateEventModifiers_20(source1, source2, Accessibility.ProtectedOrInternal);
        }

        [Fact]
        public void EventModifiers_31()
        {
            var source1 =
@"
public interface I1
{
    private protected event System.Action P1
    {
        add 
        {
            System.Console.WriteLine(""get_P1"");
        }
        remove 
        {
            System.Console.WriteLine(""set_P1"");
        }
    }

    void M2() 
    {
        P1 += null;
        P1 -= null;
    }
}
";

            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1 x = new Test1();
        x.M2();
    }
}
";

            ValidateEventModifiers_20(source1, source2, Accessibility.ProtectedAndInternal);
        }

        [Fact]
        public void NestedTypes_01()
        {
            var source1 =
@"
public interface I1
{
    interface T1
    {
        void M1();
    }

    class T2
    {}

    struct T3
    {}

    enum T4
    {
        B
    }

    delegate void T5();
}

class Test1 : I1.T1
{
    static void Main()
    {
        I1.T1 a = new Test1();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    public void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            ValidateNestedTypes_01(source1);
        }

        private void ValidateNestedTypes_01(string source1, Accessibility expected = Accessibility.Public, TargetFramework targetFramework = TargetFramework.Standard, bool execute = true, Verification verify = default)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: targetFramework);

            for (int i = 1; i <= 5; i++)
            {
                Assert.Equal(expected, compilation1.GetMember("I1.T" + i).DeclaredAccessibility);
            }

            CompileAndVerify(compilation1, expectedOutput: !execute ? null :
@"M1
I1+T2
I1+T3
B
I1+T5",
                verify: verify);
        }

        [Fact]
        public void NestedTypes_02()
        {
            var source1 =
@"
public interface I1
{
    interface T1
    {
        void M1();
    }

    class T2
    {}

    struct T3
    {}

    enum T4
    {
        B
    }
}

class Test1 : I1.T1
{
    static void Main()
    {
        I1.T1 a = new Test1();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
    }

    public void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3);

            for (int i = 1; i <= 4; i++)
            {
                Assert.Equal(Accessibility.Public, compilation1.GetMember("I1.T" + i).DeclaredAccessibility);
            }

            compilation1.VerifyDiagnostics(
                // (4,15): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     interface T1
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "T1").WithArguments("default interface implementation", "8.0").WithLocation(4, 15),
                // (9,11): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     class T2
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "T2").WithArguments("default interface implementation", "8.0").WithLocation(9, 11),
                // (12,12): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     struct T3
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "T3").WithArguments("default interface implementation", "8.0").WithLocation(12, 12),
                // (15,10): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     enum T4
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "T4").WithArguments("default interface implementation", "8.0").WithLocation(15, 10)
                );
        }

        [Fact]
        public void NestedTypes_03()
        {
            var source1 =
@"
public interface I1
{
    public interface T1
    {
        void M1();
    }

    public class T2
    {}

    public struct T3
    {}

    public enum T4
    {
        B
    }

    public delegate void T5();
}

class Test1 : I1.T1
{
    static void Main()
    {
        I1.T1 a = new Test1();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    public void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            ValidateNestedTypes_01(source1);
        }

        [Fact]
        public void NestedTypes_04()
        {
            var source0 =
@"
public interface I1
{
    protected interface T1
    {
        void M1();
    }

    protected class T2
    {}

    protected struct T3
    {}

    protected enum T4
    {
        B
    }

    protected delegate void T5();
}
";
            var source1 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.T1 a = new Test2();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    class Test2 : I1.T1
    {
        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";

            ValidateNestedTypes_01(source0 + source1, Accessibility.Protected, targetFramework: TargetFramework.NetCoreApp, execute: ExecutionConditionUtil.IsMonoOrCoreClr, verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

            var compilation1 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            for (int i = 1; i <= 5; i++)
            {
                Assert.Equal(Accessibility.Protected, compilation1.GetMember("I1.T" + i).DeclaredAccessibility);
            }

            compilation1.VerifyDiagnostics(
                // (4,25): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected interface T1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T1").WithLocation(4, 25),
                // (9,21): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected class T2
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T2").WithLocation(9, 21),
                // (12,22): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected struct T3
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T3").WithLocation(12, 22),
                // (15,20): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected enum T4
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T4").WithLocation(15, 20),
                // (20,29): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected delegate void T5();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T5").WithLocation(20, 29)
                );

            var source2 =
@"
class Test1
{
    static void Main()
    {
        I1.T1 a = new Test2();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    class Test2 : I1.T1
    {
        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";

            var compilation2 = CreateCompilation(source2 + source0, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            var expected = new DiagnosticDescription[] {
                // (6,12): error CS0122: 'I1.T1' is inaccessible due to its protection level
                //         I1.T1 a = new Test2();
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(6, 12),
                // (7,11): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         a.M1();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(7, 11),
                // (8,41): error CS0122: 'I1.T2' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T2());
                Diagnostic(ErrorCode.ERR_BadAccess, "T2").WithArguments("I1.T2").WithLocation(8, 41),
                // (9,41): error CS0122: 'I1.T3' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T3());
                Diagnostic(ErrorCode.ERR_BadAccess, "T3").WithArguments("I1.T3").WithLocation(9, 41),
                // (10,37): error CS0122: 'I1.T4' is inaccessible due to its protection level
                //         System.Console.WriteLine(I1.T4.B);
                Diagnostic(ErrorCode.ERR_BadAccess, "T4").WithArguments("I1.T4").WithLocation(10, 37),
                // (11,41): error CS0122: 'I1.T5' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T5(a.M1));
                Diagnostic(ErrorCode.ERR_BadAccess, "T5").WithArguments("I1.T5").WithLocation(11, 41),
                // (11,46): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T5(a.M1));
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(11, 46),
                // (14,22): error CS0122: 'I1.T1' is inaccessible due to its protection level
                //     class Test2 : I1.T1
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(14, 22)
                };

            compilation2.VerifyDiagnostics(expected);

            var compilation3 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            foreach (var reference in new[] { compilation3.ToMetadataReference(), compilation3.EmitToImageReference() })
            {
                var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation4.VerifyDiagnostics();
                CompileAndVerify(compilation4, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M1
I1+T2
I1+T3
B
I1+T5",
                    verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

                var compilation5 = CreateCompilation(source2, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation5.VerifyDiagnostics(expected);
            }
        }

        [Fact]
        public void NestedTypes_05()
        {
            var source0 =
@"
public interface I1
{
    protected internal interface T1
    {
        void M1();
    }

    protected internal class T2
    {}

    protected internal struct T3
    {}

    protected internal enum T4
    {
        B
    }

    protected internal delegate void T5();
}
";
            var source1 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.T1 a = new Test2();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    class Test2 : I1.T1
    {
        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";

            var source2 =
@"
class Test1
{
    static void Main()
    {
        I1.T1 a = new Test2();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    class Test2 : I1.T1
    {
        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";
            ValidateNestedTypes_01(source0 + source1, Accessibility.ProtectedOrInternal, targetFramework: TargetFramework.NetCoreApp, execute: ExecutionConditionUtil.IsMonoOrCoreClr, verify: VerifyOnMonoOrCoreClr);
            ValidateNestedTypes_01(source0 + source2, Accessibility.ProtectedOrInternal, targetFramework: TargetFramework.NetCoreApp, execute: ExecutionConditionUtil.IsMonoOrCoreClr, verify: VerifyOnMonoOrCoreClr);

            var compilation1 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            for (int i = 1; i <= 5; i++)
            {
                Assert.Equal(Accessibility.ProtectedOrInternal, compilation1.GetMember("I1.T" + i).DeclaredAccessibility);
            }

            compilation1.VerifyDiagnostics(
                // (4,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal interface T1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T1").WithLocation(4, 34),
                // (9,30): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal class T2
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T2").WithLocation(9, 30),
                // (12,31): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal struct T3
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T3").WithLocation(12, 31),
                // (15,29): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal enum T4
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T4").WithLocation(15, 29),
                // (20,38): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal delegate void T5();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T5").WithLocation(20, 38)
                );

            var compilation3 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            foreach (var reference in new[] { compilation3.ToMetadataReference(), compilation3.EmitToImageReference() })
            {
                var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation4.VerifyDiagnostics();
                CompileAndVerify(compilation4, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"M1
I1+T2
I1+T3
B
I1+T5",
                    verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

                var compilation5 = CreateCompilation(source2, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation5.VerifyDiagnostics(
                    // (6,12): error CS0122: 'I1.T1' is inaccessible due to its protection level
                    //         I1.T1 a = new Test2();
                    Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(6, 12),
                    // (7,11): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                    //         a.M1();
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(7, 11),
                    // (8,41): error CS0122: 'I1.T2' is inaccessible due to its protection level
                    //         System.Console.WriteLine(new I1.T2());
                    Diagnostic(ErrorCode.ERR_BadAccess, "T2").WithArguments("I1.T2").WithLocation(8, 41),
                    // (9,41): error CS0122: 'I1.T3' is inaccessible due to its protection level
                    //         System.Console.WriteLine(new I1.T3());
                    Diagnostic(ErrorCode.ERR_BadAccess, "T3").WithArguments("I1.T3").WithLocation(9, 41),
                    // (10,37): error CS0122: 'I1.T4' is inaccessible due to its protection level
                    //         System.Console.WriteLine(I1.T4.B);
                    Diagnostic(ErrorCode.ERR_BadAccess, "T4").WithArguments("I1.T4").WithLocation(10, 37),
                    // (11,41): error CS0122: 'I1.T5' is inaccessible due to its protection level
                    //         System.Console.WriteLine(new I1.T5(a.M1));
                    Diagnostic(ErrorCode.ERR_BadAccess, "T5").WithArguments("I1.T5").WithLocation(11, 41),
                    // (11,46): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                    //         System.Console.WriteLine(new I1.T5(a.M1));
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(11, 46),
                    // (14,22): error CS0122: 'I1.T1' is inaccessible due to its protection level
                    //     class Test2 : I1.T1
                    Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(14, 22)
                    );
            }
        }

        [Fact]
        public void NestedTypes_06()
        {
            var source1 =
@"
public interface I1
{
    internal interface T1
    {
        void M1();
    }

    internal class T2
    {}

    internal struct T3
    {}

    internal enum T4
    {
        B
    }

    internal delegate void T5();
}
";
            var source2 =
@"
class Test1 : I1.T1
{
    static void Main()
    {
        I1.T1 a = new Test1();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    public void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}
";
            ValidateNestedTypes_01(source1 + source2, Accessibility.Internal);

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe);

            var expected = new[] {
                // (2,18): error CS0122: 'I1.T1' is inaccessible due to its protection level
                // class Test1 : I1.T1
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(2, 18),
                // (6,12): error CS0122: 'I1.T1' is inaccessible due to its protection level
                //         I1.T1 a = new Test1();
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(6, 12),
                // (7,11): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         a.M1();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(7, 11),
                // (8,41): error CS0122: 'I1.T2' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T2());
                Diagnostic(ErrorCode.ERR_BadAccess, "T2").WithArguments("I1.T2").WithLocation(8, 41),
                // (9,41): error CS0122: 'I1.T3' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T3());
                Diagnostic(ErrorCode.ERR_BadAccess, "T3").WithArguments("I1.T3").WithLocation(9, 41),
                // (10,37): error CS0122: 'I1.T4' is inaccessible due to its protection level
                //         System.Console.WriteLine(I1.T4.B);
                Diagnostic(ErrorCode.ERR_BadAccess, "T4").WithArguments("I1.T4").WithLocation(10, 37),
                // (11,41): error CS0122: 'I1.T5' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T5(a.M1));
                Diagnostic(ErrorCode.ERR_BadAccess, "T5").WithArguments("I1.T5").WithLocation(11, 41),
                // (11,46): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T5(a.M1));
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(11, 46)
                };
            compilation2.VerifyDiagnostics(expected);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe);

            compilation3.VerifyDiagnostics(expected);
        }

        [Fact]
        public void NestedTypes_07()
        {
            var source1 =
@"
public interface I1
{
    private interface T1
    {
        void M1();
    }

    private class T2
    {}

    private struct T3
    {}

    private enum T4
    {
        B
    }

    private delegate void T5();

    class Test1 : I1.T1
    {
        static void Main()
        {
            I1.T1 a = new Test1();
            a.M1();
            System.Console.WriteLine(new I1.T2());
            System.Console.WriteLine(new I1.T3());
            System.Console.WriteLine(I1.T4.B);
            System.Console.WriteLine(new I1.T5(a.M1));
        }

        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";
            ValidateNestedTypes_01(source1, Accessibility.Private);
        }

        [Fact]
        public void NestedTypes_08()
        {
            var source1 =
@"
public interface I1
{
    private interface T1
    {
        void M1();
    }

    private class T2
    {}

    private struct T3
    {}

    private enum T4
    {
        B
    }
}

class Test1 : I1.T1
{
    static void Main()
    {
        I1.T1 a = new Test1();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
    }

    public void M1()
    {
        System.Console.WriteLine(""M1"");
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(
                // (21,18): error CS0122: 'I1.T1' is inaccessible due to its protection level
                // class Test1 : I1.T1
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(21, 18),
                // (25,12): error CS0122: 'I1.T1' is inaccessible due to its protection level
                //         I1.T1 a = new Test1();
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(25, 12),
                // (26,11): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         a.M1();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(26, 11),
                // (27,41): error CS0122: 'I1.T2' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T2());
                Diagnostic(ErrorCode.ERR_BadAccess, "T2").WithArguments("I1.T2").WithLocation(27, 41),
                // (28,41): error CS0122: 'I1.T3' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T3());
                Diagnostic(ErrorCode.ERR_BadAccess, "T3").WithArguments("I1.T3").WithLocation(28, 41),
                // (29,37): error CS0122: 'I1.T4' is inaccessible due to its protection level
                //         System.Console.WriteLine(I1.T4.B);
                Diagnostic(ErrorCode.ERR_BadAccess, "T4").WithArguments("I1.T4").WithLocation(29, 37)
                );
        }

        [Fact]
        public void NestedTypes_09()
        {
            var source0 =
@"
public interface I1
{
    private protected interface T1
    {
        void M1();
    }

    private protected class T2
    {}

    private protected struct T3
    {}

    private protected enum T4
    {
        B
    }

    private protected delegate void T5();
}
";
            var source1 =
@"
class Test1 : I1
{
    static void Main()
    {
        I1.T1 a = new Test2();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    class Test2 : I1.T1
    {
        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";

            var source2 =
@"
class Test1
{
    static void Main()
    {
        I1.T1 a = new Test2();
        a.M1();
        System.Console.WriteLine(new I1.T2());
        System.Console.WriteLine(new I1.T3());
        System.Console.WriteLine(I1.T4.B);
        System.Console.WriteLine(new I1.T5(a.M1));
    }

    class Test2 : I1.T1
    {
        public void M1()
        {
            System.Console.WriteLine(""M1"");
        }
    }
}
";
            ValidateNestedTypes_01(source0 + source1, Accessibility.ProtectedAndInternal, targetFramework: TargetFramework.NetCoreApp, execute: ExecutionConditionUtil.IsMonoOrCoreClr, verify: VerifyOnMonoOrCoreClr_FailsIlVerify);

            var compilation1 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            for (int i = 1; i <= 5; i++)
            {
                Assert.Equal(Accessibility.ProtectedAndInternal, compilation1.GetMember("I1.T" + i).DeclaredAccessibility);
            }

            compilation1.VerifyDiagnostics(
                // (4,33): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected interface T1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T1").WithLocation(4, 33),
                // (9,29): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected class T2
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T2").WithLocation(9, 29),
                // (12,30): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected struct T3
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T3").WithLocation(12, 30),
                // (15,28): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected enum T4
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T4").WithLocation(15, 28),
                // (20,37): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected delegate void T5();
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "T5").WithLocation(20, 37)
                );

            var compilation2 = CreateCompilation(source2 + source0, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            var expected = new DiagnosticDescription[] {
                // (6,12): error CS0122: 'I1.T1' is inaccessible due to its protection level
                //         I1.T1 a = new Test2();
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(6, 12),
                // (7,11): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         a.M1();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(7, 11),
                // (8,41): error CS0122: 'I1.T2' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T2());
                Diagnostic(ErrorCode.ERR_BadAccess, "T2").WithArguments("I1.T2").WithLocation(8, 41),
                // (9,41): error CS0122: 'I1.T3' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T3());
                Diagnostic(ErrorCode.ERR_BadAccess, "T3").WithArguments("I1.T3").WithLocation(9, 41),
                // (10,37): error CS0122: 'I1.T4' is inaccessible due to its protection level
                //         System.Console.WriteLine(I1.T4.B);
                Diagnostic(ErrorCode.ERR_BadAccess, "T4").WithArguments("I1.T4").WithLocation(10, 37),
                // (11,41): error CS0122: 'I1.T5' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T5(a.M1));
                Diagnostic(ErrorCode.ERR_BadAccess, "T5").WithArguments("I1.T5").WithLocation(11, 41),
                // (11,46): error CS0122: 'I1.T1.M1()' is inaccessible due to its protection level
                //         System.Console.WriteLine(new I1.T5(a.M1));
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I1.T1.M1()").WithLocation(11, 46),
                // (14,22): error CS0122: 'I1.T1' is inaccessible due to its protection level
                //     class Test2 : I1.T1
                Diagnostic(ErrorCode.ERR_BadAccess, "T1").WithArguments("I1.T1").WithLocation(14, 22)
                };

            compilation2.VerifyDiagnostics(expected);

            var compilation3 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            foreach (var reference in new[] { compilation3.ToMetadataReference(), compilation3.EmitToImageReference() })
            {
                var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation4.VerifyDiagnostics(expected);

                var compilation5 = CreateCompilation(source2, options: TestOptions.DebugExe,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation5.VerifyDiagnostics(expected);
            }
        }

        [Fact]
        [WorkItem(34704, "https://github.com/dotnet/roslyn/issues/34704")]
        public void NestedTypes_10()
        {
            var source1 =
@"
interface I1
{
    protected interface I2
    {
    }
}

class C1
{
    protected interface I2
    {
    }
}

interface I3 : I1, I1.I2
{
    private void M1(I1.I2 x) { }
}

interface I4 : I1, I2
{
    I2 MI4();
}

interface I5 : I1.I2, I1
{
    private void M1(I1.I2 x) { }
}

interface I6 : I2, I1
{
    I2 MI6();
}

class C3 : I1, I1.I2
{
    private void M1(I1.I2 x) { }
}

class C4 : I1, I2
{
    void MC4(I2 x) { }
}

class C5 : I1.I2, I1
{
    private void M1(I1.I2 x) { }
}

class C6 : I2, I1
{
    void MC6(I2 x) { }
}

class C33 : C1, C1.I2
{
    protected void M1(C1.I2 x) { }
}

class C44 : C1, I2
{
    void M1(I2 x) { }
}

interface I7 : I8
{
    public interface I8 : I7
    {
    }
}

class C7 : I8
{
    public interface I8
    {
    }
}

interface I9 : I9.I10
{
    public interface I10 : I9
    {
    }
}

interface I11
{
    public interface I12 : I13
    {
    }

    public interface I13
    {
    }

    public interface I14 : I11.I13
    {
    }
}

class C11
{
    public interface I12 : I13
    {
    }

    public interface I13
    {
    }

    public interface I14 : I11.I13
    {
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (16,23): error CS0122: 'I1.I2' is inaccessible due to its protection level
                // interface I3 : I1, I1.I2
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1.I2").WithLocation(16, 23),
                // (21,20): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                // interface I4 : I1, I2
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(21, 20),
                // (23,8): error CS0050: Inconsistent accessibility: return type 'I1.I2' is less accessible than method 'I4.MI4()'
                //     I2 MI4();
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "MI4").WithArguments("I4.MI4()", "I1.I2").WithLocation(23, 8),
                // (26,19): error CS0122: 'I1.I2' is inaccessible due to its protection level
                // interface I5 : I1.I2, I1
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1.I2").WithLocation(26, 19),
                // (31,16): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                // interface I6 : I2, I1
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(31, 16),
                // (33,8): error CS0050: Inconsistent accessibility: return type 'I1.I2' is less accessible than method 'I6.MI6()'
                //     I2 MI6();
                Diagnostic(ErrorCode.ERR_BadVisReturnType, "MI6").WithArguments("I6.MI6()", "I1.I2").WithLocation(33, 8),
                // (36,19): error CS0122: 'I1.I2' is inaccessible due to its protection level
                // class C3 : I1, I1.I2
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1.I2").WithLocation(36, 19),
                // (41,16): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                // class C4 : I1, I2
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(41, 16),
                // (43,14): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                //     void MC4(I2 x) { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(43, 14),
                // (46,15): error CS0122: 'I1.I2' is inaccessible due to its protection level
                // class C5 : I1.I2, I1
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("I1.I2").WithLocation(46, 15),
                // (51,12): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                // class C6 : I2, I1
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(51, 12),
                // (53,14): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                //     void MC6(I2 x) { }
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(53, 14),
                // (56,20): error CS0122: 'C1.I2' is inaccessible due to its protection level
                // class C33 : C1, C1.I2
                Diagnostic(ErrorCode.ERR_BadAccess, "I2").WithArguments("C1.I2").WithLocation(56, 20),
                // (61,17): error CS0246: The type or namespace name 'I2' could not be found (are you missing a using directive or an assembly reference?)
                // class C44 : C1, I2
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I2").WithArguments("I2").WithLocation(61, 17),
                // (66,16): error CS0246: The type or namespace name 'I8' could not be found (are you missing a using directive or an assembly reference?)
                // interface I7 : I8
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I8").WithArguments("I8").WithLocation(66, 16),
                // (73,12): error CS0246: The type or namespace name 'I8' could not be found (are you missing a using directive or an assembly reference?)
                // class C7 : I8
                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "I8").WithArguments("I8").WithLocation(73, 12),
                // (80,11): error CS0529: Inherited interface 'I9.I10' causes a cycle in the interface hierarchy of 'I9'
                // interface I9 : I9.I10
                Diagnostic(ErrorCode.ERR_CycleInInterfaceInheritance, "I9").WithArguments("I9", "I9.I10").WithLocation(80, 11),
                // (82,22): error CS0529: Inherited interface 'I9' causes a cycle in the interface hierarchy of 'I9.I10'
                //     public interface I10 : I9
                Diagnostic(ErrorCode.ERR_CycleInInterfaceInheritance, "I10").WithArguments("I9.I10", "I9").WithLocation(82, 22)
                );
        }

        [Fact]
        [WorkItem(38735, "https://github.com/dotnet/roslyn/issues/38735")]
        public void NestedTypes_52()
        {
            var source1 =
@"
interface I1 : IA<int>.NF { }

interface IQ<T> { }

interface IA<T> : IB<IQ<T>> { }

interface IB<T> : IA<IQ<T>> { }
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics(
                // (2,24): error CS0426: The type name 'NF' does not exist in the type 'IA<int>'
                // interface I1 : IA<int>.NF { }
                Diagnostic(ErrorCode.ERR_DottedTypeNameNotFoundInAgg, "NF").WithArguments("NF", "IA<int>").WithLocation(2, 24),
                // (6,11): error CS0529: Inherited interface 'IB<IQ<T>>' causes a cycle in the interface hierarchy of 'IA<T>'
                // interface IA<T> : IB<IQ<T>> { }
                Diagnostic(ErrorCode.ERR_CycleInInterfaceInheritance, "IA").WithArguments("IA<T>", "IB<IQ<T>>").WithLocation(6, 11),
                // (8,11): error CS0529: Inherited interface 'IA<IQ<T>>' causes a cycle in the interface hierarchy of 'IB<T>'
                // interface IB<T> : IA<IQ<T>> { }
                Diagnostic(ErrorCode.ERR_CycleInInterfaceInheritance, "IB").WithArguments("IB<T>", "IA<IQ<T>>").WithLocation(8, 11)
                );
        }

        [Fact]
        [WorkItem(58424, "https://github.com/dotnet/roslyn/issues/58424")]
        public void NestedTypes_53()
        {
            var source1 =
@"
public class Test : Test.IInner.IOuter
{
    public interface IInner : Other
    {
        public interface IOuter
        {
        }
    }
}

public interface Other
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
        }

        [Fact]
        [WorkItem(58424, "https://github.com/dotnet/roslyn/issues/58424")]
        public void NestedTypes_54()
        {
            var source1 =
@"
public class Test : Test.IInner.IOuter
{
    public interface IInner : Other
    {
    }
}

public interface Other
{
    public interface IOuter
    {
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (2,33): error CS0426: The type name 'IOuter' does not exist in the type 'Test.IInner'
                // public class Test : Test.IInner.IOuter
                Diagnostic(ErrorCode.ERR_DottedTypeNameNotFoundInAgg, "IOuter").WithArguments("IOuter", "Test.IInner").WithLocation(2, 33),
                // (4,31): error CS0146: Circular base type dependency involving 'Test' and 'Test.IInner'
                //     public interface IInner : Other
                Diagnostic(ErrorCode.ERR_CircularBase, "Other").WithArguments("Test", "Test.IInner").WithLocation(4, 31)
                );
        }

        [Fact]
        [WorkItem(58424, "https://github.com/dotnet/roslyn/issues/58424")]
        [WorkItem(62795, "https://github.com/dotnet/roslyn/issues/62795")]
        public void NestedTypes_55()
        {
            var source1 =
@"
namespace Ns0
{
    public class Test : Test.IInner.IOuter
    {
        public interface IInner : global::Ns0.Other
        {
            new public interface IOuter
            {
            }
        }
    }

    public interface Other
    {
        public interface IOuter
        {
            void M1();
        }
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void MethodImplementationInDerived_01(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I4
{
    " + modifiers + @"void M1(); 
}

public interface I5 : I4
{
}

public interface I1 : I2, I5
{
    " + implModifiers + @"void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"void I4.M1() 
    {
        System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";

            var source2 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I2 i2 = new Test1();
        I4 i4 = new Test1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source2 +=
@"
        i2.M1();
        i4.M1();
    }
}
";

            ValidateMethodImplementationInDerived_01(source1, source2, isStatic);
        }

        private void ValidateMethodImplementationInDerived_01(string source1, string source2, bool isStatic, string name = "M1")
        {
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateMethodImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            validateMethodImplementationInDerived_01(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            validateMethodImplementationInDerived_01(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            void validateMethodImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateMethodImplementationInDerived_01(m, i4M1IsAbstract: false, isStatic: isStatic, name: name);
            }
        }

        private static void ValidateMethodImplementationInDerived_01(ModuleSymbol m, bool isStatic)
        {
            ValidateMethodImplementationInDerived_01(m, i4M1IsAbstract: false, isStatic: isStatic);
        }

        private static void ValidateMethodImplementationInDerived_01(ModuleSymbol m, bool i4M1IsAbstract, bool isStatic, string name = "M1")
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            var i1i2m1 = i1.GetMember<MethodSymbol>(i1.OriginalDefinition.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I2").Single().ToDisplayString() + "." + name);
            var i1i4m1 = i1.GetMember<MethodSymbol>(i1.OriginalDefinition.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I4").Single().ToDisplayString() + "." + name);
            var i2 = i1.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I2").Single();
            var i2m1 = i2.GetMember<MethodSymbol>(name);
            var i4 = i1.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I4").Single();
            var i4m1 = i4.GetMember<MethodSymbol>(name);

            Assert.True(i1.IsAbstract);
            Assert.True(i1.IsMetadataAbstract);

            ValidateExplicitImplementation(i1i2m1, isStatic: isStatic);
            ValidateExplicitImplementation(i1i4m1, i4M1IsAbstract, isStatic: isStatic);

            Assert.Null(test1.FindImplementationForInterfaceMember(i1i2m1));
            Assert.Null(test1.FindImplementationForInterfaceMember(i1i4m1));
            Assert.Equal(i1i2m1, test1.FindImplementationForInterfaceMember(i2m1));
            Assert.Equal(i4M1IsAbstract ? null : i1i4m1, test1.FindImplementationForInterfaceMember(i4m1));

            Assert.Null(i1.FindImplementationForInterfaceMember(i1i2m1));
            Assert.Null(i1.FindImplementationForInterfaceMember(i1i4m1));
            Assert.Equal(i1i2m1, i1.FindImplementationForInterfaceMember(i2m1));
            Assert.Equal(i4M1IsAbstract ? null : i1i4m1, i1.FindImplementationForInterfaceMember(i4m1));

            Assert.Null(i2.FindImplementationForInterfaceMember(i2m1));
            Assert.Null(i4.FindImplementationForInterfaceMember(i4m1));

            var i3 = i1.ContainingNamespace.GetTypeMember("I3");
            i1 = i3.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            i1i2m1 = i1.GetMember<MethodSymbol>(i1.OriginalDefinition.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I2").Single().ToDisplayString() + "." + name);
            i1i4m1 = i1.GetMember<MethodSymbol>(i1.OriginalDefinition.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I4").Single().ToDisplayString() + "." + name);
            i2 = i1.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I2").Single();
            i2m1 = i2.GetMember<MethodSymbol>(name);
            i4 = i1.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I4").Single();
            i4m1 = i4.GetMember<MethodSymbol>(name);

            Assert.Equal(i1i2m1, i3.FindImplementationForInterfaceMember(i2m1));
            Assert.Equal(i4M1IsAbstract ? null : i1i4m1, i3.FindImplementationForInterfaceMember(i4m1));
        }

        private static void ValidateExplicitImplementation(MethodSymbol m1, bool isAbstract = false, bool isStatic = false)
        {
            Assert.Equal(isAbstract || !isStatic, m1.IsMetadataVirtual());
            Assert.Equal(isAbstract || !isStatic, m1.IsMetadataFinal);
            Assert.False(m1.IsMetadataNewSlot());
            Assert.Equal(isAbstract, m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.Equal(isAbstract, m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);

            if (m1.ContainingModule is PEModuleSymbol peModule)
            {
                int rva;
                peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)m1.OriginalDefinition).Handle, out _, out _, out _, out rva);
                Assert.NotEqual(0, rva);
            }
        }

        [Fact]
        public void MethodImplementationInDerived_02()
        {
            var source1 =
@"
public interface I2
{
    void M1(); 
}

public interface I4
{
    void M1(); 
}

public interface I1 : I2, I4
{
    void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,13): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     void I2.M1() 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "M1").WithArguments("default interface implementation", "8.0").WithLocation(14, 13),
                // (18,13): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     void I4.M1() 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "M1").WithArguments("default interface implementation", "8.0").WithLocation(18, 13)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, isStatic: false);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8506: 'I1.I2.M1()' cannot implement interface member 'I2.M1()' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1()", "I2.M1()", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15),
                // (6,15): error CS8506: 'I1.I4.M1()' cannot implement interface member 'I4.M1()' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1()", "I4.M1()", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15)
                );

            ValidateMethodImplementationInDerived_01(compilation2.SourceModule, isStatic: false);
        }

        [Fact]
        public void MethodImplementationInDerived_Static_02()
        {
            var source1 =
@"
public interface I2
{
    static abstract void M1(); 
}

public interface I4
{
    static abstract void M1(); 
}

public interface I1 : I2, I4
{
    static void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    static void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular10,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (4,26): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract void M1(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(4, 26),
                // (9,26): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract void M1(); 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(9, 26),
                // (14,20): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static void I2.M1() 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(14, 20),
                // (18,20): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static void I4.M1() 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(18, 20)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, isStatic: true);

            CreateCompilation(source1, options: TestOptions.DebugDll,
                              parseOptions: TestOptions.RegularPreview,
                              targetFramework: TargetFramework.Net60).VerifyEmitDiagnostics();

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular10,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8706: 'I1.I2.M1()' cannot implement interface member 'I2.M1()' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1()", "I2.M1()", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15),
                // (6,15): error CS8706: 'I1.I4.M1()' cannot implement interface member 'I4.M1()' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1()", "I4.M1()", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15)
                );

            ValidateMethodImplementationInDerived_01(compilation2.SourceModule, isStatic: true);

            CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                              parseOptions: TestOptions.RegularPreview,
                              targetFramework: TargetFramework.Net60).VerifyEmitDiagnostics();
        }

        [Fact]
        public void MethodImplementationInDerived_03()
        {
            var source1 =
@"
public interface I2
{
    void M1(); 
}

public interface I4
{
    void M1(); 
}

public interface I1 : I2, I4
{
    void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular, skipUsesIsNullable: true);
            Assert.False(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (14,13): error CS8501: Target runtime doesn't support default interface implementation.
                //     void I2.M1() 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(14, 13),
                // (18,13): error CS8501: Target runtime doesn't support default interface implementation.
                //     void I4.M1() 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(18, 13)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, isStatic: false);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular);
            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8502: 'I1.I2.M1()' cannot implement interface member 'I2.M1()' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1()", "I2.M1()", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I4.M1()' cannot implement interface member 'I4.M1()' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1()", "I4.M1()", "Test1").WithLocation(6, 15)
                );
        }

        [Fact]
        public void MethodImplementationInDerived_Static_03()
        {
            var source1 =
@"
public interface I2
{
    static abstract void M1(); 
}

public interface I4
{
    static abstract void M1(); 
}

public interface I1 : I2, I4
{
    static void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    static void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            compilation1.VerifyDiagnostics(
                // (4,26): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract void M1(); 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(4, 26),
                // (9,26): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract void M1(); 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(9, 26),
                // (14,20): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static void I2.M1() 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(14, 20),
                // (18,20): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static void I4.M1() 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(18, 20)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, isStatic: true);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8929: 'I1.I2.M1()' cannot implement interface member 'I2.M1()' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I2.M1()", "I2.M1()", "Test1").WithLocation(6, 15),
                // (6,15): error CS8929: 'I1.I4.M1()' cannot implement interface member 'I4.M1()' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I4.M1()", "I4.M1()", "Test1").WithLocation(6, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_04(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I4
{
    " + modifiers + @"void M1(); 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"void I2.M1(); 
    " + implModifiers + @"void I4.M1(); 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (15,13): error CS0501: 'I1.I4.M1()' must declare a body because it is not marked abstract, extern, or partial
                //     void I4.M1(); 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M1").WithArguments("I1.I4.M1()").WithLocation(15, 13 + implModifiers.Length),
                // (14,13): error CS0501: 'I1.I2.M1()' must declare a body because it is not marked abstract, extern, or partial
                //     void I2.M1(); 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "M1").WithArguments("I1.I2.M1()").WithLocation(14, 13 + implModifiers.Length)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_05(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I4
{
    " + modifiers + @"void M1(); 
}

public interface I1
{
    " + implModifiers + @"void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (18,10): error CS0540: 'I1.I4.M1()': containing type does not implement interface 'I4'
                //     void I4.M1() 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I4").WithArguments("I1.I4.M1()", "I4").WithLocation(18, 10 + implModifiers.Length),
                // (14,10): error CS0540: 'I1.I2.M1()': containing type does not implement interface 'I2'
                //     void I2.M1() 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I2").WithArguments("I1.I2.M1()", "I2").WithLocation(14, 10 + implModifiers.Length)
                );
        }

        [Fact]
        public void MethodImplementationInDerived_06()
        {
            var source1 =
@"
public interface I2
{
    void M1(); 
}

public interface I4
{
    void M1(); 
}

public interface I1 : I2, I4
{
    public static void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    internal virtual void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,27): error CS0106: The modifier 'public' is not valid for this item
                //     public static void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("public").WithLocation(14, 27),
                // (14,27): error CS0539: 'I1.M1()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     public static void I2.M1() 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.M1()").WithLocation(14, 27),
                // (18,30): error CS0106: The modifier 'internal' is not valid for this item
                //     internal virtual void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("internal").WithLocation(18, 30),
                // (18,30): error CS0106: The modifier 'virtual' is not valid for this item
                //     internal virtual void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("virtual").WithLocation(18, 30),
                // (28,15): error CS0535: 'Test1' does not implement interface member 'I2.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I2.M1()").WithLocation(28, 15)
                );
        }

        [Fact]
        public void MethodImplementationInDerived_Static_06()
        {
            var source1 =
@"
public interface I2
{
    static abstract void M1(); 
}

public interface I4
{
    static abstract void M1(); 
}

public interface I1 : I2, I4
{
    public void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    internal virtual static void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,20): error CS0106: The modifier 'public' is not valid for this item
                //     public void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("public").WithLocation(14, 20),
                // (14,20): error CS0539: 'I1.M1()' in explicit interface declaration is not found among members of the interface that can be implemented
                //     public void I2.M1() 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.M1()").WithLocation(14, 20),
                // (18,37): error CS0106: The modifier 'internal' is not valid for this item
                //     internal virtual static void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("internal").WithLocation(18, 37),
                // (18,37): error CS0106: The modifier 'virtual' is not valid for this item
                //     internal virtual static void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("virtual").WithLocation(18, 37),
                // (28,15): error CS0535: 'Test1' does not implement interface member 'I2.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I2.M1()").WithLocation(28, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_07(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I4
{
    " + modifiers + @"void M1(); 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"private sealed void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"protected abstract void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,28): error CS0106: The modifier 'sealed' is not valid for this item
                //     private sealed void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("sealed").WithLocation(14, 28 + implModifiers.Length),
                // (14,28): error CS0106: The modifier 'private' is not valid for this item
                //     private sealed void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private").WithLocation(14, 28 + implModifiers.Length),
                // (18,32): error CS0106: The modifier 'protected' is not valid for this item
                //     protected abstract void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected").WithLocation(18, 32 + implModifiers.Length),
                // (18,32): error CS0500: 'I1.I4.M1()' cannot declare a body because it is marked abstract
                //     protected abstract void I4.M1() 
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "M1").WithArguments("I1.I4.M1()").WithLocation(18, 32 + implModifiers.Length),
                // (28,15): error CS0535: 'Test1' does not implement interface member 'I4.M1()'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I4.M1()").WithLocation(28, 15)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, i4M1IsAbstract: true, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_08(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I4
{
    " + modifiers + @"void M1(); 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"private protected void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"internal protected override void I4.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,31): error CS0106: The modifier 'private protected' is not valid for this item
                //     private protected void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private protected").WithLocation(14, 31 + implModifiers.Length),
                // (18,41): error CS0106: The modifier 'protected internal' is not valid for this item
                //     internal protected override void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected internal").WithLocation(18, 41 + implModifiers.Length),
                // (18,41): error CS0106: The modifier 'override' is not valid for this item
                //     internal protected override void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("override").WithLocation(18, 41 + implModifiers.Length)
                );

            ValidateMethodImplementationInDerived_01(compilation1.SourceModule, isStatic: isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_09(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I1 : I2
{
    " + implModifiers + @"extern void I2.M1(); 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (9,20): warning CS0626: Method, operator, or accessor 'I1.I2.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern void I2.M1(); 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.I2.M1()").WithLocation(9, 20 + implModifiers.Length)
                );

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
                var i1i2m1 = i1.GetMember<MethodSymbol>("I2.M1");
                var i2 = i1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I2").Single();
                var i2m1 = i2.GetMembers().OfType<MethodSymbol>().Single();
                var i3 = i1.ContainingNamespace.GetTypeMember("I3");

                Assert.True(i1.IsAbstract);
                Assert.True(i1.IsMetadataAbstract);

                ValidateExplicitExternImplementation(i1i2m1, isStatic: isStatic);

                Assert.Null(test1.FindImplementationForInterfaceMember(i1i2m1));
                Assert.Same(i1i2m1, test1.FindImplementationForInterfaceMember(i2m1));

                Assert.Null(i1.FindImplementationForInterfaceMember(i1i2m1));
                Assert.Same(i1i2m1, i1.FindImplementationForInterfaceMember(i2m1));

                Assert.Null(i2.FindImplementationForInterfaceMember(i2m1));

                Assert.Same(i1i2m1, i3.FindImplementationForInterfaceMember(i2m1));
            }

            CompileAndVerify(compilation1, verify: Verify(isStatic), symbolValidator: Validate1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            if (!isStatic)
            {
                compilation2.VerifyDiagnostics(
                    // (9,20): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                    //     extern void I2.M1(); 
                    Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "M1").WithArguments("default interface implementation", "8.0").WithLocation(9, 20),
                    // (9,20): warning CS0626: Method, operator, or accessor 'I1.I2.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                    //     extern void I2.M1(); 
                    Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.I2.M1()").WithLocation(9, 20)
                    );
            }
            else
            {
                compilation2.VerifyDiagnostics(
                    // (4,26): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static abstract void M1(); 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(4, 26),
                    // (9,27): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                    //     static extern void I2.M1(); 
                    Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(9, 27),
                    // (9,27): warning CS0626: Method, operator, or accessor 'I1.I2.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                    //     static extern void I2.M1(); 
                    Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.I2.M1()").WithLocation(9, 27)
                    );
            }

            Validate1(compilation2.SourceModule);

            var compilation3 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            if (!isStatic)
            {
                compilation3.VerifyDiagnostics(
                    // (9,20): error CS8501: Target runtime doesn't support default interface implementation.
                    //     extern void I2.M1(); 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "M1").WithLocation(9, 20),
                    // (9,20): warning CS0626: Method, operator, or accessor 'I1.I2.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                    //     extern void I2.M1(); 
                    Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.I2.M1()").WithLocation(9, 20)
                    );
            }
            else
            {
                compilation3.VerifyDiagnostics(
                    // (4,26): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static abstract void M1(); 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(4, 26),
                    // (9,27): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                    //     static extern void I2.M1(); 
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(9, 27),
                    // (9,27): warning CS0626: Method, operator, or accessor 'I1.I2.M1()' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                    //     static extern void I2.M1(); 
                    Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "M1").WithArguments("I1.I2.M1()").WithLocation(9, 27)
                    );
            }

            Validate1(compilation3.SourceModule);

            var source2 =
@"
public interface I2
{
    " + modifiers + @"void M1(); 
}

public interface I1 : I2
{
    " + implModifiers + @"extern void I2.M1()
    {} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation4 = CreateCompilation(source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics(
                // (9,20): error CS0179: 'I1.I2.M1()' cannot be extern and declare a body
                //     extern void I2.M1()
                Diagnostic(ErrorCode.ERR_ExternHasBody, "M1").WithArguments("I1.I2.M1()").WithLocation(9, 20 + implModifiers.Length)
                );

            Validate1(compilation4.SourceModule);
        }

        private static void ValidateExplicitExternImplementation(MethodSymbol m1, bool isStatic = false)
        {
            Assert.Equal(!isStatic, m1.IsMetadataVirtual());
            Assert.Equal(!isStatic, m1.IsMetadataFinal);
            Assert.False(m1.IsMetadataNewSlot());
            Assert.False(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.False(m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.NotEqual(m1.OriginalDefinition is PEMethodSymbol, m1.IsExtern);
            Assert.False(m1.IsAsync);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);

            if (m1.ContainingModule is PEModuleSymbol peModule)
            {
                int rva;
                peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)m1).Handle, out _, out _, out _, out rva);
                Assert.Equal(0, rva);
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_10(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
using System.Threading.Tasks;

public interface I2
{
    " + modifiers + @"Task M1(); 
}

public interface I1 : I2
{
    " + implModifiers + @"async Task I2.M1()
    {
        await Task.Factory.StartNew(() => System.Console.WriteLine(""I2.M1""));
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Main()
    {
        I2 i2 = new Test1();
";
            }
            else
            {
                source1 +=
@"
    static void Main()
    {
        Test<Test1>();
    }

    static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        i2.M1().Wait();
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
                var i1i2m1 = i1.GetMember<MethodSymbol>("I2.M1");
                var i2 = i1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I2").Single();
                var i2m1 = i2.GetMembers().OfType<MethodSymbol>().Single();
                var i3 = i1.ContainingNamespace.GetTypeMember("I3");

                Assert.True(i1.IsAbstract);
                Assert.True(i1.IsMetadataAbstract);

                Validate2(i1i2m1);

                Assert.Null(test1.FindImplementationForInterfaceMember(i1i2m1));
                Assert.Equal("System.Threading.Tasks.Task I1.I2.M1()", test1.FindImplementationForInterfaceMember(i2m1).ToTestDisplayString());

                Assert.Null(i1.FindImplementationForInterfaceMember(i1i2m1));
                Assert.Equal("System.Threading.Tasks.Task I1.I2.M1()", i1.FindImplementationForInterfaceMember(i2m1).ToTestDisplayString());

                Assert.Null(i2.FindImplementationForInterfaceMember(i2m1));

                Assert.Equal("System.Threading.Tasks.Task I1.I2.M1()", i3.FindImplementationForInterfaceMember(i2m1).ToTestDisplayString());
            }

            void Validate2(MethodSymbol m1)
            {
                Assert.Equal(!isStatic, m1.IsMetadataVirtual());
                Assert.Equal(!isStatic, m1.IsMetadataFinal);
                Assert.False(m1.IsMetadataNewSlot());
                Assert.False(m1.IsAbstract);
                Assert.False(m1.IsVirtual);
                Assert.False(m1.IsSealed);
                Assert.Equal(isStatic, m1.IsStatic);
                Assert.False(m1.IsExtern);
                Assert.NotEqual(m1 is PEMethodSymbol, m1.IsAsync);
                Assert.False(m1.IsOverride);
                Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);

                if (m1.ContainingModule is PEModuleSymbol peModule)
                {
                    int rva;
                    peModule.Module.GetMethodDefPropsOrThrow(((PEMethodSymbol)m1).Handle, out _, out _, out _, out rva);
                    Assert.NotEqual(0, rva);
                }
            }

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null : "I2.M1",
                verify: Verify(isStatic),
                symbolValidator: Validate1);
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void MethodImplementationInDerived_11(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"internal void M1(); 
}

public interface I4
{
    " + modifiers + @"internal void M1(); 
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source1 +=
@"
        i2.M1();
        i4.M1();
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"void I4.M1() 
    {
        System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test(new Test1(), new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test<Test1, Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateMethodImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            var compilation2 = CreateCompilation(source3, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            validateMethodImplementationInDerived_01(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            var compilation3 = CreateCompilation(source3, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            validateMethodImplementationInDerived_01(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            var compilation5 = CreateCompilation(source2 + source3, new[] { compilation4.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            validateMethodImplementationInDerived_01(compilation5.SourceModule);

            compilation5.VerifyDiagnostics(
                // (8,13): error CS0122: 'I4.M1()' is inaccessible due to its protection level
                //     void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I4.M1()").WithLocation(8, 13 + implModifiers.Length),
                // (4,13): error CS0122: 'I2.M1()' is inaccessible due to its protection level
                //     void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1()").WithLocation(4, 13 + implModifiers.Length)
                );

            var compilation6 = CreateCompilation(source2 + source3, new[] { compilation4.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation6.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            validateMethodImplementationInDerived_01(compilation6.SourceModule);

            compilation6.VerifyDiagnostics(
                // (8,13): error CS0122: 'I4.M1()' is inaccessible due to its protection level
                //     void I4.M1() 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I4.M1()").WithLocation(8, 13 + implModifiers.Length),
                // (4,13): error CS0122: 'I2.M1()' is inaccessible due to its protection level
                //     void I2.M1() 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1()").WithLocation(4, 13 + implModifiers.Length)
                );

            void validateMethodImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateMethodImplementationInDerived_01(m, isStatic: isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(20083, "https://github.com/dotnet/roslyn/issues/20083")]
        public void MethodImplementationInDerived_12(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"void M1(){ System.Console.WriteLine(""Unexpected!!!""); } 
}

public interface I2 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""I2.I1.M1"");
    }
}

public interface I3 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""I3.I1.M1"");
    }
}

public interface I4 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""I4.I1.M1"");
    }
}

public interface I5 : I2, I3
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""I5.I1.M1"");
    }
}

public interface I6 : I1, I2, I3, I5
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var i1 = FindType(m, "I1");
                var i1m1 = i1.GetMember<MethodSymbol>("M1");
                var i2 = FindType(m, "I2");
                var i2m1 = i2.GetMember<MethodSymbol>("I1.M1");
                var i5 = FindType(m, "I5");
                var i5m1 = i5.GetMember<MethodSymbol>("I1.M1");
                var i6 = FindType(m, "I6");

                ValidateExplicitImplementation(i2m1, isStatic: isStatic);
                ValidateExplicitImplementation(i5m1, isStatic: isStatic);

                Assert.Same(i1m1, i1.FindImplementationForInterfaceMember(i1m1));
                Assert.Same(i2m1, i2.FindImplementationForInterfaceMember(i1m1));
                Assert.Same(i5m1, i5.FindImplementationForInterfaceMember(i1m1));
                Assert.Same(i5m1, i6.FindImplementationForInterfaceMember(i1m1));
            }

            CompileAndVerify(compilation1, verify: Verify(isStatic), symbolValidator: Validate1);

            var refs1 = new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() };

            var source2 = @"
public interface I7 : I2, I3
{}

public interface I8 : I1, I2, I3, I5, I4
{}
";

            var source3 =
@"
class Test1 : I2, I3
{}

class Test2 : I7
{}

class Test3 : I1, I2, I3, I4
{}

class Test4 : I1, I2, I3, I5, I4
{}

class Test5 : I8
{}
";

            var source4 =
@"
class Test5 : I2
{}

class Test6 : I1, I2, I3, I5
{}

class Test7 : I6
{
";
            if (!isStatic)
            {
                source4 +=
@"
    static void Main()
    {
        I1 i5 = new Test5();
        I1 i6 = new Test6();
        I1 i7 = new Test7();
";
            }
            else
            {
                source4 +=
@"
    static void Main()
    {
        Test<Test5, Test6, Test7>();
    }

    static void Test<i5, i6, i7>() where i5 : I1 where i6 : I1 where i7 : I1
    {
";
            }

            source4 +=
@"
        i5.M1();
        i6.M1();
        i7.M1();
    }
}
";
            var source5 = @"
class Test8 : I2, I3
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""Test8.I1.M1"");
    }
";
            if (!isStatic)
            {
                source5 +=
@"
    static void Main()
    {
        I1 i8 = new Test8();
        I1 i9 = new Test9();
        I1 i10 = new Test10();
        I1 i11 = new Test11();
        I1 i12 = new Test12();
";
            }
            else
            {
                source5 +=
@"
    static void Main()
    {
        Test<Test8, Test9, Test10, Test11, Test12>();
    }

    static void Test<i8, i9, i10, i11, i12>() where i8 : I1 where i9 : I1 where i10 : I1 where i11 : I1 where i12 : I1
    {
";
            }

            source5 +=
@"
        i8.M1();
        i9.M1();
        i10.M1();
        i11.M1();
        i12.M1();
    }
}

class Test9 : I7
{
    " + implModifiers + @"void I1.M1() 
    {
        System.Console.WriteLine(""Test9.I1.M1"");
    }
}

class Test10 : I1, I2, I3, I4
{
    " + implModifiers + @"public void M1() 
    {
        System.Console.WriteLine(""Test10.M1"");
    }
}

class Test11 : I1, I2, I3, I5, I4
{
    " + implModifiers + @"public void M1() 
    {
        System.Console.WriteLine(""Test11.M1"");
    }
}

class Test12 : I8
{
    " + implModifiers + @"public " + (isStatic ? "" : "virtual") + @" void M1() 
    {
        System.Console.WriteLine(""Test12.M1"");
    }
}
";

            foreach (var ref1 in refs1)
            {
                var compilation2 = CreateCompilation(source2, new[] { ref1 }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                // According to LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-06-14.md,
                // we do not require interfaces to have a most specific implementation of all members. Therefore, there are no
                // errors in this compilation.
                compilation2.VerifyDiagnostics();
                Validate2(compilation2.SourceModule);

                void Validate2(ModuleSymbol m)
                {
                    Validate1(m);

                    var i1 = FindType(m, "I1");
                    var i1m1 = i1.GetMember<MethodSymbol>("M1");
                    var i7 = FindType(m, "I7");
                    var i8 = FindType(m, "I8");

                    Assert.Null(i7.FindImplementationForInterfaceMember(i1m1));
                    Assert.Null(i8.FindImplementationForInterfaceMember(i1m1));
                }

                CompileAndVerify(compilation2, verify: Verify(isStatic), symbolValidator: Validate2);

                compilation2 = CreateCompilation(source2, new[] { ref1 }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation2.VerifyDiagnostics();
                Validate2(compilation2.SourceModule);

                CompileAndVerify(compilation2, verify: Verify(isStatic), symbolValidator: Validate2);

                var refs2 = new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() };

                var compilation4 = CreateCompilation(source4, new[] { ref1 }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation4.VerifyDiagnostics();
                Validate4(compilation4.SourceModule);

                void Validate4(ModuleSymbol m)
                {
                    Validate1(m);

                    var i1 = FindType(m, "I1");
                    var i1m1 = i1.GetMember<MethodSymbol>("M1");
                    var i2 = FindType(m, "I2");
                    var i2m1 = i2.GetMember<MethodSymbol>("I1.M1");
                    var i5 = FindType(m, "I5");
                    var i5m1 = i5.GetMember<MethodSymbol>("I1.M1");
                    var test5 = FindType(m, "Test5");
                    var test6 = FindType(m, "Test6");
                    var test7 = FindType(m, "Test7");

                    Assert.Same(i2m1, test5.FindImplementationForInterfaceMember(i1m1));
                    Assert.Same(i5m1, test6.FindImplementationForInterfaceMember(i1m1));
                    Assert.Same(i5m1, test7.FindImplementationForInterfaceMember(i1m1));
                }

                CompileAndVerify(compilation4,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.I1.M1
I5.I1.M1
I5.I1.M1
"
,
                    verify: Verify(isStatic),
                    symbolValidator: Validate4);

                foreach (var ref2 in refs2)
                {
                    var compilation3 = CreateCompilation(source3, new[] { ref1, ref2 }, options: TestOptions.DebugDll,
                                                         parseOptions: TestOptions.RegularPreview,
                                                         targetFramework: TargetFramework.Net60);
                    Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                    compilation3.VerifyDiagnostics(
                        // (2,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I2.I1.M1()', nor 'I3.I1.M1()' are most specific.
                        // class Test1 : I2, I3
                        Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1()", "I2.I1.M1()", "I3.I1.M1()").WithLocation(2, 15),
                        // (5,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I2.I1.M1()', nor 'I3.I1.M1()' are most specific.
                        // class Test2 : I7
                        Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I7").WithArguments("I1.M1()", "I2.I1.M1()", "I3.I1.M1()").WithLocation(5, 15),
                        // (8,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I2.I1.M1()', nor 'I3.I1.M1()' are most specific.
                        // class Test3 : I1, I2, I3, I4
                        Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.M1()", "I2.I1.M1()", "I3.I1.M1()").WithLocation(8, 15),
                        // (11,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I5.I1.M1()', nor 'I4.I1.M1()' are most specific.
                        // class Test4 : I1, I2, I3, I5, I4
                        Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.M1()", "I5.I1.M1()", "I4.I1.M1()").WithLocation(11, 15),
                        // (14,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I5.I1.M1()', nor 'I4.I1.M1()' are most specific.
                        // class Test5 : I8
                        Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I8").WithArguments("I1.M1()", "I5.I1.M1()", "I4.I1.M1()").WithLocation(14, 15)
                        );

                    Validate3(compilation3.SourceModule);

                    void Validate3(ModuleSymbol m)
                    {
                        Validate1(m);
                        Validate2(m);

                        var i1 = FindType(m, "I1");
                        var i1m1 = i1.GetMember<MethodSymbol>("M1");
                        var test1 = FindType(m, "Test1");
                        var test2 = FindType(m, "Test2");
                        var test3 = FindType(m, "Test3");
                        var test4 = FindType(m, "Test4");
                        var test5 = FindType(m, "Test5");

                        Assert.Null(test1.FindImplementationForInterfaceMember(i1m1));
                        Assert.Null(test2.FindImplementationForInterfaceMember(i1m1));
                        Assert.Null(test3.FindImplementationForInterfaceMember(i1m1));
                        Assert.Null(test4.FindImplementationForInterfaceMember(i1m1));
                        Assert.Null(test5.FindImplementationForInterfaceMember(i1m1));
                    }

                    var compilation5 = CreateCompilation(source5, new[] { ref1, ref2 }, options: TestOptions.DebugExe,
                                                         parseOptions: TestOptions.RegularPreview,
                                                         targetFramework: TargetFramework.Net60);
                    Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                    compilation5.VerifyDiagnostics();

                    Validate5(compilation5.SourceModule);

                    void Validate5(ModuleSymbol m)
                    {
                        var i1 = FindType(m, "I1");
                        var i1m1 = i1.GetMember<MethodSymbol>("M1");
                        var test8 = FindType(m, "Test8");
                        var test9 = FindType(m, "Test9");
                        var test10 = FindType(m, "Test10");
                        var test11 = FindType(m, "Test11");
                        var test12 = FindType(m, "Test12");

                        Assert.Same(test8.GetMember<MethodSymbol>("I1.M1"), test8.FindImplementationForInterfaceMember(i1m1));
                        Assert.Same(test9.GetMember<MethodSymbol>("I1.M1"), test9.FindImplementationForInterfaceMember(i1m1));
                        Assert.Same(test10.GetMember<MethodSymbol>("M1"), test10.FindImplementationForInterfaceMember(i1m1));
                        Assert.Same(test11.GetMember<MethodSymbol>("M1"), test11.FindImplementationForInterfaceMember(i1m1));
                        Assert.Same(test12.GetMember<MethodSymbol>("M1"), test12.FindImplementationForInterfaceMember(i1m1));
                    }

                    CompileAndVerify(compilation5,
                        expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"Test8.I1.M1
Test9.I1.M1
Test10.M1
Test11.M1
Test12.M1
",
                        verify: Verify(isStatic),
                        symbolValidator: Validate5);
                }
            }
        }

        private static NamedTypeSymbol FindType(ModuleSymbol m, string name)
        {
            var result = m.GlobalNamespace.GetMember<NamedTypeSymbol>(name);
            if ((object)result != null)
            {
                return result;
            }

            foreach (var assembly in m.ReferencedAssemblySymbols)
            {
                result = assembly.Modules[0].GlobalNamespace.GetMember<NamedTypeSymbol>(name);
                if ((object)result != null)
                {
                    return result;
                }
            }

            Assert.NotNull(result);
            return result;
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_13(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"void M1(); 
}

public interface I2 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
    }
}

public interface I3 : I1
{
    " + implModifiers + @"void I1.M1() 
    {
    }
}
";

            var source2 =
@"
class Test1 : I2, I3
{
    " + implModifiers + @"public int M1() => 1;
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (2,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I2.I1.M1()', nor 'I3.I1.M1()' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1()", "I2.I1.M1()", "I3.I1.M1()").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implement 'I1.M1()' because it does not have the matching return type of 'void'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.M1()", "Test1.M1()", "void").WithLocation(2, 15)
                );

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (2,15): error CS8505: Interface member 'I1.M1()' does not have a most specific implementation. Neither 'I2.I1.M1()', nor 'I3.I1.M1()' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1()", "I2.I1.M1()", "I3.I1.M1()").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1()'. 'Test1.M1()' cannot implement 'I1.M1()' because it does not have the matching return type of 'void'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.M1()", "Test1.M1()", "void").WithLocation(2, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(20084, "https://github.com/dotnet/roslyn/issues/20084")]
        public void MethodImplementationInDerived_14(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1<T>
{
    " + modifiers + @"void M1()
    {
        System.Console.WriteLine(""I1.M1"");
    }
    " + modifiers + @"void M2<S>()
    {
        System.Console.WriteLine(""I1.M2"");
    }
}

public interface I2<T> : I1<T>
{
    " + implModifiers + @"void I1<T>.M1() 
    {
        System.Console.WriteLine(""I2.I1.M1"");
    }
    " + implModifiers + @"void I1<T>.M2<S>() 
    {
        System.Console.WriteLine(""I2.I1.M2"");
    }
}

public interface I3<T> : I1<T>
{}
";

            var source2 =
@"
class Test1 : I1<int>
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1<int> i1Int = new Test1();
        I1<long> i1Long = new Test2();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test2>();
    }

    static void Test<i1Int, i1Long>() where i1Int : I1<int> where i1Long : I1<long>
    {
";
            }

            source2 +=
@"
        i1Int.M1();
        i1Int.M2<int>();
        i1Long.M1();
        i1Long.M2<int>();
    }
}
class Test2 : I2<long>
{}
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var i1 = FindType(m, "I1");
                var i1m1 = i1.GetMember<MethodSymbol>("M1");
                var i1m2 = i1.GetMember<MethodSymbol>("M2");
                var i2 = FindType(m, "I2");
                var i2m1 = i2.GetMember<MethodSymbol>("I1<T>.M1");
                var i2m2 = i2.GetMember<MethodSymbol>("I1<T>.M2");
                var i2i1 = i2.InterfacesNoUseSiteDiagnostics().Single();
                var i2i1m1 = i2i1.GetMember<MethodSymbol>("M1");
                var i2i1m2 = i2i1.GetMember<MethodSymbol>("M2");
                var i3 = FindType(m, "I3");
                var i3i1 = i3.InterfacesNoUseSiteDiagnostics().Single();
                var i3i1m1 = i3i1.GetMember<MethodSymbol>("M1");
                var i3i1m2 = i3i1.GetMember<MethodSymbol>("M2");

                Assert.Same(i1m1, i1.FindImplementationForInterfaceMember(i1m1));
                Assert.Same(i1m2, i1.FindImplementationForInterfaceMember(i1m2));

                Assert.Null(i2.FindImplementationForInterfaceMember(i1m1));
                Assert.Null(i2.FindImplementationForInterfaceMember(i1m2));
                Assert.Null(i2.FindImplementationForInterfaceMember(i3i1m1));
                Assert.Null(i2.FindImplementationForInterfaceMember(i3i1m2));
                Assert.Same(i2m1, i2.FindImplementationForInterfaceMember(i2i1m1));
                Assert.Same(i2m2, i2.FindImplementationForInterfaceMember(i2i1m2));

                Assert.Null(i3.FindImplementationForInterfaceMember(i1m1));
                Assert.Null(i3.FindImplementationForInterfaceMember(i1m2));
                Assert.Null(i3.FindImplementationForInterfaceMember(i2i1m1));
                Assert.Null(i3.FindImplementationForInterfaceMember(i2i1m2));
                Assert.Same(i3i1m1, i3.FindImplementationForInterfaceMember(i3i1m1));
                Assert.Same(i3i1m2, i3.FindImplementationForInterfaceMember(i3i1m2));

                ValidateExplicitImplementation(i2m1, isStatic: isStatic);
                ValidateExplicitImplementation(i2m2, isStatic: isStatic);

                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var test1i1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32));
                var test1i1m1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32)).GetMember<MethodSymbol>("M1");
                var test1i1m2 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32)).GetMember<MethodSymbol>("M2");
                var test2 = m.GlobalNamespace.GetTypeMember("Test2");
                var test2i1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64));
                var test2i1m1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)).GetMember<MethodSymbol>("M1");
                var test2i1m2 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)).GetMember<MethodSymbol>("M2");
                var test2i2 = i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64));
                var test2i2m1 = i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)).GetMember<MethodSymbol>("I1<T>.M1");
                var test2i2m2 = i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)).GetMember<MethodSymbol>("I1<T>.M2");

                Assert.NotSame(test1i1, test1i1m1.ContainingType);
                Assert.NotSame(test1i1, test1i1m2.ContainingType);
                Assert.NotSame(test2i1, test2i1m1.ContainingType);
                Assert.NotSame(test2i1, test2i1m2.ContainingType);
                Assert.NotSame(test2i2, test2i2m1.ContainingType);
                Assert.NotSame(test2i2, test2i2m2.ContainingType);

                Assert.Null(test1i1.FindImplementationForInterfaceMember(i1m1));
                Assert.Null(test1i1.FindImplementationForInterfaceMember(i1m2));
                Assert.Equal(test1i1m1, test1i1.FindImplementationForInterfaceMember(test1i1m1));
                Assert.Equal(test1i1m2, test1i1.FindImplementationForInterfaceMember(test1i1m2));
                Assert.Equal(test2i1m1, test2i1.FindImplementationForInterfaceMember(test2i1m1));
                Assert.Equal(test2i1m2, test2i1.FindImplementationForInterfaceMember(test2i1m2));
                Assert.Null(test2i2.FindImplementationForInterfaceMember(i1m1));
                Assert.Null(test2i2.FindImplementationForInterfaceMember(i1m2));
                Assert.Equal(test2i2m1, test2i2.FindImplementationForInterfaceMember(test2i1m1));
                Assert.Equal(test2i2m2, test2i2.FindImplementationForInterfaceMember(test2i1m2));

                ValidateExplicitImplementation(test2i2m1, isStatic: isStatic);
                ValidateExplicitImplementation(test2i2m2, isStatic: isStatic);

                Assert.Null(test1.FindImplementationForInterfaceMember(i1m1));
                Assert.Null(test1.FindImplementationForInterfaceMember(i1m2));
                Assert.Equal(test1i1m1, test1.FindImplementationForInterfaceMember(test1i1m1));
                Assert.Equal(test1i1m2, test1.FindImplementationForInterfaceMember(test1i1m2));
                Assert.Null(test2.FindImplementationForInterfaceMember(i1m1));
                Assert.Null(test2.FindImplementationForInterfaceMember(i1m2));
                Assert.Equal(test2i2m1, test2.FindImplementationForInterfaceMember(test2i1m1));
                Assert.Equal(test2i2m2, test2.FindImplementationForInterfaceMember(test2i1m2));
            }

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1
I1.M2
I2.I1.M1
I2.I1.M2
",
                verify: Verify(isStatic),
                symbolValidator: Validate);

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1
I1.M2
I2.I1.M1
I2.I1.M2
",
                verify: Verify(isStatic),
                symbolValidator: Validate);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            Validate(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1
I1.M2
I2.I1.M1
I2.I1.M2
",
                verify: Verify(isStatic),
                symbolValidator: Validate);
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_15(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1(); 
}

public interface I2 : I1<string>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
    }
}

public interface I3 : I1<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
    }
}
";

            var source2 =
@"#nullable enable warnings
class Test1 : I2, I3
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (2,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(2, 7),
                // (2,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(2, 15),
                // (2,19): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I3").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(2, 19)
                );

            var test1 = compilation2.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation3.VerifyDiagnostics(
                // (2,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(2, 7),
                // (2,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(2, 15),
                // (2,19): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I3").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(2, 19)
                );

            test1 = compilation3.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_16(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable
public interface I1<T>
{
    " + modifiers + @"void M1(); 
}

public interface I2 : I1<string>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
    }
}

public interface I3 : I1<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
    }
}
";

            var source2 =
@"#nullable enable warnings
class Test1 : I2, I3, I1<string>
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (2,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(2, 7),
                // (2,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(2, 15),
                // (2,19): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I3").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(2, 19),
                // (2,23): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1<string>").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(2, 23)
                );

            var test1 = compilation2.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>", "I1<System.String>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[4].GetMember("M1")));

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation3.VerifyDiagnostics(
                // (2,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(2, 7),
                // (2,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(2, 15),
                // (2,19): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I3").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(2, 19),
                // (2,23): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1<string>").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(2, 23)
                );

            test1 = compilation3.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>", "I1<System.String>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[4].GetMember("M1")));
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_17(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1<T>
{
    " + modifiers + @"void M1(); 
}

public interface I2 : I1<string>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
    }
}

#nullable enable

public interface I3 : I1<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
    }
}
";

            var source2 =
@"
#nullable enable
class Test1 : I2, I3, I1<string>
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (3,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(3, 15),
                // (3,19): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I3").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(3, 19),
                // (3,23): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1<string>").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(3, 23)
                );

            var test1 = compilation2.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>", "I1<System.String>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[4].GetMember("M1")));

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation3.VerifyDiagnostics(
                // (3,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(3, 15),
                // (3,19): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I3").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(3, 19),
                // (3,23): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1<string>").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(3, 23)
                );

            test1 = compilation3.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>", "I1<System.String>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[4].GetMember("M1")));
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_18(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1(); 
}

public interface I2 : I1<string>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
    }
}

public interface I3 : I1<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
    }
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2, I3, I1<string?>
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I2, I3, I1<string?>
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                // (4,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string?>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(4, 15),
                // (4,23): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string?>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string?>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1<string?>").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string?>.M1()").WithLocation(4, 23)
                );

            var test1 = compilation2.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation3.VerifyDiagnostics(
                // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I2, I3, I1<string?>
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                // (4,15): error CS8705: Interface member 'I1<string>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string?>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1<string>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(4, 15),
                // (4,23): error CS8705: Interface member 'I1<string?>.M1()' does not have a most specific implementation. Neither 'I2.I1<string>.M1()', nor 'I3.I1<string>.M1()' are most specific.
                // class Test1 : I2, I3, I1<string?>
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1<string?>").WithArguments("I1<string?>.M1()", "I2.I1<string>.M1()", "I3.I1<string>.M1()").WithLocation(4, 23)
                );

            test1 = compilation3.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_19(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1(); 
}

public interface I2 : I1<string>
{
}

public interface I3 : I2, I1<string?>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
    }

    " + implModifiers + @"void I1<string?>.M1() 
    {
    }
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I3
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (13,18): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'I3' with different nullability of reference types.
                // public interface I3 : I2, I1<string?>
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "I3").WithArguments("I1<string?>", "I3").WithLocation(13, 18),
                // (13,18): error CS8646: 'I1<string>.M1()' is explicitly implemented more than once.
                // public interface I3 : I2, I1<string?>
                Diagnostic(ErrorCode.ERR_DuplicateExplicitImpl, "I3").WithArguments("I1<string>.M1()").WithLocation(13, 18),
                // (13,18): error CS8646: 'I1<string?>.M1()' is explicitly implemented more than once.
                // public interface I3 : I2, I1<string?>
                Diagnostic(ErrorCode.ERR_DuplicateExplicitImpl, "I3").WithArguments("I1<string?>.M1()").WithLocation(13, 18)
                );

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I3
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                // (4,15): error CS0535: 'Test1' does not implement interface member 'I1<string>.M1()'
                // class Test1 : I3
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test1", "I1<string>.M1()").WithLocation(4, 15),
                // (4,15): error CS0535: 'Test1' does not implement interface member 'I1<string?>.M1()'
                // class Test1 : I3
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test1", "I1<string?>.M1()").WithLocation(4, 15)
                );

            var test1 = compilation2.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I3", "I2", "I1<System.String>", "I1<System.String?>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_20(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2 : I1<string>
{
}

public interface I3 : I1<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
        System.Console.WriteLine(""I3.M1"");
    } 
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2, I3
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I2", "I1<System.String>", "I3", "I1<System.String?>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I3.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I3.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2, I3
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                    // (4,15): warning CS8644: 'Test1' does not implement interface member 'I1<string>.M1()'. Nullability of reference types in interface implemented by the base type doesn't match.
                    // class Test1 : I2, I3
                    Diagnostic(ErrorCode.WRN_NullabilityMismatchInInterfaceImplementedByBase, "I2").WithArguments("Test1", "I1<string>.M1()").WithLocation(4, 15)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I3.M1
I3.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_21(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2 : I1<string>
{
}

public interface I3 : I1<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
        System.Console.WriteLine(""I3.M1"");
    } 
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I3, I2
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I3", "I1<System.String?>", "I2", "I1<System.String>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I3.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I3.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I3, I2
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string>", "Test1").WithLocation(4, 7),
                    // (4,19): warning CS8644: 'Test1' does not implement interface member 'I1<string>.M1()'. Nullability of reference types in interface implemented by the base type doesn't match.
                    // class Test1 : I3, I2
                    Diagnostic(ErrorCode.WRN_NullabilityMismatchInInterfaceImplementedByBase, "I2").WithArguments("Test1", "I1<string>.M1()").WithLocation(4, 19)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I3.M1
I3.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_22(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2<T> : I1<T>
{
    " + implModifiers + @"void I1<T>.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    } 
}

public interface I3 : I2<string>
{
}

public interface I4 : I2<string?>
{
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I3, I4
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I3", "I2<System.String>", "I1<System.String>", "I4", "I2<System.String?>", "I1<System.String?>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I2<System.String?>.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I2<System.String?>.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[5].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I2<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I3, I4
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I2<string?>", "Test1").WithLocation(4, 7),
                    // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I3, I4
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I2.M1
I2.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_23(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1() 
    {
        System.Console.WriteLine(""I1.M1"");
    } 
}

public interface I2<T> : I1<T>
{
    " + implModifiers + @"void I1<T>.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    } 
}

public interface I3 : I2<string>
{
}

public interface I4 : I2<string?>
{
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I4, I3
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I4", "I2<System.String?>", "I1<System.String?>", "I3", "I2<System.String>", "I1<System.String>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I2<System.String>.I1<System.String>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I2<System.String>.I1<System.String>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[5].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I2<string>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I4, I3
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I2<string>", "Test1").WithLocation(4, 7),
                    // (4,7): warning CS8645: 'I1<string>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I4, I3
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string>", "Test1").WithLocation(4, 7)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I2.M1
I2.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_24(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1();
}

public interface I2 : I1<string>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    } 
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (11,10): warning CS8643: Nullability of reference types in explicit interface specifier doesn't match interface implemented by the type.
                //     void I1<string?>.M1() 
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInExplicitlyImplementedInterface, "I1<string?>").WithLocation(11, 10 + implModifiers.Length)
                );

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I2", "I1<System.String>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I2.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[1].GetMember("M1")).ToTestDisplayString());

                if (!isStatic)
                {
                    compilation2.VerifyDiagnostics();
                }
                else
                {
                    compilation2.VerifyDiagnostics(
                        // (9,9): warning CS8631: The type 'Test1' cannot be used as type parameter 'i11' in the generic type or method 'Test1.Test<i11, i12>()'. Nullability of type argument 'Test1' doesn't match constraint type 'I1<string?>'.
                        //         Test<Test1, Test1>();
                        Diagnostic(ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint, "Test<Test1, Test1>").WithArguments("Test1.Test<i11, i12>()", "I1<string?>", "i11", "Test1").WithLocation(9, 9)
                        );
                }

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I2.M1
I2.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_25(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1();
}

public interface I2 : I1<string>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    } 
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2, I1<string?>
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2, I1<string?>
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                    // (4,19): warning CS8644: 'Test1' does not implement interface member 'I1<string?>.M1()'. Nullability of reference types in interface implemented by the base type doesn't match.
                    // class Test1 : I2, I1<string?>
                    Diagnostic(ErrorCode.WRN_NullabilityMismatchInInterfaceImplementedByBase, "I1<string?>").WithArguments("Test1", "I1<string?>.M1()").WithLocation(4, 19)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I2.M1
I2.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_26(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1();
}

public interface I2 : I1<string?>
{
    " + implModifiers + @"void I1<string>.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    } 
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2, I1<string?>
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (11,10): warning CS8643: Nullability of reference types in explicit interface specifier doesn't match interface implemented by the type.
                //     void I1<string>.M1() 
                Diagnostic(ErrorCode.WRN_NullabilityMismatchInExplicitlyImplementedInterface, "I1<string>").WithLocation(11, 10 + implModifiers.Length)
                );

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                if (!isStatic)
                {
                    compilation2.VerifyDiagnostics();
                }
                else
                {
                    compilation2.VerifyDiagnostics(
                        // (9,9): warning CS8631: The type 'Test1' cannot be used as type parameter 'i12' in the generic type or method 'Test1.Test<i11, i12>()'. Nullability of type argument 'Test1' doesn't match constraint type 'I1<string>'.
                        //         Test<Test1, Test1>();
                        Diagnostic(ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint, "Test<Test1, Test1>").WithArguments("Test1.Test<i11, i12>()", "I1<string>", "i12", "Test1").WithLocation(9, 9)
                        );
                }

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I2.M1
I2.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_27(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"void M1();
}

public interface I2<T>: I1<T>
{
    " + implModifiers + @"void I1<T>.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    } 
}

public interface I3<T> : I1<T>
{
    " + implModifiers + @"void I1<T>.M1() 
    {
        System.Console.WriteLine(""I3.M1"");
    } 
}

public interface I4 : I2<string?>, I3<string?>
{
    " + implModifiers + @"void I1<string?>.M1() 
    {
        System.Console.WriteLine(""I4.M1"");
    } 
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I2<string>, I3<string>, I4
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        ((I1<string?>)new Test1()).M1();
        ((I1<string>)new Test1()).M1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i11, i12>() where i11 : I1<string?> where i12 : I1<string>
    {
        i11.M1();
        i12.M1();
";
            }

            source2 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics();

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                        parseOptions: TestOptions.RegularPreview,
                                                        targetFramework: TargetFramework.Net60);

                var test1 = compilation2.GetTypeByMetadataName("Test1");

                Assert.Equal(new[] { "I2<System.String>", "I3<System.String>", "I1<System.String>", "I4", "I2<System.String?>", "I3<System.String?>", "I1<System.String?>" },
                             test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

                Assert.Equal("void I4.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")).ToTestDisplayString());
                Assert.Equal("void I4.I1<System.String?>.M1()", test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[6].GetMember("M1")).ToTestDisplayString());

                compilation2.VerifyDiagnostics(
                    // (4,7): warning CS8645: 'I2<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2<string>, I3<string>, I4
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I2<string?>", "Test1").WithLocation(4, 7),
                    // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2<string>, I3<string>, I4
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                    // (4,7): warning CS8645: 'I3<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                    // class Test1 : I2<string>, I3<string>, I4
                    Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I3<string?>", "Test1").WithLocation(4, 7),
                    // (4,15): warning CS8644: 'Test1' does not implement interface member 'I1<string>.M1()'. Nullability of reference types in interface implemented by the base type doesn't match.
                    // class Test1 : I2<string>, I3<string>, I4
                    Diagnostic(ErrorCode.WRN_NullabilityMismatchInInterfaceImplementedByBase, "I2<string>").WithArguments("Test1", "I1<string>.M1()").WithLocation(4, 15)
                    );

                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"
I4.M1
I4.M1
",
                    verify: Verify(isStatic));
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_28(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"protected void M1(); 
}

public interface I4
{
    " + modifiers + @"protected void M1(); 
}

public interface I5 : I4
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Test(I5 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    static void Test<i4>() where i4 : I5
    {
";
            }

            source1 +=
@"
        i4.M1();
    }
}

public interface I6 : I2
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Test(I6 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    static void Test<i2>() where i2 : I6
    {
";
            }

            source1 +=
@"
        i2.M1();
    }
}
";
            var source2 =
@"
public interface I1 : I6, I5
{
    " + implModifiers + @"void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"void I4.M1() 
    {
        System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        I6.Test(new Test1());
        I5.Test(new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        I6.Test<Test1>();
        I5.Test<Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateMethodImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: VerifyFailsIlVerify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateMethodImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validateMethodImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateMethodImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics();
                CompileAndVerify(compilation5,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validateMethodImplementationInDerived_01);
            }

            void validateMethodImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateMethodImplementationInDerived_01(m, isStatic: isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_29(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"protected internal void M1(); 
}

public interface I4
{
    " + modifiers + @"protected internal void M1(); 
}

public interface I5 : I4
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Test(I5 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    static void Test<i4>() where i4 : I5
    {
";
            }

            source1 +=
@"
        i4.M1();
    }
}

public interface I6 : I2
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Test(I6 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    static void Test<i2>() where i2 : I6
    {
";
            }

            source1 +=
@"
        i2.M1();
    }
}
";
            var source2 =
@"
public interface I1 : I6, I5
{
    " + implModifiers + @"void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"void I4.M1() 
    {
        System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        I6.Test(new Test1());
        I5.Test(new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        I6.Test<Test1>();
        I5.Test<Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateMethodImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: Verify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateMethodImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validateMethodImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateMethodImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics();
                CompileAndVerify(compilation5,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validateMethodImplementationInDerived_01);
            }

            void validateMethodImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateMethodImplementationInDerived_01(m, isStatic: isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_30(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"private protected void M1(); 
}

public interface I4
{
    " + modifiers + @"private protected void M1(); 
}

public interface I5 : I4
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Test(I5 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    static void Test<i4>() where i4 : I5
    {
";
            }

            source1 +=
@"
        i4.M1();
    }
}

public interface I6 : I2
{
";
            if (!isStatic)
            {
                source1 +=
@"
    static void Test(I6 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    static void Test<i2>() where i2 : I6
    {
";
            }

            source1 +=
@"
        i2.M1();
    }
}
";
            var source2 =
@"
public interface I1 : I6, I5
{
    " + implModifiers + @"void I2.M1() 
    {
        System.Console.WriteLine(""I2.M1"");
    }
    " + implModifiers + @"void I4.M1() 
    {
        System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        I6.Test(new Test1());
        I5.Test(new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        I6.Test<Test1>();
        I5.Test<Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateMethodImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                verify: VerifyFailsIlVerify(isStatic),
                symbolValidator: validateMethodImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateMethodImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validateMethodImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateMethodImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics(
                    // (4,13): error CS0122: 'I2.M1()' is inaccessible due to its protection level
                    //     void I2.M1() 
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1()").WithLocation(4, 13 + implModifiers.Length),
                    // (8,13): error CS0122: 'I4.M1()' is inaccessible due to its protection level
                    //     void I4.M1() 
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I4.M1()").WithLocation(8, 13 + implModifiers.Length)
                    );
            }

            void validateMethodImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateMethodImplementationInDerived_01(m, isStatic: isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        public void MethodImplementationInDerived_31(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"void M1(); 
    " + modifiers + @"void M2(); 
}

public interface I2 : I1
{
    " + implModifiers + @"public virtual void M1()
    {}
    " + implModifiers + @"new public virtual void M2()
    {}
}

public interface I3 : I1, I2
{
}

class Test1 : I1, I2
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (10,25): warning CS0108: 'I2.M1()' hides inherited member 'I1.M1()'. Use the new keyword if hiding was intended.
                //     public virtual void M1()
                Diagnostic(ErrorCode.WRN_NewRequired, "M1").WithArguments("I2.M1()", "I1.M1()").WithLocation(10, 25 + implModifiers.Length),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M2()'
                // class Test1 : I1, I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M2()").WithLocation(20, 15),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M1()'
                // class Test1 : I1, I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I1.M1()").WithLocation(20, 15)
                );
        }

        [Fact]
        public void PartialMethodAccessibility()
        {
            var source1 =
@"
public partial interface I
{
    partial void M();
    partial void M() { }
}
";

            var comp = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(comp.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            comp.VerifyDiagnostics();

            var @interface = comp.SourceModule.GlobalNamespace.GetTypeMember("I");
            var method = @interface.GetMembers().OfType<MethodSymbol>().Single();
            Assert.Equal(Accessibility.Private, method.DeclaredAccessibility);
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void PropertyImplementationInDerived_01(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get;} 
}

public interface I4
{
    " + modifiers + @"int M1 {set;} 
}

public interface I5 : I4
{
}

public interface I1 : I2, I5
{
    " + implModifiers + @"int I2.M1
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
    " + implModifiers + @"int I4.M1 
    {
        set => System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";
            var source2 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I2 i2 = new Test1();
        I4 i4 = new Test1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source2 +=
@"
        _ = i2.M1;
        i4.M1 = 0;
    }
}
";

            ValidatePropertyImplementationInDerived_01(source1, source2, isStatic);
        }

        private void ValidatePropertyImplementationInDerived_01(string source1, string source2, bool isStatic = false)
        {
            foreach (var options in new[] { TestOptions.DebugExe, TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All) })
            {
                var compilation1 = CreateCompilation(source1 + source2, options: options,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation1.VerifyDiagnostics();

                validatePropertyImplementationInDerived_01(compilation1.SourceModule);

                CompileAndVerify(compilation1,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validatePropertyImplementationInDerived_01);

                var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: options,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validatePropertyImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validatePropertyImplementationInDerived_01);

                var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: options,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                validatePropertyImplementationInDerived_01(compilation3.SourceModule);

                compilation3.VerifyDiagnostics();
                CompileAndVerify(compilation3,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
",
                    verify: Verify(isStatic),
                    symbolValidator: validatePropertyImplementationInDerived_01);
            }

            void validatePropertyImplementationInDerived_01(ModuleSymbol m)
            {
                ValidatePropertyImplementationInDerived_01(m, isStatic: isStatic);
            }
        }

        private static void ValidatePropertyImplementationInDerived_01(ModuleSymbol m, bool isStatic)
        {
            ValidatePropertyImplementationInDerived_01(m, i4M1IsAbstract: false, isStatic: isStatic);
        }

        private static void ValidatePropertyImplementationInDerived_01(ModuleSymbol m, bool i4M1IsAbstract, bool isStatic)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            var i1i2m1 = i1.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I2.")).Single();
            var i1i4m1 = i1.GetMembers().OfType<PropertySymbol>().Where(p => p.Name.StartsWith("I4.")).Single();
            var i2 = i1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I2").Single();
            var i2m1 = i2.GetMembers().OfType<PropertySymbol>().Single();
            var i4 = i1.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I4").Single();
            var i4m1 = i4.GetMembers().OfType<PropertySymbol>().Single();
            var i3 = i1.ContainingNamespace.GetTypeMember("I3");

            Assert.True(i1.IsAbstract);
            Assert.True(i1.IsMetadataAbstract);

            ValidateExplicitImplementation(i1i2m1, isStatic: isStatic);
            ValidateExplicitImplementation(i1i4m1, i4M1IsAbstract, isStatic: isStatic);

            VerifyFindImplementationForInterfaceMemberSame(null, test1, i1i2m1);
            VerifyFindImplementationForInterfaceMemberSame(null, test1, i1i4m1);

            VerifyFindImplementationForInterfaceMemberSame(i1i2m1, test1, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4M1IsAbstract ? null : i1i4m1, test1, i4m1);

            VerifyFindImplementationForInterfaceMemberSame(null, i1, i1i2m1);
            VerifyFindImplementationForInterfaceMemberSame(null, i1, i1i4m1);
            VerifyFindImplementationForInterfaceMemberSame(i1i2m1, i1, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4M1IsAbstract ? null : i1i4m1, i1, i4m1);

            VerifyFindImplementationForInterfaceMemberSame(i2m1.IsAbstract ? null : i2m1, i2, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4m1.IsAbstract ? null : i4m1, i4, i4m1);

            VerifyFindImplementationForInterfaceMemberSame(i1i2m1, i3, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4M1IsAbstract ? null : i1i4m1, i3, i4m1);
        }

        private static void VerifyFindImplementationForInterfaceMemberSame(PropertySymbol expected, NamedTypeSymbol implementingType, PropertySymbol interfaceProperty)
        {
            Assert.Same(expected, implementingType.FindImplementationForInterfaceMember(interfaceProperty));
            ValidateAccessor(expected?.GetMethod, interfaceProperty.GetMethod);
            ValidateAccessor(expected?.SetMethod, interfaceProperty.SetMethod);

            void ValidateAccessor(MethodSymbol accessor, MethodSymbol interfaceAccessor)
            {
                if ((object)interfaceAccessor != null)
                {
                    Assert.Same(interfaceAccessor.DeclaredAccessibility == Accessibility.Private ? null : accessor, implementingType.FindImplementationForInterfaceMember(interfaceAccessor));
                }
                else
                {
                    Assert.Null(accessor);
                }
            }
        }

        private static void VerifyFindImplementationForInterfaceMemberEqual(PropertySymbol expected, NamedTypeSymbol implementingType, PropertySymbol interfaceProperty)
        {
            Assert.Equal(expected, implementingType.FindImplementationForInterfaceMember(interfaceProperty));
            ValidateAccessor(expected?.GetMethod, interfaceProperty.GetMethod);
            ValidateAccessor(expected?.SetMethod, interfaceProperty.SetMethod);

            void ValidateAccessor(MethodSymbol accessor, MethodSymbol interfaceAccessor)
            {
                if ((object)interfaceAccessor != null)
                {
                    Assert.Equal(accessor, implementingType.FindImplementationForInterfaceMember(interfaceAccessor));
                }
                else
                {
                    Assert.Null(accessor);
                }
            }
        }

        private static void ValidateExplicitImplementation(PropertySymbol m1, bool isAbstract = false, bool isStatic = false)
        {
            Assert.Equal(isAbstract, m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.Equal(isAbstract, m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);

            ValidateAccessor(m1.GetMethod, isAbstract, isStatic);
            ValidateAccessor(m1.SetMethod, isAbstract, isStatic);

            static void ValidateAccessor(MethodSymbol accessor, bool isAbstract, bool isStatic)
            {
                if ((object)accessor != null)
                {
                    ValidateExplicitImplementation(accessor, isAbstract, isStatic);
                }
            }
        }

        [Fact]
        public void PropertyImplementationInDerived_02()
        {
            var source1 =
@"
public interface I2
{
    int M1 {get;} 
}

public interface I4
{
    int M1 {set;} 
}

public interface I1 : I2, I4
{
    int I2.M1 => Getter();

    private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }

    int I4.M1 
    {
        set 
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_02(source1, isStatic: false,
                new DiagnosticDescription[]
                {
                // (14,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     int I2.M1 => Getter();
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "Getter()").WithArguments("default interface implementation", "8.0").WithLocation(14, 18),
                // (16,17): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     private int Getter()
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "Getter").WithArguments("default interface implementation", "8.0").WithLocation(16, 17),
                // (24,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         set 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(24, 9)
                },
                // (6,15): error CS8506: 'I1.I2.M1.get' cannot implement interface member 'I2.M1.get' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.get", "I2.M1.get", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15),
                // (6,15): error CS8506: 'I1.I4.M1.set' cannot implement interface member 'I4.M1.set' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.set", "I4.M1.set", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15)
                );
        }

        [Fact]
        public void PropertyImplementationInDerived_Static_02()
        {
            var source1 =
@"
public interface I2
{
    static abstract int M1 {get;} 
}

public interface I4
{
    static abstract int M1 {set;} 
}

public interface I1 : I2, I4
{
    static int I2.M1 => Getter();

    static private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }

    static int I4.M1 
    {
        set 
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_02(source1, isStatic: true,
                new DiagnosticDescription[]
                {
                // (4,25): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract int M1 {get;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(4, 25),
                // (9,25): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract int M1 {set;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(9, 25),
                // (14,19): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static int I2.M1 => Getter();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(14, 19),
                // (22,19): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static int I4.M1 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(22, 19)
                },
                // (6,15): error CS8706: 'I1.I2.M1.get' cannot implement interface member 'I2.M1.get' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.get", "I2.M1.get", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15),
                // (6,15): error CS8706: 'I1.I4.M1.set' cannot implement interface member 'I4.M1.set' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.set", "I4.M1.set", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15)
                );
        }

        private void ValidatePropertyImplementationInDerived_02(string source1, bool isStatic, DiagnosticDescription[] expected1, params DiagnosticDescription[] expected2)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected1);

            ValidatePropertyImplementationInDerived_01(compilation1.SourceModule, isStatic: isStatic);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(expected2);

            ValidatePropertyImplementationInDerived_01(compilation2.SourceModule, isStatic: isStatic);
        }

        [Fact]
        public void PropertyImplementationInDerived_03()
        {
            var source1 =
@"
public interface I2
{
    int M1 {get; set;} 
}

public interface I4
{
    int M1 {set;} 
}

public interface I1 : I2, I4
{
    int I2.M1 
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }

    int I4.M1 
    {
        set 
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_03(source1, isStatic: false,
                new DiagnosticDescription[]
                {
                // (16,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         get
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(16, 9),
                // (21,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         set
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(21, 9),
                // (29,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         set 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(29, 9)
                },
                // (6,15): error CS8502: 'I1.I2.M1.set' cannot implement interface member 'I2.M1.set' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.set", "I2.M1.set", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I2.M1.get' cannot implement interface member 'I2.M1.get' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.get", "I2.M1.get", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I4.M1.set' cannot implement interface member 'I4.M1.set' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.set", "I4.M1.set", "Test1").WithLocation(6, 15)
                );
        }

        private void ValidatePropertyImplementationInDerived_03(string source1, bool isStatic, DiagnosticDescription[] expected1, params DiagnosticDescription[] expected2)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            compilation1.VerifyDiagnostics(expected1);

            ValidatePropertyImplementationInDerived_01(compilation1.SourceModule, isStatic);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation2.VerifyDiagnostics(expected2);
        }

        [Fact]
        public void PropertyImplementationInDerived_Static_03()
        {
            var source1 =
@"
public interface I2
{
    static abstract int M1 {get; set;} 
}

public interface I4
{
    static abstract int M1 {set;} 
}

public interface I1 : I2, I4
{
    static int I2.M1 
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }

    static int I4.M1 
    {
        set 
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_03(source1, isStatic: true,
                new DiagnosticDescription[]
                {
                // (4,29): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract int M1 {get; set;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "get").WithLocation(4, 29),
                // (4,34): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract int M1 {get; set;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "set").WithLocation(4, 34),
                // (9,29): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract int M1 {set;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "set").WithLocation(9, 29),
                // (14,19): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static int I2.M1 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(14, 19),
                // (27,19): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static int I4.M1 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(27, 19)
                },
                // (6,15): error CS8929: 'I1.I2.M1.get' cannot implement interface member 'I2.M1.get' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I2.M1.get", "I2.M1.get", "Test1").WithLocation(6, 15),
                // (6,15): error CS8929: 'I1.I2.M1.set' cannot implement interface member 'I2.M1.set' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I2.M1.set", "I2.M1.set", "Test1").WithLocation(6, 15),
                // (6,15): error CS8929: 'I1.I4.M1.set' cannot implement interface member 'I4.M1.set' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I4.M1.set", "I4.M1.set", "Test1").WithLocation(6, 15)
                );
        }

        [Fact]
        public void PropertyImplementationInDerived_04()
        {
            var source1 =
@"
public interface I2
{
    int M1 {get;} 
}

public interface I4
{
    int M1 {set;} 
}

public interface I1 : I2, I4
{
    int I2.M1 {get;} 
    int I4.M1 {set;} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic: false,
                // (14,16): error CS0501: 'I1.I2.M1.get' must declare a body because it is not marked abstract, extern, or partial
                //     int I2.M1 {get;} 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.I2.M1.get").WithLocation(14, 16),
                // (15,16): error CS0501: 'I1.I4.M1.set' must declare a body because it is not marked abstract, extern, or partial
                //     int I4.M1 {set;} 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.I4.M1.set").WithLocation(15, 16)
                );
        }

        private void ValidatePropertyImplementationInDerived_04(string source1, bool isStatic, params DiagnosticDescription[] expected)
        {
            ValidatePropertyImplementationInDerived_04(source1, i4M1IsAbstract: false, isStatic, expected);
        }

        private void ValidatePropertyImplementationInDerived_04(string source1, bool i4M1IsAbstract, bool isStatic, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);

            ValidatePropertyImplementationInDerived_01(compilation1.SourceModule, i4M1IsAbstract, isStatic: isStatic);
        }

        [Fact]
        public void PropertyImplementationInDerived_Static_04()
        {
            var source1 =
@"
public interface I2
{
    static abstract int M1 {get;} 
}

public interface I4
{
    static abstract int M1 {set;} 
}

public interface I1 : I2, I4
{
    static int I2.M1 {get;} 
    static int I4.M1 {set;} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic: true,
                // (15,23): error CS8051: Auto-implemented properties must have get accessors.
                //     static int I4.M1 {set;} 
                Diagnostic(ErrorCode.ERR_AutoPropertyMustHaveGetAccessor, "set").WithLocation(15, 23)
                );
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_05(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get;} 
}

public interface I4
{
    " + modifiers + @"int M1 {set;} 
}

public interface I1
{
    " + implModifiers + @"int I2.M1 {get => throw null;} 
    " + implModifiers + @"int I4.M1 {set => throw null;} 
}
";

            ValidatePropertyImplementationInDerived_05(source1,
                // (14,9): error CS0540: 'I1.I2.M1': containing type does not implement interface 'I2'
                //     int I2.M1 {get => throw null;} 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I2").WithArguments("I1.I2.M1", "I2").WithLocation(14, 9 + implModifiers.Length),
                // (15,9): error CS0540: 'I1.I4.M1': containing type does not implement interface 'I4'
                //     int I4.M1 {set => throw null;} 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I4").WithArguments("I1.I4.M1", "I4").WithLocation(15, 9 + implModifiers.Length)
                );
        }

        private void ValidatePropertyImplementationInDerived_05(string source1, params DiagnosticDescription[] expected)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected);
        }

        [Fact]
        public void PropertyImplementationInDerived_06()
        {
            var source1 =
@"
public interface I2
{
    int M1 {get; set;} 
}

public interface I4
{
    int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    public static int I2.M1 
    {
        internal get => throw null;
        set => throw null;
    }
    internal virtual int I4.M1
    {
        public get => throw null;
        set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (14,26): error CS0106: The modifier 'public' is not valid for this item
                //     public static int I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("public").WithLocation(14, 26),
                // (14,26): error CS0539: 'I1.M1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     public static int I2.M1 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.M1").WithLocation(14, 26),
                // (16,18): error CS0106: The modifier 'internal' is not valid for this item
                //         internal get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("internal").WithLocation(16, 18),
                // (19,29): error CS0106: The modifier 'internal' is not valid for this item
                //     internal virtual int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("internal").WithLocation(19, 29),
                // (19,29): error CS0106: The modifier 'virtual' is not valid for this item
                //     internal virtual int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("virtual").WithLocation(19, 29),
                // (21,16): error CS0106: The modifier 'public' is not valid for this item
                //         public get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("public").WithLocation(21, 16),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I2.M1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I2.M1").WithLocation(30, 15)
                );
        }

        [Fact]
        public void PropertyImplementationInDerived_Static_06()
        {
            var source1 =
@"
public interface I2
{
    static abstract int M1 {get; set;} 
}

public interface I4
{
    static abstract int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    public int I2.M1 
    {
        internal get => throw null;
        set => throw null;
    }
    internal static virtual int I4.M1
    {
        public get => throw null;
        set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (14,19): error CS0106: The modifier 'public' is not valid for this item
                //     public int I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("public").WithLocation(14, 19),
                // (14,19): error CS0539: 'I1.M1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     public int I2.M1 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.M1").WithLocation(14, 19),
                // (16,18): error CS0106: The modifier 'internal' is not valid for this item
                //         internal get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("internal").WithLocation(16, 18),
                // (19,36): error CS0106: The modifier 'internal' is not valid for this item
                //     internal static virtual int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("internal").WithLocation(19, 36),
                // (19,36): error CS0106: The modifier 'virtual' is not valid for this item
                //     internal static virtual int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("virtual").WithLocation(19, 36),
                // (21,16): error CS0106: The modifier 'public' is not valid for this item
                //         public get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("public").WithLocation(21, 16),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I2.M1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I2.M1").WithLocation(30, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_07(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I4
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"private sealed int I2.M1 
    {
        private get => throw null;
        set => throw null;
    }
    " + implModifiers + @"protected abstract int I4.M1
    {
        get => throw null;
        protected set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, i4M1IsAbstract: true, isStatic: isStatic,
                // (14,27): error CS0106: The modifier 'sealed' is not valid for this item
                //     private sealed int I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("sealed").WithLocation(14, 27 + implModifiers.Length),
                // (14,27): error CS0106: The modifier 'private' is not valid for this item
                //     private sealed int I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private").WithLocation(14, 27 + implModifiers.Length),
                // (16,17): error CS0106: The modifier 'private' is not valid for this item
                //         private get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("private").WithLocation(16, 17),
                // (19,31): error CS0106: The modifier 'protected' is not valid for this item
                //     protected abstract int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected").WithLocation(19, 31 + implModifiers.Length),
                // (21,9): error CS0500: 'I1.I4.M1.get' cannot declare a body because it is marked abstract
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I1.I4.M1.get").WithLocation(21, 9),
                // (22,19): error CS0106: The modifier 'protected' is not valid for this item
                //         protected set => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "set").WithArguments("protected").WithLocation(22, 19),
                // (22,19): error CS0500: 'I1.I4.M1.set' cannot declare a body because it is marked abstract
                //         protected set => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I1.I4.M1.set").WithLocation(22, 19),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I4.M1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I4.M1").WithLocation(30, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_08(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I4
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"private protected int I2.M1 
    {
        private protected get => throw null;
        set => throw null;
    }
    " + implModifiers + @"internal protected override int I4.M1 
    {
        get => throw null;
        internal protected set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic: isStatic,
                // (14,30): error CS0106: The modifier 'private protected' is not valid for this item
                //     private protected int I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private protected").WithLocation(14, 30 + implModifiers.Length),
                // (16,27): error CS0106: The modifier 'private protected' is not valid for this item
                //         private protected get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("private protected").WithLocation(16, 27),
                // (19,40): error CS0106: The modifier 'protected internal' is not valid for this item
                //     internal protected override int I4.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected internal").WithLocation(19, 40 + implModifiers.Length),
                // (19,40): error CS0106: The modifier 'override' is not valid for this item
                //     internal protected override int I4.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("override").WithLocation(19, 40 + implModifiers.Length),
                // (22,28): error CS0106: The modifier 'protected internal' is not valid for this item
                //         internal protected set => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "set").WithArguments("protected internal").WithLocation(22, 28)
                );
        }

        [Fact]
        public void PropertyImplementationInDerived_09()
        {
            var source1 =
@"
public interface I2
{
    int M1 {get;} 
}

public interface I1 : I2
{
    extern int I2.M1 {get;} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var source2 =
@"
public interface I2
{
    int M1 {set;} 
}

public interface I1 : I2
{
    extern int I2.M1
    { set {}} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_09(source1, source2, isStatic: false,
                new DiagnosticDescription[]
                {
                // (9,23): warning CS0626: Method, operator, or accessor 'I1.I2.M1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.M1.get").WithLocation(9, 23)
                },
                new DiagnosticDescription[]
                {
                // (9,23): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(9, 23),
                // (9,23): warning CS0626: Method, operator, or accessor 'I1.I2.M1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.M1.get").WithLocation(9, 23)
                },
                new DiagnosticDescription[]
                {
                // (9,23): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(9, 23),
                // (9,23): warning CS0626: Method, operator, or accessor 'I1.I2.M1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.M1.get").WithLocation(9, 23)
                },
                // (10,7): error CS0179: 'I1.I2.M1.set' cannot be extern and declare a body
                //     { set {}} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I1.I2.M1.set").WithLocation(10, 7)
                );
        }

        private void ValidatePropertyImplementationInDerived_09(string source1, string source2, bool isStatic,
            DiagnosticDescription[] expected1, DiagnosticDescription[] expected2,
            DiagnosticDescription[] expected3, params DiagnosticDescription[] expected4)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(expected1);

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
                var i1i2m1 = GetSingleProperty(i1);
                var i2 = i1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I2").Single();
                var i2m1 = GetSingleProperty(i2);
                var i3 = i1.ContainingNamespace.GetTypeMember("I3");

                Assert.True(i1.IsAbstract);
                Assert.True(i1.IsMetadataAbstract);

                ValidateExplicitExternImplementation(i1i2m1, isStatic: isStatic);

                VerifyFindImplementationForInterfaceMemberSame(null, test1, i1i2m1);
                VerifyFindImplementationForInterfaceMemberSame(i1i2m1, test1, i2m1);

                VerifyFindImplementationForInterfaceMemberSame(null, i1, i1i2m1);
                VerifyFindImplementationForInterfaceMemberSame(i1i2m1, i1, i2m1);

                VerifyFindImplementationForInterfaceMemberSame(null, i2, i2m1);

                VerifyFindImplementationForInterfaceMemberSame(i1i2m1, i3, i2m1);
            }

            CompileAndVerify(compilation1, verify: Verify(isStatic), symbolValidator: Validate1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation2.VerifyDiagnostics(expected2);

            Validate1(compilation2.SourceModule);

            var compilation3 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: isStatic ? TargetFramework.Net50 : TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            compilation3.VerifyDiagnostics(expected3);

            Validate1(compilation3.SourceModule);

            var compilation4 = CreateCompilation(source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics(expected4);

            Validate1(compilation4.SourceModule);
        }

        private static void ValidateExplicitExternImplementation(PropertySymbol m1, bool isStatic)
        {
            Assert.False(m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.False(m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.NotEqual(m1.OriginalDefinition is PEPropertySymbol, m1.IsExtern);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);

            ValidateAccessor(m1.GetMethod);
            ValidateAccessor(m1.SetMethod);

            void ValidateAccessor(MethodSymbol accessor)
            {
                if ((object)accessor != null)
                {
                    ValidateExplicitExternImplementation(accessor, isStatic: isStatic);
                }
            }
        }

        [Fact]
        public void PropertyImplementationInDerived_Static_09()
        {
            var source1 =
@"
public interface I2
{
    static abstract int M1 {get;} 
}

public interface I1 : I2
{
    static extern int I2.M1 {get;} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var source2 =
@"
public interface I2
{
    static abstract int M1 {set;} 
}

public interface I1 : I2
{
    static extern int I2.M1
    { set {}} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_09(source1, source2, isStatic: true,
                new DiagnosticDescription[]
                {
                // (9,30): warning CS0626: Method, operator, or accessor 'I1.I2.M1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.M1.get").WithLocation(9, 30)
                },
                new DiagnosticDescription[]
                {
                // (4,25): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract int M1 {get;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(4, 25),
                // (9,26): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(9, 26),
                // (9,30): warning CS0626: Method, operator, or accessor 'I1.I2.M1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.M1.get").WithLocation(9, 30)
                },
                new DiagnosticDescription[]
                {
                // (4,29): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract int M1 {get;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "get").WithLocation(4, 29),
                // (9,26): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(9, 26),
                // (9,30): warning CS0626: Method, operator, or accessor 'I1.I2.M1.get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     static extern int I2.M1 {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.M1.get").WithLocation(9, 30)
                },
                // (10,7): error CS0179: 'I1.I2.M1.set' cannot be extern and declare a body
                //     { set {}} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I1.I2.M1.set").WithLocation(10, 7)
                );
        }

        [Fact]
        public void PropertyImplementationInDerived_10()
        {
            var source1 =
@"
public interface I1
{
    int M1 {get;set;} 
    int M2 {get;set;} 
}

public interface I2 : I1
{
    int I1.M1
    {
        get => throw null;
    }
    int I1.M2 
    {
        set => throw null;
    }
}

class Test1 : I2
{}

public interface I3
{
    int M1 {get;} 
    int M2 {set;} 
    int M3 {get;set;} 
    int M4 {get;set;} 
}

public interface I4 : I3
{
    int I3.M1
    {
        get => throw null;
        set => throw null;
    }
    int I3.M2 
    {
        get => throw null;
        set => throw null;
    }
    int I3.M3 {get; set;}
    int I3.M4 {get; set;} = 0;
}

class Test2 : I4
{}
";

            ValidatePropertyImplementationInDerived_05(source1,
                // (10,12): error CS0551: Explicit interface implementation 'I2.I1.M1' is missing accessor 'I1.M1.set'
                //     int I1.M1
                Diagnostic(ErrorCode.ERR_ExplicitPropertyMissingAccessor, "M1").WithArguments("I2.I1.M1", "I1.M1.set").WithLocation(10, 12),
                // (14,12): error CS0551: Explicit interface implementation 'I2.I1.M2' is missing accessor 'I1.M2.get'
                //     int I1.M2 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyMissingAccessor, "M2").WithArguments("I2.I1.M2", "I1.M2.get").WithLocation(14, 12),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M1'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.M1").WithLocation(20, 15),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M2'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.M2").WithLocation(20, 15),
                // (36,9): error CS0550: 'I4.I3.M1.set' adds an accessor not found in interface member 'I3.M1'
                //         set => throw null;
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I4.I3.M1.set", "I3.M1").WithLocation(36, 9),
                // (40,9): error CS0550: 'I4.I3.M2.get' adds an accessor not found in interface member 'I3.M2'
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I4.I3.M2.get", "I3.M2").WithLocation(40, 9),
                // (43,16): error CS0501: 'I4.I3.M3.get' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.M3 {get; set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I4.I3.M3.get").WithLocation(43, 16),
                // (43,21): error CS0501: 'I4.I3.M3.set' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.M3 {get; set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I4.I3.M3.set").WithLocation(43, 21),
                // (44,12): error CS8053: Instance properties in interfaces cannot have initializers.
                //     int I3.M4 {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "M4").WithLocation(44, 12),
                // (44,16): error CS0501: 'I4.I3.M4.get' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.M4 {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I4.I3.M4.get").WithLocation(44, 16),
                // (44,21): error CS0501: 'I4.I3.M4.set' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.M4 {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I4.I3.M4.set").WithLocation(44, 21)
                );
        }

        [Fact]
        public void PropertyImplementationInDerived_Static_10()
        {
            var source1 =
@"
public interface I1
{
    static abstract int M1 {get;set;} 
    static abstract int M2 {get;set;} 
}

public interface I2 : I1
{
    static int I1.M1
    {
        get => throw null;
    }
    static int I1.M2 
    {
        set => throw null;
    }
}

class Test1 : I2
{}

public interface I3
{
    static abstract int M1 {get;} 
    static abstract int M2 {set;} 
    static abstract int M3 {get;set;} 
    static abstract int M4 {get;set;} 
}

public interface I4 : I3
{
    static int I3.M1
    {
        get => throw null;
        set => throw null;
    }
    static int I3.M2 
    {
        get => throw null;
        set => throw null;
    }
    static int I3.M3 {get; set;}
    static int I3.M4 {get; set;} = 0;
}

class Test2 : I4
{}
";

            ValidatePropertyImplementationInDerived_05(source1,
                // (10,19): error CS0551: Explicit interface implementation 'I2.I1.M1' is missing accessor 'I1.M1.set'
                //     static int I1.M1
                Diagnostic(ErrorCode.ERR_ExplicitPropertyMissingAccessor, "M1").WithArguments("I2.I1.M1", "I1.M1.set").WithLocation(10, 19),
                // (14,19): error CS0551: Explicit interface implementation 'I2.I1.M2' is missing accessor 'I1.M2.get'
                //     static int I1.M2 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyMissingAccessor, "M2").WithArguments("I2.I1.M2", "I1.M2.get").WithLocation(14, 19),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M1'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.M1").WithLocation(20, 15),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M2'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.M2").WithLocation(20, 15),
                // (36,9): error CS0550: 'I4.I3.M1.set' adds an accessor not found in interface member 'I3.M1'
                //         set => throw null;
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I4.I3.M1.set", "I3.M1").WithLocation(36, 9),
                // (40,9): error CS0550: 'I4.I3.M2.get' adds an accessor not found in interface member 'I3.M2'
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I4.I3.M2.get", "I3.M2").WithLocation(40, 9)
                );
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void PropertyImplementationInDerived_11(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"internal int M1 {get;} 
}

public interface I4
{
    " + modifiers + @"int M1 {get; internal set;} 
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source1 +=
@"
        _ = i2.M1;
        i4.M1 = i4.M1;
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"int I2.M1 => Getter();

    " + implModifiers + @"private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }
    " + implModifiers + @"int I4.M1 
    {
        get 
        {
            System.Console.WriteLine(""I4.M1"");
            return 1;
        }
        set => System.Console.WriteLine(""I4.M1.set"");
    }
}

public interface I3 : I1
{
}
";

            ValidatePropertyImplementationInDerived_11(source1, source2, isStatic,
                // (4,12): error CS0122: 'I2.M1' is inaccessible due to its protection level
                //     int I2.M1 => Getter();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1").WithLocation(4, 12 + implModifiers.Length),
                // (11,12): error CS0122: 'I4.M1.set' is inaccessible due to its protection level
                //     int I4.M1 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I4.M1.set").WithLocation(11, 12 + implModifiers.Length)
                );
        }

        private void ValidatePropertyImplementationInDerived_11(string source1, string source2, bool isStatic, params DiagnosticDescription[] expected)
        {
            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test(new Test1(), new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test<Test1, Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validatePropertyImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
I4.M1.set
",
                verify: Verify(isStatic),
                symbolValidator: validatePropertyImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validatePropertyImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
I4.M1.set
",
                    verify: Verify(isStatic),
                    symbolValidator: validatePropertyImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validatePropertyImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics(expected);

                if (expected.Length == 0)
                {
                    CompileAndVerify(compilation5,
                        expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1
I4.M1
I4.M1.set
",
                        verify: Verify(isStatic),
                        symbolValidator: validatePropertyImplementationInDerived_01);
                }
            }

            void validatePropertyImplementationInDerived_01(ModuleSymbol m)
            {
                ValidatePropertyImplementationInDerived_01(m, isStatic: isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(20083, "https://github.com/dotnet/roslyn/issues/20083")]
        public void PropertyImplementationInDerived_12(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"int M1 { get => throw null; set => throw null;} 
}

public interface I2 : I1
{
    " + implModifiers + @"int I1.M1
    {
        get
        {
            System.Console.WriteLine(""I2.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I2.I1.M1.set"");
        }
    }
}

public interface I3 : I1
{
    " + implModifiers + @"int I1.M1
    {
        get
        {
            System.Console.WriteLine(""I3.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I3.I1.M1.set"");
        }
    }
}

public interface I4 : I1
{
    " + implModifiers + @"int I1.M1
    {
        get
        {
            System.Console.WriteLine(""I4.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I4.I1.M1.set"");
        }
    }
}

public interface I5 : I2, I3
{
    " + implModifiers + @"int I1.M1
    {
        get
        {
            System.Console.WriteLine(""I5.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I5.I1.M1.set"");
        }
    }
}

public interface I6 : I1, I2, I3, I5
{}
";
            var source4 =
@"
class Test5 : I2
{}

class Test6 : I1, I2, I3, I5
{}

class Test7 : I6
{
";
            if (!isStatic)
            {
                source4 +=
@"
    static void Main()
    {
        I1 i5 = new Test5();
        I1 i6 = new Test6();
        I1 i7 = new Test7();
";
            }
            else
            {
                source4 +=
@"
    static void Main()
    {
        Test<Test5, Test6, Test7>();
    }

    static void Test<i5, i6, i7>() where i5 : I1 where i6 : I1 where i7 : I1
    {
";
            }

            source4 +=
@"
        i5.M1 = i5.M1;
        i6.M1 = i6.M1;
        i7.M1 = i7.M1;
    }
}
";
            var source5 = @"
class Test8 : I2, I3
{
    " + implModifiers + @"int I1.M1
    {
        get
        {
            System.Console.WriteLine(""Test8.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test8.I1.M1.set"");
        }
    }
";
            if (!isStatic)
            {
                source5 +=
@"
    static void Main()
    {
        I1 i8 = new Test8();
        I1 i9 = new Test9();
        I1 i10 = new Test10();
        I1 i11 = new Test11();
        I1 i12 = new Test12();
";
            }
            else
            {
                source5 +=
@"
    static void Main()
    {
        Test<Test8, Test9, Test10, Test11, Test12>();
    }

    static void Test<i8, i9, i10, i11, i12>() where i8 : I1 where i9 : I1 where i10 : I1 where i11 : I1 where i12 : I1
    {
";
            }

            source5 +=
@"
        i8.M1 = i8.M1;
        i9.M1 = i9.M1;
        i10.M1 = i10.M1;
        i11.M1 = i11.M1;
        i12.M1 = i12.M1;
    }
}

class Test9 : I7
{
    " + implModifiers + @"int I1.M1
    {
        get
        {
            System.Console.WriteLine(""Test9.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test9.I1.M1.set"");
        }
    }
}

class Test10 : I1, I2, I3, I4
{
    " + implModifiers + @"public int M1
    {
        get
        {
            System.Console.WriteLine(""Test10.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test10.M1.set"");
        }
    }
}

class Test11 : I1, I2, I3, I5, I4
{
    " + implModifiers + @"public int M1
    {
        get
        {
            System.Console.WriteLine(""Test11.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test11.M1.set"");
        }
    }
}

class Test12 : I8
{
    " + implModifiers + @"public " + (isStatic ? "" : "virtual") + @" int M1
    {
        get
        {
            System.Console.WriteLine(""Test12.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test12.M1.set"");
        }
    }
}
";
            ValidatePropertyImplementationInDerived_12(source1, source4, source5, isStatic,
                new DiagnosticDescription[]
                {
                // (2,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(2, 15),
                // (5,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test2 : I7
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I7").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(5, 15),
                // (8,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test3 : I1, I2, I3, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(8, 15),
                // (11,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I5.I1.M1', nor 'I4.I1.M1' are most specific.
                // class Test4 : I1, I2, I3, I5, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.M1", "I5.I1.M1", "I4.I1.M1").WithLocation(11, 15),
                // (14,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I5.I1.M1', nor 'I4.I1.M1' are most specific.
                // class Test5 : I8
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I8").WithArguments("I1.M1", "I5.I1.M1", "I4.I1.M1").WithLocation(14, 15)
                }
                );
        }

        private void ValidatePropertyImplementationInDerived_12(string source1, string source4, string source5, bool isStatic, DiagnosticDescription[] expected1, DiagnosticDescription[] expected2 = null)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var i1 = FindType(m, "I1");
                var i1m1 = GetSingleProperty(i1);
                var i2 = FindType(m, "I2");
                var i2m1 = GetSingleProperty(i2);
                var i5 = FindType(m, "I5");
                var i5m1 = GetSingleProperty(i5);
                var i6 = FindType(m, "I6");

                ValidateExplicitImplementation(i2m1, isStatic: isStatic);
                ValidateExplicitImplementation(i5m1, isStatic: isStatic);

                VerifyFindImplementationForInterfaceMemberSame(i1m1, i1, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i2m1, i2, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i5m1, i5, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i5m1, i6, i1m1);
            }

            CompileAndVerify(compilation1, verify: Verify(isStatic), symbolValidator: Validate1);

            var refs1 = new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() };

            var source2 = @"
public interface I7 : I2, I3
{}

public interface I8 : I1, I2, I3, I5, I4
{}
";

            var source3 =
@"
class Test1 : I2, I3
{}

class Test2 : I7
{}

class Test3 : I1, I2, I3, I4
{}

class Test4 : I1, I2, I3, I5, I4
{}

class Test5 : I8
{}
";

            foreach (var ref1 in refs1)
            {
                var compilation2 = CreateCompilation(source2, new[] { ref1 }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                // According to LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-06-14.md,
                // we do not require interfaces to have a most specific implementation of all members. Therefore, there are no
                // errors in this compilation.
                compilation2.VerifyDiagnostics();
                Validate2(compilation2.SourceModule);

                void Validate2(ModuleSymbol m)
                {
                    Validate1(m);

                    var i1 = FindType(m, "I1");
                    var i1m1 = GetSingleProperty(i1);
                    var i7 = FindType(m, "I7");
                    var i8 = FindType(m, "I8");

                    VerifyFindImplementationForInterfaceMemberSame(null, i7, i1m1);
                    VerifyFindImplementationForInterfaceMemberSame(null, i8, i1m1);
                }

                CompileAndVerify(compilation2, verify: Verify(isStatic), symbolValidator: Validate2);

                compilation2 = CreateCompilation(source2, new[] { ref1 }, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation2.VerifyDiagnostics();
                Validate2(compilation2.SourceModule);

                CompileAndVerify(compilation2, verify: Verify(isStatic), symbolValidator: Validate2);

                var refs2 = new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() };

                var compilation4 = CreateCompilation(source4, new[] { ref1 }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation4.VerifyDiagnostics();
                Validate4(compilation4.SourceModule);

                void Validate4(ModuleSymbol m)
                {
                    Validate1(m);

                    var i1 = FindType(m, "I1");
                    var i1m1 = GetSingleProperty(i1);
                    var i2 = FindType(m, "I2");
                    var i2m1 = GetSingleProperty(i2);
                    var i5 = FindType(m, "I5");
                    var i5m1 = GetSingleProperty(i5);
                    var test5 = FindType(m, "Test5");
                    var test6 = FindType(m, "Test6");
                    var test7 = FindType(m, "Test7");

                    VerifyFindImplementationForInterfaceMemberSame(i2m1, test5, i1m1);
                    VerifyFindImplementationForInterfaceMemberSame(i5m1, test6, i1m1);
                    VerifyFindImplementationForInterfaceMemberSame(i5m1, test7, i1m1);
                }

                CompileAndVerify(compilation4,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.I1.M1.get
I2.I1.M1.set
I5.I1.M1.get
I5.I1.M1.set
I5.I1.M1.get
I5.I1.M1.set
"
,
                    verify: Verify(isStatic),
                    symbolValidator: Validate4);

                foreach (var ref2 in refs2)
                {
                    var compilation3 = CreateCompilation(source3, new[] { ref1, ref2 }, options: TestOptions.DebugDll.WithConcurrentBuild(false),
                                                         parseOptions: TestOptions.RegularPreview,
                                                         targetFramework: TargetFramework.Net60);
                    Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                    compilation3.VerifyDiagnostics(ref1 is CompilationReference ? expected1 : expected2 ?? expected1);

                    Validate3(compilation3.SourceModule);

                    void Validate3(ModuleSymbol m)
                    {
                        Validate1(m);
                        Validate2(m);

                        var i1 = FindType(m, "I1");
                        var i1m1 = GetSingleProperty(i1);
                        var test1 = FindType(m, "Test1");
                        var test2 = FindType(m, "Test2");
                        var test3 = FindType(m, "Test3");
                        var test4 = FindType(m, "Test4");
                        var test5 = FindType(m, "Test5");

                        VerifyFindImplementationForInterfaceMemberSame(null, test1, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test2, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test3, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test4, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test5, i1m1);
                    }

                    var compilation5 = CreateCompilation(source5, new[] { ref1, ref2 }, options: TestOptions.DebugExe,
                                                         parseOptions: TestOptions.RegularPreview,
                                                         targetFramework: TargetFramework.Net60);
                    Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                    compilation5.VerifyDiagnostics();

                    Validate5(compilation5.SourceModule);

                    void Validate5(ModuleSymbol m)
                    {
                        var i1 = FindType(m, "I1");
                        var i1m1 = GetSingleProperty(i1);
                        var test8 = FindType(m, "Test8");
                        var test9 = FindType(m, "Test9");
                        var test10 = FindType(m, "Test10");
                        var test11 = FindType(m, "Test11");
                        var test12 = FindType(m, "Test12");

                        VerifyFindImplementationForInterfaceMemberSame(GetSingleProperty(test8), test8, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleProperty(test9), test9, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleProperty(test10), test10, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleProperty(test11), test11, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleProperty(test12), test12, i1m1);
                    }

                    CompileAndVerify(compilation5,
                        expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"Test8.I1.M1.get
Test8.I1.M1.set
Test9.I1.M1.get
Test9.I1.M1.set
Test10.M1.get
Test10.M1.set
Test11.M1.get
Test11.M1.set
Test12.M1.get
Test12.M1.set
",
                        verify: Verify(isStatic),
                        symbolValidator: Validate5);
                }
            }
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_13(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"int M1 {get;} 
}

public interface I2 : I1
{
    " + implModifiers + @"int I1.M1 => throw null; 
}

public interface I3 : I1
{
    " + implModifiers + @"int I1.M1 => throw null; 
}
";

            var source2 =
@"
class Test1 : I2, I3
{
    " + implModifiers + @"public long M1 => 1;
}
";

            ValidatePropertyImplementationInDerived_13(source1, source2,
                new DiagnosticDescription[]
                {
                // (2,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1'. 'Test1.M1' cannot implement 'I1.M1' because it does not have the matching return type of 'int'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.M1", "Test1.M1", "int").WithLocation(2, 15)
                }
                );
        }

        private void ValidatePropertyImplementationInDerived_13(string source1, string source2, DiagnosticDescription[] expected1, DiagnosticDescription[] expected2 = null)
        {
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(expected1);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(expected2 ?? expected1);
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(20084, "https://github.com/dotnet/roslyn/issues/20084")]
        public void PropertyImplementationInDerived_14(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1<T>
{
    " + modifiers + @"int M1
    {
        get
        {
            System.Console.WriteLine(""I1.M1.get"");
            return 1;
        }
        set => System.Console.WriteLine(""I1.M1.set"");
    }
}

public interface I2<T> : I1<T>
{
    " + implModifiers + @"int I1<T>.M1
    {
        get
        {
            System.Console.WriteLine(""I2.I1.M1.get"");
            return 1;
        }
        set => System.Console.WriteLine(""I2.I1.M1.set"");
    }
}

public interface I3<T> : I1<T>
{}
";
            var source2 =
@"
class Test1 : I1<int>
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1<int> i1Int = new Test1();
        I1<long> i1Long = new Test2();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test2>();
    }

    static void Test<i1Int, i1Long>() where i1Int : I1<int> where i1Long : I1<long>
    {
";
            }

            source2 +=
@"
        i1Int.M1 = i1Int.M1;
        i1Long.M1 = i1Long.M1;
    }
}
class Test2 : I2<long>
{}
";

            ValidatePropertyImplementationInDerived_14(source1, source2, isStatic);
        }

        private void ValidatePropertyImplementationInDerived_14(string source1, string source2, bool isStatic = false)
        {
            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var i1 = FindType(m, "I1");
                var i1m1 = GetSingleProperty(i1);
                var i2 = FindType(m, "I2");
                var i2m1 = GetSingleProperty(i2);
                var i2i1 = i2.InterfacesNoUseSiteDiagnostics().Single();
                var i2i1m1 = GetSingleProperty(i2i1);
                var i3 = FindType(m, "I3");
                var i3i1 = i3.InterfacesNoUseSiteDiagnostics().Single();
                var i3i1m1 = GetSingleProperty(i3i1);

                VerifyFindImplementationForInterfaceMemberSame(i1m1, i1, i1m1);

                VerifyFindImplementationForInterfaceMemberSame(null, i2, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, i2, i3i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i2m1, i2, i2i1m1);

                VerifyFindImplementationForInterfaceMemberSame(null, i3, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, i3, i2i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(i3i1m1, i3, i3i1m1);

                ValidateExplicitImplementation(i2m1, isStatic: isStatic);

                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var test1i1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32));
                var test1i1m1 = GetSingleProperty(i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32)));
                var test2 = m.GlobalNamespace.GetTypeMember("Test2");
                var test2i1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64));
                var test2i1m1 = GetSingleProperty(i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)));
                var test2i2 = i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64));
                var test2i2m1 = GetSingleProperty(i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)));

                Assert.NotSame(test1i1, test1i1m1.ContainingType);
                Assert.NotSame(test2i1, test2i1m1.ContainingType);
                Assert.NotSame(test2i2, test2i2m1.ContainingType);

                VerifyFindImplementationForInterfaceMemberSame(null, test1i1, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test1i1m1, test1i1, test1i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test2i1m1, test2i1, test2i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, test2i2, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test2i2m1, test2i2, test2i1m1);

                ValidateExplicitImplementation(test2i2m1, isStatic: isStatic);

                VerifyFindImplementationForInterfaceMemberSame(null, test1, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test1i1m1, test1, test1i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, test2, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test2i2m1, test2, test2i1m1);
            }

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1.get
I1.M1.set
I2.I1.M1.get
I2.I1.M1.set
",
                verify: Verify(isStatic),
                symbolValidator: Validate);

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1.get
I1.M1.set
I2.I1.M1.get
I2.I1.M1.set
",
                verify: Verify(isStatic),
                symbolValidator: Validate);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            Validate(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1.get
I1.M1.set
I2.I1.M1.get
I2.I1.M1.set
",
                verify: Verify(isStatic),
                symbolValidator: Validate);
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_15(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get => throw null; private set => throw null;} 
}

public interface I4
{
    " + modifiers + @"int M1 {set => throw null; private get => throw null;} 
}

public interface I5 : I4
{
}

public interface I1 : I2, I5
{
    " + implModifiers + @"int I2.M1
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
    " + implModifiers + @"int I4.M1 
    {
        set => System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";
            var source2 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I2 i2 = new Test1();
        I4 i4 = new Test1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source2 +=
@"
        _ = i2.M1;
        i4.M1 = 0;
    }
}
";

            ValidatePropertyImplementationInDerived_01(source1, source2, isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_16(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I4
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"int I2.M1 
    {
        protected get => throw null;
        set => throw null;
    }
    " + implModifiers + @"protected int I4.M1
    {
        get => throw null;
        set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic,
                // (16,19): error CS0106: The modifier 'protected' is not valid for this item
                //         protected get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("protected").WithLocation(16, 19),
                // (19,22): error CS0106: The modifier 'protected' is not valid for this item
                //     protected int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected").WithLocation(19, 22 + implModifiers.Length)
                );
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_17(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I4
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"int I2.M1 
    {
        get => throw null;
        protected internal set => throw null;
    }
    " + implModifiers + @"protected internal int I4.M1
    {
        get => throw null;
        set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic,
                // (17,28): error CS0106: The modifier 'protected internal' is not valid for this item
                //         protected internal set => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "set").WithArguments("protected internal").WithLocation(17, 28),
                // (19,31): error CS0106: The modifier 'protected internal' is not valid for this item
                //     protected internal int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected internal").WithLocation(19, 31 + implModifiers.Length)
                );
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_18(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I4
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"int I2.M1 
    {
        private protected get => throw null;
        set => throw null;
    }
    " + implModifiers + @"private protected int I4.M1
    {
        get => throw null;
        set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic,
                // (16,27): error CS0106: The modifier 'private protected' is not valid for this item
                //         private protected get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("private protected").WithLocation(16, 27),
                // (19,30): error CS0106: The modifier 'private protected' is not valid for this item
                //     private protected int I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private protected").WithLocation(19, 30 + implModifiers.Length)
                );
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_19(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
#nullable enable

public interface I1<T>
{
    " + modifiers + @"int M1 {get; set;} 
}

public interface I2 : I1<string>
{
}

public interface I3 : I2, I1<string?>
{
    " + implModifiers + @"int I1<string>.M1 
    {
        get => throw null!;
        set => throw null!;
    }

    " + implModifiers + @"int I1<string?>.M1 
    {
        get => throw null!;
        set => throw null!;
    }
}
";

            var source2 =
@"
#nullable enable 

class Test1 : I3
{
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (13,18): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'I3' with different nullability of reference types.
                // public interface I3 : I2, I1<string?>
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "I3").WithArguments("I1<string?>", "I3").WithLocation(13, 18),
                // (13,18): error CS8646: 'I1<string>.M1' is explicitly implemented more than once.
                // public interface I3 : I2, I1<string?>
                Diagnostic(ErrorCode.ERR_DuplicateExplicitImpl, "I3").WithArguments("I1<string>.M1").WithLocation(13, 18),
                // (13,18): error CS8646: 'I1<string?>.M1' is explicitly implemented more than once.
                // public interface I3 : I2, I1<string?>
                Diagnostic(ErrorCode.ERR_DuplicateExplicitImpl, "I3").WithArguments("I1<string?>.M1").WithLocation(13, 18),
                // (21,21): error CS0102: The type 'I3' already contains a definition for 'I1<System.String>.M1'
                //     int I1<string?>.M1 
                Diagnostic(ErrorCode.ERR_DuplicateNameInClass, "M1").WithArguments("I3", "I1<System.String>.M1").WithLocation(21, 21 + implModifiers.Length)
                );

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                    parseOptions: TestOptions.RegularPreview,
                                                    targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (4,7): warning CS8645: 'I1<string?>' is already listed in the interface list on type 'Test1' with different nullability of reference types.
                // class Test1 : I3
                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "Test1").WithArguments("I1<string?>", "Test1").WithLocation(4, 7),
                // (4,15): error CS0535: 'Test1' does not implement interface member 'I1<string>.M1'
                // class Test1 : I3
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test1", "I1<string>.M1").WithLocation(4, 15),
                // (4,15): error CS0535: 'Test1' does not implement interface member 'I1<string?>.M1'
                // class Test1 : I3
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I3").WithArguments("Test1", "I1<string?>.M1").WithLocation(4, 15)
                );

            var test1 = compilation2.GetTypeByMetadataName("Test1");

            Assert.Equal(new[] { "I3", "I2", "I1<System.String>", "I1<System.String?>" },
                         test1.AllInterfacesNoUseSiteDiagnostics.Select(i => i.ToTestDisplayString()));

            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[2].GetMember("M1")));
            Assert.Null(test1.FindImplementationForInterfaceMember(test1.AllInterfacesNoUseSiteDiagnostics[3].GetMember("M1")));
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_20(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"protected int M1 {get;} 

";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        _ = i2.M1;
    }
}

public interface I4
{
    " + modifiers + @"int M1 {get; protected set;} 

";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I4 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i4>() where i4 : I4
    {
";
            }

            source1 +=
@"
        i4.M1 = i4.M1;
    }
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
        I2.Test(i2);
        I4.Test(i4);
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
        I2.Test<i2>();
        I4.Test<i4>();
";
            }

            source1 +=
@"
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"int I2.M1 => Getter();

    " + implModifiers + @"private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }
    " + implModifiers + @"int I4.M1 
    {
        get 
        {
            System.Console.WriteLine(""I4.M1"");
            return 1;
        }
        set => System.Console.WriteLine(""I4.M1.set"");
    }
}

public interface I3 : I1
{
}
";

            ValidatePropertyImplementationInDerived_11(source1, source2, isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_21(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"protected internal int M1 {get;} 

";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        _ = i2.M1;
    }
}

public interface I4
{
    " + modifiers + @"int M1 {get; protected internal set;} 

";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I4 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i4>() where i4 : I4
    {
";
            }

            source1 +=
@"
        i4.M1 = i4.M1;
    }
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
        I2.Test(i2);
        I4.Test(i4);
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
        I2.Test<i2>();
        I4.Test<i4>();
";
            }

            source1 +=
@"
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"int I2.M1 => Getter();

    " + implModifiers + @"private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }
    " + implModifiers + @"int I4.M1 
    {
        get 
        {
            System.Console.WriteLine(""I4.M1"");
            return 1;
        }
        set => System.Console.WriteLine(""I4.M1.set"");
    }
}

public interface I3 : I1
{
}
";

            ValidatePropertyImplementationInDerived_11(source1, source2, isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void PropertyImplementationInDerived_22(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"private protected int M1 {get;} 

";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        _ = i2.M1;
    }
}

public interface I4
{
    " + modifiers + @"int M1 {get; private protected set;} 

";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I4 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i4>() where i4 : I4
    {
";
            }

            source1 +=
@"
        i4.M1 = i4.M1;
    }
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
        I2.Test(i2);
        I4.Test(i4);
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
        I2.Test<i2>();
        I4.Test<i4>();
";
            }

            source1 +=
@"
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"int I2.M1 => Getter();

    " + implModifiers + @"private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }
    " + implModifiers + @"int I4.M1 
    {
        get 
        {
            System.Console.WriteLine(""I4.M1"");
            return 1;
        }
        set => System.Console.WriteLine(""I4.M1.set"");
    }
}

public interface I3 : I1
{
}
";

            ValidatePropertyImplementationInDerived_11(source1, source2, isStatic,
                // (4,12): error CS0122: 'I2.M1' is inaccessible due to its protection level
                //     int I2.M1 => Getter();
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1").WithLocation(4, 12 + implModifiers.Length),
                // (11,12): error CS0122: 'I4.M1.set' is inaccessible due to its protection level
                //     int I4.M1 
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I4.M1.set").WithLocation(11, 12 + implModifiers.Length)
                );
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void EventImplementationInDerived_01(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"event System.Action M1; 
}

public interface I4
{
    " + modifiers + @"event System.Action M1; 
}

public interface I5 : I4
{
}

public interface I1 : I2, I5
{
    " + implModifiers + @"event System.Action I2.M1
    {
        add
        {
            System.Console.WriteLine(""I2.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""I2.M1.remove"");
        }
    }
    " + implModifiers + @"event System.Action I4.M1 
    {
        add => System.Console.WriteLine(""I4.M1.add"");
        remove => System.Console.WriteLine(""I4.M1.remove"");
    }
}

public interface I3 : I1
{
}
";

            var source2 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I2 i2 = new Test1();
        I4 i4 = new Test1();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test1>();
    }

    static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source2 +=
@"
        i2.M1 += null;
        i2.M1 -= null;
        i4.M1 += null;
        i4.M1 -= null;
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateEventImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            validateEventImplementationInDerived_01(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            validateEventImplementationInDerived_01(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            void validateEventImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateEventImplementationInDerived_01(m, isStatic);
            }
        }

        private static void ValidateEventImplementationInDerived_01(ModuleSymbol m, bool isStatic)
        {
            ValidateEventImplementationInDerived_01(m, i4M1IsAbstract: false, isStatic);
        }

        private static void ValidateEventImplementationInDerived_01(ModuleSymbol m, bool i4M1IsAbstract, bool isStatic)
        {
            var test1 = m.GlobalNamespace.GetTypeMember("Test1");
            var i1 = test1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I1").Single();
            var i1i2m1 = i1.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I2.")).Single();
            var i1i4m1 = i1.GetMembers().OfType<EventSymbol>().Where(p => p.Name.StartsWith("I4.")).Single();
            var i2 = i1.InterfacesNoUseSiteDiagnostics().Where(i => i.Name == "I2").Single();
            var i2m1 = i2.GetMembers().OfType<EventSymbol>().Single();
            var i4 = i1.AllInterfacesNoUseSiteDiagnostics.Where(i => i.Name == "I4").Single();
            var i4m1 = i4.GetMembers().OfType<EventSymbol>().Single();
            var i3 = i1.ContainingNamespace.GetTypeMember("I3");

            Assert.True(i1.IsAbstract);
            Assert.True(i1.IsMetadataAbstract);

            ValidateExplicitImplementation(i1i2m1, isStatic: isStatic);
            ValidateExplicitImplementation(i1i4m1, i4M1IsAbstract, isStatic: isStatic);

            VerifyFindImplementationForInterfaceMemberSame(null, test1, i1i2m1);
            VerifyFindImplementationForInterfaceMemberSame(null, test1, i1i4m1);

            VerifyFindImplementationForInterfaceMemberSame(i1i2m1, test1, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4M1IsAbstract ? null : i1i4m1, test1, i4m1);

            VerifyFindImplementationForInterfaceMemberSame(null, i1, i1i2m1);
            VerifyFindImplementationForInterfaceMemberSame(null, i1, i1i4m1);
            VerifyFindImplementationForInterfaceMemberSame(i1i2m1, i1, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4M1IsAbstract ? null : i1i4m1, i1, i4m1);

            VerifyFindImplementationForInterfaceMemberSame(null, i2, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(null, i4, i4m1);

            VerifyFindImplementationForInterfaceMemberSame(i1i2m1, i3, i2m1);
            VerifyFindImplementationForInterfaceMemberSame(i4M1IsAbstract ? null : i1i4m1, i3, i4m1);
        }

        private static void VerifyFindImplementationForInterfaceMemberSame(EventSymbol expected, NamedTypeSymbol implementingType, EventSymbol interfaceEvent)
        {
            Assert.Same(expected, implementingType.FindImplementationForInterfaceMember(interfaceEvent));
            ValidateAccessor(expected?.AddMethod, interfaceEvent.AddMethod);
            ValidateAccessor(expected?.RemoveMethod, interfaceEvent.RemoveMethod);

            void ValidateAccessor(MethodSymbol accessor, MethodSymbol interfaceAccessor)
            {
                if ((object)interfaceAccessor != null)
                {
                    Assert.Same(accessor, implementingType.FindImplementationForInterfaceMember(interfaceAccessor));
                }
                else
                {
                    Assert.Null(accessor);
                }
            }
        }

        private static void VerifyFindImplementationForInterfaceMemberEqual(EventSymbol expected, NamedTypeSymbol implementingType, EventSymbol interfaceEvent)
        {
            Assert.Equal(expected, implementingType.FindImplementationForInterfaceMember(interfaceEvent));
            ValidateAccessor(expected?.AddMethod, interfaceEvent.AddMethod);
            ValidateAccessor(expected?.RemoveMethod, interfaceEvent.RemoveMethod);

            void ValidateAccessor(MethodSymbol accessor, MethodSymbol interfaceAccessor)
            {
                if ((object)interfaceAccessor != null)
                {
                    Assert.Equal(accessor, implementingType.FindImplementationForInterfaceMember(interfaceAccessor));
                }
                else
                {
                    Assert.Null(accessor);
                }
            }
        }

        private static void ValidateExplicitImplementation(EventSymbol m1, bool isAbstract = false, bool isStatic = false)
        {
            Assert.Equal(isAbstract, m1.IsAbstract);
            Assert.False(m1.IsVirtual);
            Assert.Equal(isAbstract, m1.IsSealed);
            Assert.Equal(isStatic, m1.IsStatic);
            Assert.False(m1.IsExtern);
            Assert.False(m1.IsOverride);
            Assert.Equal(Accessibility.Private, m1.DeclaredAccessibility);

            ValidateAccessor(m1.AddMethod, isAbstract);
            ValidateAccessor(m1.RemoveMethod, isAbstract);

            void ValidateAccessor(MethodSymbol accessor, bool isAbstract)
            {
                if ((object)accessor != null)
                {
                    ValidateExplicitImplementation(accessor, isAbstract, isStatic);
                }
            }
        }

        [Fact]
        public void EventImplementationInDerived_02()
        {
            var source1 =
@"
public interface I2
{
    event System.Action M1; 
}

public interface I4
{
    event System.Action M1; 
}

public interface I1 : I2, I4
{
    event System.Action I2.M1
    {
        add => throw null;
        remove => throw null;
    }

    event System.Action I4.M1 
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (16,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         add => throw null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(16, 9),
                // (17,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         remove => throw null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(17, 9),
                // (22,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         add => throw null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "add").WithArguments("default interface implementation", "8.0").WithLocation(22, 9),
                // (23,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         remove => throw null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "remove").WithArguments("default interface implementation", "8.0").WithLocation(23, 9)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, isStatic: false);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8506: 'I1.I2.M1.remove' cannot implement interface member 'I2.M1.remove' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.remove", "I2.M1.remove", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15),
                // (6,15): error CS8506: 'I1.I2.M1.add' cannot implement interface member 'I2.M1.add' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.add", "I2.M1.add", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15),
                // (6,15): error CS8506: 'I1.I4.M1.remove' cannot implement interface member 'I4.M1.remove' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.remove", "I4.M1.remove", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15),
                // (6,15): error CS8506: 'I1.I4.M1.add' cannot implement interface member 'I4.M1.add' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.add", "I4.M1.add", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15)
                );

            ValidateEventImplementationInDerived_01(compilation2.SourceModule, isStatic: false);
        }

        [Fact]
        public void EventImplementationInDerived_Static_02()
        {
            var source1 =
@"
public interface I2
{
    static abstract event System.Action M1; 
}

public interface I4
{
    static abstract event System.Action M1; 
}

public interface I1 : I2, I4
{
    static event System.Action I2.M1
    {
        add => throw null;
        remove => throw null;
    }

    static event System.Action I4.M1 
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular10,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (4,41): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract event System.Action M1; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(4, 41),
                // (9,41): error CS8703: The modifier 'abstract' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static abstract event System.Action M1; 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("abstract", "10.0", "11.0").WithLocation(9, 41),
                // (14,35): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static event System.Action I2.M1
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(14, 35),
                // (20,35): error CS8703: The modifier 'static' is not valid for this item in C# 10.0. Please use language version '11.0' or greater.
                //     static event System.Action I4.M1 
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "M1").WithArguments("static", "10.0", "11.0").WithLocation(20, 35)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, isStatic: true);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular10,
                                                 targetFramework: TargetFramework.Net60);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8706: 'I1.I2.M1.add' cannot implement interface member 'I2.M1.add' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.add", "I2.M1.add", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15),
                // (6,15): error CS8706: 'I1.I2.M1.remove' cannot implement interface member 'I2.M1.remove' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.remove", "I2.M1.remove", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15),
                // (6,15): error CS8706: 'I1.I4.M1.add' cannot implement interface member 'I4.M1.add' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.add", "I4.M1.add", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15),
                // (6,15): error CS8706: 'I1.I4.M1.remove' cannot implement interface member 'I4.M1.remove' in type 'Test1' because feature 'static abstract members in interfaces' is not available in C# 10.0. Please use language version '11.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.remove", "I4.M1.remove", "Test1", "static abstract members in interfaces", "10.0", "11.0").WithLocation(6, 15)
                );

            ValidateEventImplementationInDerived_01(compilation2.SourceModule, isStatic: true);
        }

        [Fact]
        public void EventImplementationInDerived_03()
        {
            var source1 =
@"
public interface I2
{
    event System.Action M1;
}

public interface I4
{
    event System.Action M1; 
}

public interface I1 : I2, I4
{
    event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    }

    event System.Action I4.M1 
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular, skipUsesIsNullable: true);
            Assert.False(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation1.VerifyDiagnostics(
                // (16,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         add => throw null;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(16, 9),
                // (17,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         remove => throw null;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(17, 9),
                // (22,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         add => throw null;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "add").WithLocation(22, 9),
                // (23,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         remove => throw null;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "remove").WithLocation(23, 9)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, isStatic: false);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular);
            Assert.False(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8502: 'I1.I2.M1.remove' cannot implement interface member 'I2.M1.remove' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.remove", "I2.M1.remove", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I2.M1.add' cannot implement interface member 'I2.M1.add' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.M1.add", "I2.M1.add", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I4.M1.remove' cannot implement interface member 'I4.M1.remove' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.remove", "I4.M1.remove", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I4.M1.add' cannot implement interface member 'I4.M1.add' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I4.M1.add", "I4.M1.add", "Test1").WithLocation(6, 15)
                );
        }

        [Fact]
        public void EventImplementationInDerived_Static_03()
        {
            var source1 =
@"
public interface I2
{
    static abstract event System.Action M1;
}

public interface I4
{
    static abstract event System.Action M1; 
}

public interface I1 : I2, I4
{
    static event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    }

    static event System.Action I4.M1 
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                 parseOptions: TestOptions.RegularPreview, skipUsesIsNullable: true);

            compilation1.VerifyDiagnostics(
                // (4,41): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract event System.Action M1;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(4, 41),
                // (9,41): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static abstract event System.Action M1; 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(9, 41),
                // (14,35): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(14, 35),
                // (20,35): error CS8919: Target runtime doesn't support static abstract members in interfaces.
                //     static event System.Action I4.M1 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfaces, "M1").WithLocation(20, 35)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, isStatic: true);

            var source2 = @"
public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() },
                                                 options: TestOptions.DebugDll, targetFramework: TargetFramework.Net50,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation2.VerifyDiagnostics(
                // (6,15): error CS8929: 'I1.I2.M1.add' cannot implement interface member 'I2.M1.add' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I2.M1.add", "I2.M1.add", "Test1").WithLocation(6, 15),
                // (6,15): error CS8929: 'I1.I2.M1.remove' cannot implement interface member 'I2.M1.remove' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I2.M1.remove", "I2.M1.remove", "Test1").WithLocation(6, 15),
                // (6,15): error CS8929: 'I1.I4.M1.add' cannot implement interface member 'I4.M1.add' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I4.M1.add", "I4.M1.add", "Test1").WithLocation(6, 15),
                // (6,15): error CS8929: 'I1.I4.M1.remove' cannot implement interface member 'I4.M1.remove' in type 'Test1' because the target runtime doesn't support static abstract members in interfaces.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportStaticAbstractMembersInInterfacesForMember, "I1").WithArguments("I1.I4.M1.remove", "I4.M1.remove", "Test1").WithLocation(6, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_04(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"event System.Action M1;
}

public interface I4
{
    " + modifiers + @"event System.Action M1;
}

public interface I1 : I2, I4
{
    " + implModifiers + @"event System.Action I2.M1
    {
        add; 
        remove;
    } 
    " + implModifiers + @"event System.Action I4.M1 
    {
        add; 
        remove {}
    } 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (16,12): error CS0073: An add or remove accessor must have a body
                //         add; 
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(16, 12),
                // (17,15): error CS0073: An add or remove accessor must have a body
                //         remove;
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(17, 15),
                // (21,12): error CS0073: An add or remove accessor must have a body
                //         add; 
                Diagnostic(ErrorCode.ERR_AddRemoveMustHaveBody, ";").WithLocation(21, 12)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_05(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"event System.Action M1; 
}

public interface I1
{
    " + implModifiers + @"event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    } 
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (9,25): error CS0540: 'I1.I2.M1': containing type does not implement interface 'I2'
                //     event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I2").WithArguments("I1.I2.M1", "I2").WithLocation(9, 25 + implModifiers.Length)
                );
        }

        [Fact]
        public void EventImplementationInDerived_06()
        {
            var source1 =
@"
public interface I2
{
    event System.Action M1; 
}

public interface I4
{
    event System.Action M1; 
}

public interface I1 : I2, I4
{
    public static event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    }
    internal virtual event System.Action I4.M1
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,42): error CS0106: The modifier 'public' is not valid for this item
                //     public static event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("public").WithLocation(14, 42),
                // (14,42): error CS0539: 'I1.M1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     public static event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.M1").WithLocation(14, 42),
                // (19,45): error CS0106: The modifier 'internal' is not valid for this item
                //     internal virtual event System.Action I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("internal").WithLocation(19, 45),
                // (19,45): error CS0106: The modifier 'virtual' is not valid for this item
                //     internal virtual event System.Action I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("virtual").WithLocation(19, 45),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I2.M1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I2.M1").WithLocation(30, 15)
                );
        }

        [Fact]
        public void EventImplementationInDerived_Static_06()
        {
            var source1 =
@"
public interface I2
{
    static abstract event System.Action M1; 
}

public interface I4
{
    static abstract event System.Action M1; 
}

public interface I1 : I2, I4
{
    public event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    }
    static internal virtual event System.Action I4.M1
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);

            compilation1.VerifyDiagnostics(
                // (14,35): error CS0106: The modifier 'public' is not valid for this item
                //     public event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("public").WithLocation(14, 35),
                // (14,35): error CS0539: 'I1.M1' in explicit interface declaration is not found among members of the interface that can be implemented
                //     public event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_InterfaceMemberNotFound, "M1").WithArguments("I1.M1").WithLocation(14, 35),
                // (19,52): error CS0106: The modifier 'internal' is not valid for this item
                //     static internal virtual event System.Action I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("internal").WithLocation(19, 52),
                // (19,52): error CS0106: The modifier 'virtual' is not valid for this item
                //     static internal virtual event System.Action I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("virtual").WithLocation(19, 52),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I2.M1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I2.M1").WithLocation(30, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_07(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"event System.Action M1;
}

public interface I4
{
    " + modifiers + @"event System.Action M1; 
}

public interface I1 : I2, I4
{
    " + implModifiers + @"private sealed event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    }
    " + implModifiers + @"protected abstract event System.Action I4.M1
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,43): error CS0106: The modifier 'sealed' is not valid for this item
                //     private sealed event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("sealed").WithLocation(14, 43 + implModifiers.Length),
                // (14,43): error CS0106: The modifier 'private' is not valid for this item
                //     private sealed event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private").WithLocation(14, 43 + implModifiers.Length),
                // (19,47): error CS0106: The modifier 'protected' is not valid for this item
                //     protected abstract event System.Action I4.M1
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected").WithLocation(19, 47 + implModifiers.Length),
                // (20,5): error CS8712: 'I1.I4.M1': abstract event cannot use event accessor syntax
                //     {
                Diagnostic(ErrorCode.ERR_AbstractEventHasAccessors, "{").WithArguments("I1.I4.M1").WithLocation(20, 5),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I4.M1'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I4.M1").WithLocation(30, 15)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, i4M1IsAbstract: true, isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_08(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"event System.Action M1; 
}

public interface I4
{
    " + modifiers + @"event System.Action M1;
}

public interface I1 : I2, I4
{
    " + implModifiers + @"private protected extern event System.Action I2.M1 
    {
        add => throw null;
        remove => throw null;
    }
    " + implModifiers + @"internal protected override event System.Action I4.M1 
    {
        add => throw null;
        remove => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (14,53): error CS0106: The modifier 'private protected' is not valid for this item
                //     private protected extern event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("private protected").WithLocation(14, 53 + implModifiers.Length),
                // (14,53): error CS0106: The modifier 'extern' is not valid for this item
                //     private protected extern event System.Action I2.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("extern").WithLocation(14, 53 + implModifiers.Length),
                // (19,56): error CS0106: The modifier 'protected internal' is not valid for this item
                //     internal protected override event System.Action I4.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("protected internal").WithLocation(19, 56 + implModifiers.Length),
                // (19,56): error CS0106: The modifier 'override' is not valid for this item
                //     internal protected override event System.Action I4.M1 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "M1").WithArguments("override").WithLocation(19, 56 + implModifiers.Length)
                );

            ValidateEventImplementationInDerived_01(compilation1.SourceModule, isStatic);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_10(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"event System.Action M1; 
    " + modifiers + @"event System.Action M2;
}

public interface I2 : I1
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add => throw null;
    }
    " + implModifiers + @"event System.Action I1.M2 
    {
        remove => throw null;
    }
}

class Test1 : I2
{}

public interface I3
{
    " + modifiers + @"event System.Action M1; 
}

public interface I4 : I3
{
    " + implModifiers + @"event System.Action I3.M1;
}

class Test2 : I4
{}

public interface I5 : I3
{
    " + implModifiers + @"event System.Action I3.M1
}

class Test3 : I5
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics(
                // (30,28): error CS0071: An explicit interface implementation of an event must use event accessor syntax
                //     event System.Action I3.M1;
                Diagnostic(ErrorCode.ERR_ExplicitEventFieldImpl, "M1").WithLocation(30, 28 + implModifiers.Length),
                // (10,28): error CS0065: 'I2.I1.M1': event property must have both add and remove accessors
                //     event System.Action I1.M1
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "M1").WithArguments("I2.I1.M1").WithLocation(10, 28 + implModifiers.Length),
                // (14,28): error CS0065: 'I2.I1.M2': event property must have both add and remove accessors
                //     event System.Action I1.M2 
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "M2").WithArguments("I2.I1.M2").WithLocation(14, 28 + implModifiers.Length),
                // (33,15): error CS0535: 'Test2' does not implement interface member 'I3.M1'
                // class Test2 : I4
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I4").WithArguments("Test2", "I3.M1").WithLocation(33, 15),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M1'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.M1").WithLocation(20, 15),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.M2'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.M2").WithLocation(20, 15),
                // (38,27): error CS0071: An explicit interface implementation of an event must use event accessor syntax
                //     event System.Action I3.M1
                Diagnostic(ErrorCode.ERR_ExplicitEventFieldImpl, ".").WithLocation(38, 27 + implModifiers.Length),
                // (41,15): error CS0535: 'Test3' does not implement interface member 'I3.M1'
                // class Test3 : I5
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I5").WithArguments("Test3", "I3.M1").WithLocation(41, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void EventImplementationInDerived_11(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"internal event System.Action M1;
}

public interface I4
{
    " + modifiers + @"event System.Action M1;
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
";
            }

            source1 +=
@"
        i2.M1 += null;
        i2.M1 -= null;
        i4.M1 += null;
        i4.M1 -= null;
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"event System.Action I2.M1
    {
        add 
        {
            System.Console.WriteLine(""I2.M1.add"");
        }
        remove => System.Console.WriteLine(""I2.M1.remove"");
    }
    " + implModifiers + @"event System.Action I4.M1 
    {
        add 
        {
            System.Console.WriteLine(""I4.M1.add"");
        }
        remove => System.Console.WriteLine(""I4.M1.remove"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test(new Test1(), new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test<Test1, Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateEventImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            var compilation2 = CreateCompilation(source3, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            validateEventImplementationInDerived_01(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            var compilation3 = CreateCompilation(source3, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            validateEventImplementationInDerived_01(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            var compilation5 = CreateCompilation(source2 + source3, new[] { compilation4.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            validateEventImplementationInDerived_01(compilation5.SourceModule);

            compilation5.VerifyDiagnostics(
                // (4,28): error CS0122: 'I2.M1' is inaccessible due to its protection level
                //     event System.Action I2.M1
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1").WithLocation(4, 28 + implModifiers.Length)
                );

            var compilation6 = CreateCompilation(source2 + source3, new[] { compilation4.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation6.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            validateEventImplementationInDerived_01(compilation6.SourceModule);

            compilation6.VerifyDiagnostics(
                // (4,28): error CS0122: 'I2.M1' is inaccessible due to its protection level
                //     event System.Action I2.M1
                Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1").WithLocation(4, 28 + implModifiers.Length)
                );

            void validateEventImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateEventImplementationInDerived_01(m, isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(20083, "https://github.com/dotnet/roslyn/issues/20083")]
        public void EventImplementationInDerived_12(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"event System.Action M1 { add => throw null; remove => throw null;} 
}

public interface I2 : I1
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add
        {
            System.Console.WriteLine(""I2.I1.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""I2.I1.M1.remove"");
        }
    }
}

public interface I3 : I1
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add
        {
            System.Console.WriteLine(""I3.I1.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""I3.I1.M1.remove"");
        }
    }
}

public interface I4 : I1
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add
        {
            System.Console.WriteLine(""I4.I1.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""I4.I1.M1.remove"");
        }
    }
}

public interface I5 : I2, I3
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add
        {
            System.Console.WriteLine(""I5.I1.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""I5.I1.M1.remove"");
        }
    }
}

public interface I6 : I1, I2, I3, I5
{}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate1(compilation1.SourceModule);

            void Validate1(ModuleSymbol m)
            {
                var i1 = FindType(m, "I1");
                var i1m1 = GetSingleEvent(i1);
                var i2 = FindType(m, "I2");
                var i2m1 = GetSingleEvent(i2);
                var i5 = FindType(m, "I5");
                var i5m1 = GetSingleEvent(i5);
                var i6 = FindType(m, "I6");

                ValidateExplicitImplementation(i2m1, isStatic: isStatic);
                ValidateExplicitImplementation(i5m1, isStatic: isStatic);

                VerifyFindImplementationForInterfaceMemberSame(i1m1, i1, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i2m1, i2, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i5m1, i5, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i5m1, i6, i1m1);
            }

            CompileAndVerify(compilation1, verify: Verify(isStatic), symbolValidator: Validate1);

            var refs1 = new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() };

            var source2 = @"
public interface I7 : I2, I3
{}

public interface I8 : I1, I2, I3, I5, I4
{}
";

            var source3 =
@"
class Test1 : I2, I3
{}

class Test2 : I7
{}

class Test3 : I1, I2, I3, I4
{}

class Test4 : I1, I2, I3, I5, I4
{}

class Test5 : I8
{}
";

            var source4 =
@"
class Test5 : I2
{}

class Test6 : I1, I2, I3, I5
{}

class Test7 : I6
{
";
            if (!isStatic)
            {
                source4 +=
@"
    static void Main()
    {
        I1 i5 = new Test5();
        I1 i6 = new Test6();
        I1 i7 = new Test7();
";
            }
            else
            {
                source4 +=
@"
    static void Main()
    {
        Test<Test5, Test6, Test7>();
    }

    static void Test<i5, i6, i7>() where i5 : I1 where i6 : I1 where i7 : I1
    {
";
            }

            source4 +=
@"
        i5.M1 += null;
        i5.M1 -= null;
        i6.M1 += null;
        i6.M1 -= null;
        i7.M1 += null;
        i7.M1 -= null;
    }
}
";
            var source5 = @"
class Test8 : I2, I3
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add
        {
            System.Console.WriteLine(""Test8.I1.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""Test8.I1.M1.remove"");
        }
    }
";
            if (!isStatic)
            {
                source5 +=
@"
    static void Main()
    {
        I1 i8 = new Test8();
        I1 i9 = new Test9();
        I1 i10 = new Test10();
        I1 i11 = new Test11();
        I1 i12 = new Test12();
";
            }
            else
            {
                source5 +=
@"
    static void Main()
    {
        Test<Test8, Test9, Test10, Test11, Test12>();
    }

    static void Test<i8, i9, i10, i11, i12>() where i8 : I1 where i9 : I1 where i10 : I1 where i11 : I1 where i12 : I1
    {
";
            }

            source5 +=
@"
        i8.M1 += null;
        i8.M1 -= null;
        i9.M1 += null;
        i9.M1 -= null;
        i10.M1 += null;
        i10.M1 -= null;
        i11.M1 += null;
        i11.M1 -= null;
        i12.M1 += null;
        i12.M1 -= null;
    }
}

class Test9 : I7
{
    " + implModifiers + @"event System.Action I1.M1
    {
        add
        {
            System.Console.WriteLine(""Test9.I1.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""Test9.I1.M1.remove"");
        }
    }
}

class Test10 : I1, I2, I3, I4
{
    " + implModifiers + @"public event System.Action M1
    {
        add
        {
            System.Console.WriteLine(""Test10.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""Test10.M1.remove"");
        }
    }
}

class Test11 : I1, I2, I3, I5, I4
{
    " + implModifiers + @"public event System.Action M1
    {
        add
        {
            System.Console.WriteLine(""Test11.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""Test11.M1.remove"");
        }
    }
}

class Test12 : I8
{
    " + implModifiers + @"public " + (isStatic ? "" : "virtual") + @" event System.Action M1
    {
        add
        {
            System.Console.WriteLine(""Test12.M1.add"");
        }
        remove
        {
            System.Console.WriteLine(""Test12.M1.remove"");
        }
    }
}
";

            foreach (var ref1 in refs1)
            {
                var compilation2 = CreateCompilation(source2, new[] { ref1 }, options: TestOptions.DebugDll,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                // According to LDM decision captured at https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-06-14.md,
                // we do not require interfaces to have a most specific implementation of all members. Therefore, there are no
                // errors in this compilation.
                compilation2.VerifyDiagnostics();
                Validate2(compilation2.SourceModule);

                void Validate2(ModuleSymbol m)
                {
                    Validate1(m);

                    var i1 = FindType(m, "I1");
                    var i1m1 = GetSingleEvent(i1);
                    var i7 = FindType(m, "I7");
                    var i8 = FindType(m, "I8");

                    VerifyFindImplementationForInterfaceMemberSame(null, i7, i1m1);
                    VerifyFindImplementationForInterfaceMemberSame(null, i8, i1m1);
                }

                CompileAndVerify(compilation2, verify: Verify(isStatic), symbolValidator: Validate2);

                compilation2 = CreateCompilation(source2, new[] { ref1 }, options: TestOptions.DebugDll,
                                                 parseOptions: isStatic ? TestOptions.Regular10 : TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation2.VerifyDiagnostics();
                Validate2(compilation2.SourceModule);

                CompileAndVerify(compilation2, verify: Verify(isStatic), symbolValidator: Validate2);

                var refs2 = new[] { compilation2.ToMetadataReference(), compilation2.EmitToImageReference() };

                var compilation4 = CreateCompilation(source4, new[] { ref1 }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                compilation4.VerifyDiagnostics();
                Validate4(compilation4.SourceModule);

                void Validate4(ModuleSymbol m)
                {
                    Validate1(m);

                    var i1 = FindType(m, "I1");
                    var i1m1 = GetSingleEvent(i1);
                    var i2 = FindType(m, "I2");
                    var i2m1 = GetSingleEvent(i2);
                    var i5 = FindType(m, "I5");
                    var i5m1 = GetSingleEvent(i5);
                    var test5 = FindType(m, "Test5");
                    var test6 = FindType(m, "Test6");
                    var test7 = FindType(m, "Test7");

                    VerifyFindImplementationForInterfaceMemberSame(i2m1, test5, i1m1);
                    VerifyFindImplementationForInterfaceMemberSame(i5m1, test6, i1m1);
                    VerifyFindImplementationForInterfaceMemberSame(i5m1, test7, i1m1);
                }

                CompileAndVerify(compilation4,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.I1.M1.add
I2.I1.M1.remove
I5.I1.M1.add
I5.I1.M1.remove
I5.I1.M1.add
I5.I1.M1.remove
"
,
                    verify: Verify(isStatic),
                    symbolValidator: Validate4);

                foreach (var ref2 in refs2)
                {
                    var compilation3 = CreateCompilation(source3, new[] { ref1, ref2 }, options: TestOptions.DebugDll.WithConcurrentBuild(false),
                                                         parseOptions: TestOptions.RegularPreview,
                                                         targetFramework: TargetFramework.Net60);
                    Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                    compilation3.VerifyDiagnostics(
                // (2,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(2, 15),
                // (5,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test2 : I7
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I7").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(5, 15),
                // (8,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test3 : I1, I2, I3, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(8, 15),
                // (11,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I5.I1.M1', nor 'I4.I1.M1' are most specific.
                // class Test4 : I1, I2, I3, I5, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.M1", "I5.I1.M1", "I4.I1.M1").WithLocation(11, 15),
                // (14,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I5.I1.M1', nor 'I4.I1.M1' are most specific.
                // class Test5 : I8
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I8").WithArguments("I1.M1", "I5.I1.M1", "I4.I1.M1").WithLocation(14, 15)
                        );

                    Validate3(compilation3.SourceModule);

                    void Validate3(ModuleSymbol m)
                    {
                        Validate1(m);
                        Validate2(m);

                        var i1 = FindType(m, "I1");
                        var i1m1 = GetSingleEvent(i1);
                        var test1 = FindType(m, "Test1");
                        var test2 = FindType(m, "Test2");
                        var test3 = FindType(m, "Test3");
                        var test4 = FindType(m, "Test4");
                        var test5 = FindType(m, "Test5");

                        VerifyFindImplementationForInterfaceMemberSame(null, test1, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test2, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test3, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test4, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(null, test5, i1m1);
                    }

                    var compilation5 = CreateCompilation(source5, new[] { ref1, ref2 }, options: TestOptions.DebugExe,
                                                         parseOptions: TestOptions.RegularPreview,
                                                         targetFramework: TargetFramework.Net60);
                    Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                    compilation5.VerifyDiagnostics();

                    Validate5(compilation5.SourceModule);

                    void Validate5(ModuleSymbol m)
                    {
                        var i1 = FindType(m, "I1");
                        var i1m1 = GetSingleEvent(i1);
                        var test8 = FindType(m, "Test8");
                        var test9 = FindType(m, "Test9");
                        var test10 = FindType(m, "Test10");
                        var test11 = FindType(m, "Test11");
                        var test12 = FindType(m, "Test12");

                        VerifyFindImplementationForInterfaceMemberSame(GetSingleEvent(test8), test8, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleEvent(test9), test9, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleEvent(test10), test10, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleEvent(test11), test11, i1m1);
                        VerifyFindImplementationForInterfaceMemberSame(GetSingleEvent(test12), test12, i1m1);
                    }

                    CompileAndVerify(compilation5,
                        expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"Test8.I1.M1.add
Test8.I1.M1.remove
Test9.I1.M1.add
Test9.I1.M1.remove
Test10.M1.add
Test10.M1.remove
Test11.M1.add
Test11.M1.remove
Test12.M1.add
Test12.M1.remove
",
                        verify: Verify(isStatic),
                        symbolValidator: Validate5);
                }
            }
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_13(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1
{
    " + modifiers + @"event System.Action M1; 
}

public interface I2 : I1
{
    " + implModifiers + @"event System.Action I1.M1
    { add => throw null; remove => throw null;} 
}

public interface I3 : I1
{
    " + implModifiers + @"event System.Action I1.M1 
    { add => throw null; remove => throw null;} 
}
";

            var source2 =
@"
class Test1 : I2, I3
{
    " + implModifiers + @"public event System.Action<object> M1
    { add => throw null; remove => throw null;} 
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation2.VerifyDiagnostics(
                // (2,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1'. 'Test1.M1' cannot implement 'I1.M1' because it does not have the matching return type of 'Action'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.M1", "Test1.M1", "System.Action").WithLocation(2, 15)
                );

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            compilation3.VerifyDiagnostics(
                // (2,15): error CS8505: Interface member 'I1.M1' does not have a most specific implementation. Neither 'I2.I1.M1', nor 'I3.I1.M1' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.M1", "I2.I1.M1", "I3.I1.M1").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.M1'. 'Test1.M1' cannot implement 'I1.M1' because it does not have the matching return type of 'Action'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.M1", "Test1.M1", "System.Action").WithLocation(2, 15)
                );
        }

        [Theory]
        [CombinatorialData]
        [WorkItem(20084, "https://github.com/dotnet/roslyn/issues/20084")]
        public void EventImplementationInDerived_14(bool isStatic)
        {
            string modifiers = isStatic ? "static virtual " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I1<T>
{
    " + modifiers + @"event System.Action M1
    {
        add
        {
            System.Console.WriteLine(""I1.M1.add"");
        }
        remove => System.Console.WriteLine(""I1.M1.remove"");
    }
}

public interface I2<T> : I1<T>
{
    " + implModifiers + @"event System.Action I1<T>.M1
    {
        add
        {
            System.Console.WriteLine(""I2.I1.M1.add"");
        }
        remove => System.Console.WriteLine(""I2.I1.M1.remove"");
    }
}

public interface I3<T> : I1<T>
{}
";

            var source2 =
@"
class Test1 : I1<int>
{
";
            if (!isStatic)
            {
                source2 +=
@"
    static void Main()
    {
        I1<int> i1Int = new Test1();
        I1<long> i1Long = new Test2();
";
            }
            else
            {
                source2 +=
@"
    static void Main()
    {
        Test<Test1, Test2>();
    }

    static void Test<i1Int, i1Long>() where i1Int : I1<int> where i1Long : I1<long>
    {
";
            }

            source2 +=
@"
        i1Int.M1 += null;
        i1Int.M1 -= null;
        i1Long.M1 += null;
        i1Long.M1 -= null;
    }
}
class Test2 : I2<long>
{}
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            Validate(compilation1.SourceModule);

            void Validate(ModuleSymbol m)
            {
                var i1 = FindType(m, "I1");
                var i1m1 = GetSingleEvent(i1);
                var i2 = FindType(m, "I2");
                var i2m1 = GetSingleEvent(i2);
                var i2i1 = i2.InterfacesNoUseSiteDiagnostics().Single();
                var i2i1m1 = GetSingleEvent(i2i1);
                var i3 = FindType(m, "I3");
                var i3i1 = i3.InterfacesNoUseSiteDiagnostics().Single();
                var i3i1m1 = GetSingleEvent(i3i1);

                VerifyFindImplementationForInterfaceMemberSame(i1m1, i1, i1m1);

                VerifyFindImplementationForInterfaceMemberSame(null, i2, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, i2, i3i1m1);
                VerifyFindImplementationForInterfaceMemberSame(i2m1, i2, i2i1m1);

                VerifyFindImplementationForInterfaceMemberSame(null, i3, i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, i3, i2i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(i3i1m1, i3, i3i1m1);

                ValidateExplicitImplementation(i2m1, isStatic: isStatic);

                var test1 = m.GlobalNamespace.GetTypeMember("Test1");
                var test1i1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32));
                var test1i1m1 = GetSingleEvent(i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int32)));
                var test2 = m.GlobalNamespace.GetTypeMember("Test2");
                var test2i1 = i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64));
                var test2i1m1 = GetSingleEvent(i1.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)));
                var test2i2 = i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64));
                var test2i2m1 = GetSingleEvent(i2.Construct(m.ContainingAssembly.GetSpecialType(SpecialType.System_Int64)));

                Assert.NotSame(test1i1, test1i1m1.ContainingType);
                Assert.NotSame(test2i1, test2i1m1.ContainingType);
                Assert.NotSame(test2i2, test2i2m1.ContainingType);

                VerifyFindImplementationForInterfaceMemberSame(null, test1i1, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test1i1m1, test1i1, test1i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test2i1m1, test2i1, test2i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, test2i2, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test2i2m1, test2i2, test2i1m1);

                ValidateExplicitImplementation(test2i2m1, isStatic: isStatic);

                VerifyFindImplementationForInterfaceMemberSame(null, test1, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test1i1m1, test1, test1i1m1);
                VerifyFindImplementationForInterfaceMemberSame(null, test2, i1m1);
                VerifyFindImplementationForInterfaceMemberEqual(test2i2m1, test2, test2i1m1);
            }

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1.add
I1.M1.remove
I2.I1.M1.add
I2.I1.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: Validate);

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

            Validate(compilation2.SourceModule);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1.add
I1.M1.remove
I2.I1.M1.add
I2.I1.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: Validate);

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation3.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            Validate(compilation3.SourceModule);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I1.M1.add
I1.M1.remove
I2.I1.M1.add
I2.I1.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: Validate);
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_15(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"protected event System.Action M1;
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        i2.M1 += null;
        i2.M1 -= null;
    }
}

public interface I4
{
    " + modifiers + @"event System.Action M1;
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
        I2.Test(i2);
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
        I2.Test<i2>();
";
            }

            source1 +=
@"
        i4.M1 += null;
        i4.M1 -= null;
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"event System.Action I2.M1
    {
        add 
        {
            System.Console.WriteLine(""I2.M1.add"");
        }
        remove => System.Console.WriteLine(""I2.M1.remove"");
    }
    " + implModifiers + @"event System.Action I4.M1 
    {
        add 
        {
            System.Console.WriteLine(""I4.M1.add"");
        }
        remove => System.Console.WriteLine(""I4.M1.remove"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test(new Test1(), new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test<Test1, Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateEventImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateEventImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                    verify: Verify(isStatic),
                    symbolValidator: validateEventImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateEventImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics();
                CompileAndVerify(compilation5,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                    verify: Verify(isStatic),
                    symbolValidator: validateEventImplementationInDerived_01);
            }

            void validateEventImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateEventImplementationInDerived_01(m, isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_16(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"protected internal event System.Action M1;
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        i2.M1 += null;
        i2.M1 -= null;
    }
}

public interface I4
{
    " + modifiers + @"event System.Action M1;
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
        I2.Test(i2);
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
        I2.Test<i2>();
";
            }

            source1 +=
@"
        i4.M1 += null;
        i4.M1 -= null;
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"event System.Action I2.M1
    {
        add 
        {
            System.Console.WriteLine(""I2.M1.add"");
        }
        remove => System.Console.WriteLine(""I2.M1.remove"");
    }
    " + implModifiers + @"event System.Action I4.M1 
    {
        add 
        {
            System.Console.WriteLine(""I4.M1.add"");
        }
        remove => System.Console.WriteLine(""I4.M1.remove"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test(new Test1(), new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test<Test1, Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateEventImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateEventImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                    verify: Verify(isStatic),
                    symbolValidator: validateEventImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateEventImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics();
                CompileAndVerify(compilation5,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                    verify: Verify(isStatic),
                    symbolValidator: validateEventImplementationInDerived_01);
            }

            void validateEventImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateEventImplementationInDerived_01(m, isStatic);
            }
        }

        [Theory]
        [CombinatorialData]
        public void EventImplementationInDerived_17(bool isStatic)
        {
            string modifiers = isStatic ? "static abstract " : "";
            string implModifiers = isStatic ? "static " : "";

            var source1 =
@"
public interface I2
{
    " + modifiers + @"private protected event System.Action M1;
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2)
    {
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2>() where i2 : I2
    {
";
            }

            source1 +=
@"
        i2.M1 += null;
        i2.M1 -= null;
    }
}

public interface I4
{
    " + modifiers + @"event System.Action M1;
}

public interface I5 : I4
{
}

public class TestHelper
{
";
            if (!isStatic)
            {
                source1 +=
@"
    public static void Test(I2 i2, I4 i4)
    {
        I2.Test(i2);
";
            }
            else
            {
                source1 +=
@"
    public static void Test<i2, i4>() where i2 : I2 where i4 : I4
    {
        I2.Test<i2>();
";
            }

            source1 +=
@"
        i4.M1 += null;
        i4.M1 -= null;
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    " + implModifiers + @"event System.Action I2.M1
    {
        add 
        {
            System.Console.WriteLine(""I2.M1.add"");
        }
        remove => System.Console.WriteLine(""I2.M1.remove"");
    }
    " + implModifiers + @"event System.Action I4.M1 
    {
        add 
        {
            System.Console.WriteLine(""I4.M1.add"");
        }
        remove => System.Console.WriteLine(""I4.M1.remove"");
    }
}

public interface I3 : I1
{
}
";

            var source3 =
@"
class Test1 : I1
{
";
            if (!isStatic)
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test(new Test1(), new Test1());
";
            }
            else
            {
                source3 +=
@"
    static void Main()
    {
        TestHelper.Test<Test1, Test1>();
";
            }

            source3 +=
@"
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            validateEventImplementationInDerived_01(compilation1.SourceModule);

            CompileAndVerify(compilation1,
                expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                verify: Verify(isStatic),
                symbolValidator: validateEventImplementationInDerived_01);

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source3, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.RegularPreview,
                                                     targetFramework: TargetFramework.Net60);
                Assert.True(compilation2.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateEventImplementationInDerived_01(compilation2.SourceModule);

                compilation2.VerifyDiagnostics();
                CompileAndVerify(compilation2,
                    expectedOutput: !Execute(isStatic, haveImplementationInDerivedInterface: true) ? null :
@"I2.M1.add
I2.M1.remove
I4.M1.add
I4.M1.remove
",
                    verify: Verify(isStatic),
                    symbolValidator: validateEventImplementationInDerived_01);
            }

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            Assert.True(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation4.VerifyDiagnostics();

            foreach (var reference in new[] { compilation4.ToMetadataReference(), compilation4.EmitToImageReference() })
            {
                var compilation5 = CreateCompilation(source2 + source3, new[] { reference }, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
                Assert.True(compilation5.Assembly.RuntimeSupportsDefaultInterfaceImplementation);

                validateEventImplementationInDerived_01(compilation5.SourceModule);

                compilation5.VerifyDiagnostics(
                    // (4,28): error CS0122: 'I2.M1' is inaccessible due to its protection level
                    //     event System.Action I2.M1
                    Diagnostic(ErrorCode.ERR_BadAccess, "M1").WithArguments("I2.M1").WithLocation(4, 28 + implModifiers.Length)
                    );
            }

            void validateEventImplementationInDerived_01(ModuleSymbol m)
            {
                ValidateEventImplementationInDerived_01(m, isStatic);
            }
        }

        [Fact]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void IndexerImplementationInDerived_01()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get;} 
}

public interface I4
{
    int this[int x] {set;} 
}

public interface I5 : I4
{
}

public interface I1 : I2, I5
{
    int I2.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
    int I4.this[int x] 
    {
        set => System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";
            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I2 i2 = new Test1();
        _ = i2[0];
        I4 i4 = new Test1();
        i4[0] = 0;
    }
}
";
            ValidatePropertyImplementationInDerived_01(source1, source2);
        }

        [Fact]
        public void IndexerImplementationInDerived_02()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get;} 
}

public interface I4
{
    int this[int x] {set;} 
}

public interface I1 : I2, I4
{
    int I2.this[int x] => Getter();

    private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }

    int I4.this[int x] 
    {
        set 
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_02(source1, isStatic: false,
                new DiagnosticDescription[]
                {
                // (16,17): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     private int Getter()
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "Getter").WithArguments("default interface implementation", "8.0").WithLocation(16, 17),
                // (14,27): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     int I2.this[int x] => Getter();
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "Getter()").WithArguments("default interface implementation", "8.0").WithLocation(14, 27),
                // (24,9): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         set 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "set").WithArguments("default interface implementation", "8.0").WithLocation(24, 9)
                },
                // (6,15): error CS8506: 'I1.I2.this[int].get' cannot implement interface member 'I2.this[int].get' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I2.this[int].get", "I2.this[int].get", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15),
                // (6,15): error CS8506: 'I1.I4.this[int].set' cannot implement interface member 'I4.this[int].set' in type 'Test1' because feature 'default interface implementation' is not available in C# 7.3. Please use language version '8.0' or greater.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_LanguageVersionDoesNotSupportInterfaceImplementationForMember, "I1").WithArguments("I1.I4.this[int].set", "I4.this[int].set", "Test1", "default interface implementation", "7.3", "8.0").WithLocation(6, 15)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_03()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get; set;} 
}

public interface I4
{
    int this[int x] {set;} 
}

public interface I1 : I2, I4
{
    int I2.this[int x] 
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }

    int I4.this[int x] 
    {
        set 
        {
            System.Console.WriteLine(""I2.M1"");
        }
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_03(source1, isStatic: false,
                new DiagnosticDescription[]
                {
                // (16,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         get
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(16, 9),
                // (21,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         set
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(21, 9),
                // (29,9): error CS8501: Target runtime doesn't support default interface implementation.
                //         set 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "set").WithLocation(29, 9)
                },
                // (6,15): error CS8502: 'I1.I2.this[int].set' cannot implement interface member 'I2.this[int].set' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.this[int].set", "I2.this[int].set", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I2.this[int].get' cannot implement interface member 'I2.this[int].get' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I2.this[int].get", "I2.this[int].get", "Test1").WithLocation(6, 15),
                // (6,15): error CS8502: 'I1.I4.this[int].set' cannot implement interface member 'I4.this[int].set' in type 'Test1' because the target runtime doesn't support default interface implementation.
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementationForMember, "I1").WithArguments("I1.I4.this[int].set", "I4.this[int].set", "Test1").WithLocation(6, 15)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_04()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get;} 
}

public interface I4
{
    int this[int x] {set;} 
}

public interface I1 : I2, I4
{
    int I2.this[int x] {get;} 
    int I4.this[int x] {set;} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic: false,
                // (14,25): error CS0501: 'I1.I2.this[int].get' must declare a body because it is not marked abstract, extern, or partial
                //     int I2.this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.I2.this[int].get").WithLocation(14, 25),
                // (15,25): error CS0501: 'I1.I4.this[int].set' must declare a body because it is not marked abstract, extern, or partial
                //     int I4.this[int x] {set;} 
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.I4.this[int].set").WithLocation(15, 25)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_05()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get;} 
}

public interface I4
{
    int this[int x] {set;} 
}

public interface I1
{
    int I2.this[int x] {get => throw null;} 
    int I4.this[int x] {set => throw null;} 
}
";

            ValidatePropertyImplementationInDerived_05(source1,
                // (14,9): error CS0540: 'I1.I2.this[int]': containing type does not implement interface 'I2'
                //     int I2.this[int x] {get => throw null;} 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I2").WithArguments("I1.I2.this[int]", "I2").WithLocation(14, 9),
                // (15,9): error CS0540: 'I1.I4.this[int]': containing type does not implement interface 'I4'
                //     int I4.this[int x] {set => throw null;} 
                Diagnostic(ErrorCode.ERR_ClassDoesntImplementInterface, "I4").WithArguments("I1.I4.this[int]", "I4").WithLocation(15, 9)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_06()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get; set;} 
}

public interface I4
{
    int this[int x] {get; set;} 
}

public interface I1 : I2, I4
{
    public static int I2.this[int x] 
    {
        internal get => throw null;
        set => throw null;
    }
    internal virtual int I4.this[int x]
    {
        public get => throw null;
        set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic: false,
                // (14,26): error CS0106: The modifier 'static' is not valid for this item
                //     public static int I2.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(14, 26),
                // (14,26): error CS0106: The modifier 'public' is not valid for this item
                //     public static int I2.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("public").WithLocation(14, 26),
                // (16,18): error CS0106: The modifier 'internal' is not valid for this item
                //         internal get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("internal").WithLocation(16, 18),
                // (19,29): error CS0106: The modifier 'internal' is not valid for this item
                //     internal virtual int I4.this[int x]
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("internal").WithLocation(19, 29),
                // (19,29): error CS0106: The modifier 'virtual' is not valid for this item
                //     internal virtual int I4.this[int x]
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("virtual").WithLocation(19, 29),
                // (21,16): error CS0106: The modifier 'public' is not valid for this item
                //         public get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("public").WithLocation(21, 16)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_07()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get; set;} 
}

public interface I4
{
    int this[int x] {get; set;} 
}

public interface I1 : I2, I4
{
    private sealed int I2.this[int x] 
    {
        private get => throw null;
        set => throw null;
    }
    protected abstract int I4.this[int x]
    {
        get => throw null;
        protected set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, i4M1IsAbstract: true, isStatic: false,
                // (14,27): error CS0106: The modifier 'sealed' is not valid for this item
                //     private sealed int I2.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("sealed").WithLocation(14, 27),
                // (14,27): error CS0106: The modifier 'private' is not valid for this item
                //     private sealed int I2.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("private").WithLocation(14, 27),
                // (16,17): error CS0106: The modifier 'private' is not valid for this item
                //         private get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("private").WithLocation(16, 17),
                // (19,31): error CS0106: The modifier 'protected' is not valid for this item
                //     protected abstract int I4.this[int x]
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("protected").WithLocation(19, 31),
                // (21,9): error CS0500: 'I1.I4.this[int].get' cannot declare a body because it is marked abstract
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "get").WithArguments("I1.I4.this[int].get").WithLocation(21, 9),
                // (22,19): error CS0106: The modifier 'protected' is not valid for this item
                //         protected set => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "set").WithArguments("protected").WithLocation(22, 19),
                // (22,19): error CS0500: 'I1.I4.this[int].set' cannot declare a body because it is marked abstract
                //         protected set => throw null;
                Diagnostic(ErrorCode.ERR_AbstractHasBody, "set").WithArguments("I1.I4.this[int].set").WithLocation(22, 19),
                // (30,15): error CS0535: 'Test1' does not implement interface member 'I4.this[int]'
                // class Test1 : I1
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I1").WithArguments("Test1", "I4.this[int]").WithLocation(30, 15)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_08()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get; set;} 
}

public interface I4
{
    int this[int x] {get; set;} 
}

public interface I1 : I2, I4
{
    private protected int I2.this[int x] 
    {
        private protected get => throw null;
        set => throw null;
    }
    internal protected override int I4.this[int x] 
    {
        get => throw null;
        internal protected set => throw null;
    }
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_04(source1, isStatic: false,
                // (14,30): error CS0106: The modifier 'private protected' is not valid for this item
                //     private protected int I2.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("private protected").WithLocation(14, 30),
                // (16,27): error CS0106: The modifier 'private protected' is not valid for this item
                //         private protected get => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "get").WithArguments("private protected").WithLocation(16, 27),
                // (19,40): error CS0106: The modifier 'protected internal' is not valid for this item
                //     internal protected override int I4.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("protected internal").WithLocation(19, 40),
                // (19,40): error CS0106: The modifier 'override' is not valid for this item
                //     internal protected override int I4.this[int x] 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("override").WithLocation(19, 40),
                // (22,28): error CS0106: The modifier 'protected internal' is not valid for this item
                //         internal protected set => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "set").WithArguments("protected internal").WithLocation(22, 28)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_09()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get;} 
}

public interface I1 : I2
{
    extern int I2.this[int x] {get;} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            var source2 =
@"
public interface I2
{
    int this[int x] {set;} 
}

public interface I1 : I2
{
    extern int I2.this[int x]
    { set {}} 
}

public interface I3 : I1
{
}

class Test1 : I1
{}
";

            ValidatePropertyImplementationInDerived_09(source1, source2, isStatic: false,
                new DiagnosticDescription[]
                {
                // (9,32): warning CS0626: Method, operator, or accessor 'I1.I2.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int I2.this[int x] {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.this[int].get").WithLocation(9, 32)
                },
                new DiagnosticDescription[]
                {
                // (9,32): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     extern int I2.this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "get").WithArguments("default interface implementation", "8.0").WithLocation(9, 32),
                // (9,32): warning CS0626: Method, operator, or accessor 'I1.I2.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int I2.this[int x] {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.this[int].get").WithLocation(9, 32)
                },
                new DiagnosticDescription[]
                {
                // (9,32): error CS8501: Target runtime doesn't support default interface implementation.
                //     extern int I2.this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "get").WithLocation(9, 32),
                // (9,32): warning CS0626: Method, operator, or accessor 'I1.I2.this[int].get' is marked external and has no attributes on it. Consider adding a DllImport attribute to specify the external implementation.
                //     extern int I2.this[int x] {get;} 
                Diagnostic(ErrorCode.WRN_ExternMethodNoImplementation, "get").WithArguments("I1.I2.this[int].get").WithLocation(9, 32)
                },
                // (10,7): error CS0179: 'I1.I2.this[int].set' cannot be extern and declare a body
                //     { set {}} 
                Diagnostic(ErrorCode.ERR_ExternHasBody, "set").WithArguments("I1.I2.this[int].set").WithLocation(10, 7)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_10()
        {
            var source1 =
@"
public interface I1
{
    int this[int x] {get;set;} 
    int this[long x] {get;set;} 
}

public interface I2 : I1
{
    int I1.this[int x]
    {
        get => throw null;
    }
    int I1.this[long x] 
    {
        set => throw null;
    }
}

class Test1 : I2
{}

public interface I3
{
    int this[int x] {get;} 
    int this[long x] {set;} 
    int this[byte x] {get;set;} 
    int this[short x] {get;set;} 
}

public interface I4 : I3
{
    int I3.this[int x]
    {
        get => throw null;
        set => throw null;
    }
    int I3.this[long x] 
    {
        get => throw null;
        set => throw null;
    }
    int I3.this[byte x] {get; set;}
    int I3.this[short x] {get; set;} = 0;
}

class Test2 : I4
{}
";

            ValidatePropertyImplementationInDerived_05(source1,
                // (44,38): error CS1519: Invalid token '=' in class, record, struct, or interface member declaration
                //     int I3.this[short x] {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_InvalidMemberDecl, "=").WithArguments("=").WithLocation(44, 38),
                // (10,12): error CS0551: Explicit interface implementation 'I2.I1.this[int]' is missing accessor 'I1.this[int].set'
                //     int I1.this[int x]
                Diagnostic(ErrorCode.ERR_ExplicitPropertyMissingAccessor, "this").WithArguments("I2.I1.this[int]", "I1.this[int].set").WithLocation(10, 12),
                // (14,12): error CS0551: Explicit interface implementation 'I2.I1.this[long]' is missing accessor 'I1.this[long].get'
                //     int I1.this[long x] 
                Diagnostic(ErrorCode.ERR_ExplicitPropertyMissingAccessor, "this").WithArguments("I2.I1.this[long]", "I1.this[long].get").WithLocation(14, 12),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.this[long]'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.this[long]").WithLocation(20, 15),
                // (20,15): error CS0535: 'Test1' does not implement interface member 'I1.this[int]'
                // class Test1 : I2
                Diagnostic(ErrorCode.ERR_UnimplementedInterfaceMember, "I2").WithArguments("Test1", "I1.this[int]").WithLocation(20, 15),
                // (36,9): error CS0550: 'I4.I3.this[int].set' adds an accessor not found in interface member 'I3.this[int]'
                //         set => throw null;
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "set").WithArguments("I4.I3.this[int].set", "I3.this[int]").WithLocation(36, 9),
                // (40,9): error CS0550: 'I4.I3.this[long].get' adds an accessor not found in interface member 'I3.this[long]'
                //         get => throw null;
                Diagnostic(ErrorCode.ERR_ExplicitPropertyAddingAccessor, "get").WithArguments("I4.I3.this[long].get", "I3.this[long]").WithLocation(40, 9),
                // (43,26): error CS0501: 'I4.I3.this[byte].get' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.this[byte x] {get; set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I4.I3.this[byte].get").WithLocation(43, 26),
                // (43,31): error CS0501: 'I4.I3.this[byte].set' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.this[byte x] {get; set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I4.I3.this[byte].set").WithLocation(43, 31),
                // (44,27): error CS0501: 'I4.I3.this[short].get' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.this[short x] {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I4.I3.this[short].get").WithLocation(44, 27),
                // (44,32): error CS0501: 'I4.I3.this[short].set' must declare a body because it is not marked abstract, extern, or partial
                //     int I3.this[short x] {get; set;} = 0;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I4.I3.this[short].set").WithLocation(44, 32)
                );
        }

        [Fact]
        [WorkItem(32540, "https://github.com/dotnet/roslyn/issues/32540")]
        public void IndexerImplementationInDerived_11()
        {
            var source1 =
@"
public interface I2
{
    internal int this[int x] {get;} 
}

public interface I4
{
    int this[int x] {get; internal set;} 
}

public interface I5 : I4
{
}

public class TestHelper
{
    public static void Test(I2 i2, I4 i4)
    {
        _ = i2[0];
        i4[0] = i4[0];
    }
}
";
            var source2 =
@"
public interface I1 : I2, I5
{
    int I2.this[int x] => Getter();

    private int Getter()
    {
        System.Console.WriteLine(""I2.M1"");
        return 1;
    }
    int I4.this[int x] 
    {
        get 
        {
            System.Console.WriteLine(""I4.M1"");
            return 1;
        }
        set => System.Console.WriteLine(""I4.M1.set"");
    }
}

public interface I3 : I1
{
}
";

            ValidatePropertyImplementationInDerived_11(source1, source2, isStatic: false,
                // (4,12): error CS0122: 'I2.this[int]' is inaccessible due to its protection level
                //     int I2.this[int x] => Getter();
                Diagnostic(ErrorCode.ERR_BadAccess, "this").WithArguments("I2.this[int]").WithLocation(4, 12),
                // (11,12): error CS0122: 'I4.this[int].set' is inaccessible due to its protection level
                //     int I4.this[int x] 
                Diagnostic(ErrorCode.ERR_BadAccess, "this").WithArguments("I4.this[int].set").WithLocation(11, 12)
                );
        }

        [Fact]
        [WorkItem(20083, "https://github.com/dotnet/roslyn/issues/20083")]
        public void IndexerImplementationInDerived_12()
        {
            var source1 =
@"
public interface I1
{
    int this[int x] { get => throw null; set => throw null;} 
}

public interface I2 : I1
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I2.I1.M1.set"");
        }
    }
}

public interface I3 : I1
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I3.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I3.I1.M1.set"");
        }
    }
}

public interface I4 : I1
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I4.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I4.I1.M1.set"");
        }
    }
}

public interface I5 : I2, I3
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I5.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""I5.I1.M1.set"");
        }
    }
}

public interface I6 : I1, I2, I3, I5
{}
";
            var source4 =
@"
class Test5 : I2
{}

class Test6 : I1, I2, I3, I5
{}

class Test7 : I6
{
    static void Main()
    {
        I1 i1 = new Test5();
        i1[0] = i1[0];
        i1 = new Test6();
        i1[0] = i1[0];
        i1 = new Test7();
        i1[0] = i1[0];
    }
}
";
            var source5 = @"
class Test8 : I2, I3
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""Test8.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test8.I1.M1.set"");
        }
    }
    static void Main()
    {
        I1 i1 = new Test8();
        i1[0] = i1[0];
        i1 = new Test9();
        i1[0] = i1[0];
        i1 = new Test10();
        i1[0] = i1[0];
        i1 = new Test11();
        i1[0] = i1[0];
        i1 = new Test12();
        i1[0] = i1[0];
    }
}

class Test9 : I7
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""Test9.I1.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test9.I1.M1.set"");
        }
    }
}

class Test10 : I1, I2, I3, I4
{
    public int this[int x]
    {
        get
        {
            System.Console.WriteLine(""Test10.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test10.M1.set"");
        }
    }
}

class Test11 : I1, I2, I3, I5, I4
{
    public int this[int x]
    {
        get
        {
            System.Console.WriteLine(""Test11.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test11.M1.set"");
        }
    }
}

class Test12 : I8
{
    public virtual int this[int x]
    {
        get
        {
            System.Console.WriteLine(""Test12.M1.get"");
            return 1;
        }
        set
        {
            System.Console.WriteLine(""Test12.M1.set"");
        }
    }
}
";
            ValidatePropertyImplementationInDerived_12(source1, source4, source5, isStatic: false,
                new DiagnosticDescription[]
                {
                // (2,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.this[int]', nor 'I3.I1.this[int]' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.this[int]", "I2.I1.this[int]", "I3.I1.this[int]").WithLocation(2, 15),
                // (5,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.this[int]', nor 'I3.I1.this[int]' are most specific.
                // class Test2 : I7
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I7").WithArguments("I1.this[int]", "I2.I1.this[int]", "I3.I1.this[int]").WithLocation(5, 15),
                // (8,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.this[int]', nor 'I3.I1.this[int]' are most specific.
                // class Test3 : I1, I2, I3, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.this[int]", "I2.I1.this[int]", "I3.I1.this[int]").WithLocation(8, 15),
                // (11,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I5.I1.this[int]', nor 'I4.I1.this[int]' are most specific.
                // class Test4 : I1, I2, I3, I5, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.this[int]", "I5.I1.this[int]", "I4.I1.this[int]").WithLocation(11, 15),
                // (14,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I5.I1.this[int]', nor 'I4.I1.this[int]' are most specific.
                // class Test5 : I8
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I8").WithArguments("I1.this[int]", "I5.I1.this[int]", "I4.I1.this[int]").WithLocation(14, 15)
                },
                new DiagnosticDescription[]
                {
                // (2,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.Item[int]', nor 'I3.I1.Item[int]' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.this[int]", "I2.I1.Item[int]", "I3.I1.Item[int]").WithLocation(2, 15),
                // (5,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.Item[int]', nor 'I3.I1.Item[int]' are most specific.
                // class Test2 : I7
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I7").WithArguments("I1.this[int]", "I2.I1.Item[int]", "I3.I1.Item[int]").WithLocation(5, 15),
                // (8,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.Item[int]', nor 'I3.I1.Item[int]' are most specific.
                // class Test3 : I1, I2, I3, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.this[int]", "I2.I1.Item[int]", "I3.I1.Item[int]").WithLocation(8, 15),
                // (11,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I5.I1.Item[int]', nor 'I4.I1.Item[int]' are most specific.
                // class Test4 : I1, I2, I3, I5, I4
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I1").WithArguments("I1.this[int]", "I5.I1.Item[int]", "I4.I1.Item[int]").WithLocation(11, 15),
                // (14,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I5.I1.Item[int]', nor 'I4.I1.Item[int]' are most specific.
                // class Test5 : I8
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I8").WithArguments("I1.this[int]", "I5.I1.Item[int]", "I4.I1.Item[int]").WithLocation(14, 15)
                }
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_13()
        {
            var source1 =
@"
public interface I1
{
    int this[int x] {get;} 
}

public interface I2 : I1
{
    int I1.this[int x] => throw null; 
}

public interface I3 : I1
{
    int I1.this[int x] => throw null; 
}
";

            var source2 =
@"
class Test1 : I2, I3
{
    public long this[int x] => 1;
}
";

            ValidatePropertyImplementationInDerived_13(source1, source2,
                new DiagnosticDescription[]
                {
                // (2,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.this[int]', nor 'I3.I1.this[int]' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.this[int]", "I2.I1.this[int]", "I3.I1.this[int]").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.this[int]'. 'Test1.this[int]' cannot implement 'I1.this[int]' because it does not have the matching return type of 'int'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.this[int]", "Test1.this[int]", "int").WithLocation(2, 15)
                },
                new DiagnosticDescription[]
                {
                // (2,15): error CS8505: Interface member 'I1.this[int]' does not have a most specific implementation. Neither 'I2.I1.Item[int]', nor 'I3.I1.Item[int]' are most specific.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_MostSpecificImplementationIsNotFound, "I2").WithArguments("I1.this[int]", "I2.I1.Item[int]", "I3.I1.Item[int]").WithLocation(2, 15),
                // (2,15): error CS0738: 'Test1' does not implement interface member 'I1.this[int]'. 'Test1.this[int]' cannot implement 'I1.this[int]' because it does not have the matching return type of 'int'.
                // class Test1 : I2, I3
                Diagnostic(ErrorCode.ERR_CloseUnimplementedInterfaceMemberWrongReturnType, "I2").WithArguments("Test1", "I1.this[int]", "Test1.this[int]", "int").WithLocation(2, 15)
                }
                );
        }

        [Fact]
        [WorkItem(20084, "https://github.com/dotnet/roslyn/issues/20084")]
        public void IndexerImplementationInDerived_14()
        {
            var source1 =
@"
public interface I1<T>
{
    int this[int x]
    {
        get
        {
            System.Console.WriteLine(""I1.M1.get"");
            return 1;
        }
        set => System.Console.WriteLine(""I1.M1.set"");
    }
}

public interface I2<T> : I1<T>
{
    int I1<T>.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.I1.M1.get"");
            return 1;
        }
        set => System.Console.WriteLine(""I2.I1.M1.set"");
    }
}

public interface I3<T> : I1<T>
{}
";
            var source2 =
@"
class Test1 : I1<int>
{
    static void Main()
    {
        I1<int> i1Int = new Test1();
        i1Int[0] = i1Int[0];
        I1<long> i1Long = new Test2();
        i1Long[0] = i1Long[0];
    }
}
class Test2 : I2<long>
{}
";

            ValidatePropertyImplementationInDerived_14(source1, source2);
        }

        [Fact]
        public void IndexerImplementationInDerived_15()
        {
            var source1 =
@"
public interface I2
{
    int this[int x] {get => throw null; private set => throw null;} 
}

public interface I4
{
    int this[int x] {set => throw null; private get => throw null;} 
}

public interface I5 : I4
{
}

public interface I1 : I2, I5
{
    int I2.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
    int I4.this[int x] 
    {
        set => System.Console.WriteLine(""I4.M1"");
    }
}

public interface I3 : I1
{
}
";
            var source2 =
@"
class Test1 : I1
{
    static void Main()
    {
        I2 i2 = new Test1();
        _ = i2[0];
        I4 i4 = new Test1();
        i4[0] = 0;
    }
}
";
            ValidatePropertyImplementationInDerived_01(source1, source2);
        }

        [Fact]
        public void IndexerImplementationInDerived_16()
        {
            var source1 =
@"
public interface I1
{
    static int this[int x] {get;} 
}

public interface I2 : I1
{
    int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
}

public interface I3 : I1
{
    abstract int I1.this[int x] {get;} 
}
";
            var compilation1 = CreateCompilation(source1,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (4,16): error CS0106: The modifier 'static' is not valid for this item
                //     static int this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(4, 16)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_17()
        {
            var source1 =
@"
public interface I1
{
    static virtual int this[int x] {get => 0;} 
}

public interface I2 : I1
{
    static int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
}

public interface I3 : I1
{
    abstract static int I1.this[int x] {get;} 
}
";
            var compilation1 = CreateCompilation(source1,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (4,24): error CS0106: The modifier 'static' is not valid for this item
                //     static virtual int this[int x] {get => 0;} 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(4, 24),
                // (9,19): error CS0106: The modifier 'static' is not valid for this item
                //     static int I1.this[int x]
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(9, 19),
                // (21,28): error CS0106: The modifier 'static' is not valid for this item
                //     abstract static int I1.this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(21, 28)
                );
        }

        [Fact]
        public void IndexerImplementationInDerived_18()
        {
            var source1 =
@"
public interface I1
{
    int this[int x] {get;} 
}

public interface I2 : I1
{
    static int I1.this[int x]
    {
        get
        {
            System.Console.WriteLine(""I2.M1"");
            return 1;
        }
    }
}

public interface I3 : I1
{
    abstract static int I1.this[int x] {get;} 
}
";
            var compilation1 = CreateCompilation(source1,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Net60);
            compilation1.VerifyDiagnostics(
                // (9,19): error CS0106: The modifier 'static' is not valid for this item
                //     static int I1.this[int x]
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(9, 19),
                // (21,28): error CS0106: The modifier 'static' is not valid for this item
                //     abstract static int I1.this[int x] {get;} 
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "this").WithArguments("static").WithLocation(21, 28)
                );
        }

        [Theory]
        [CombinatorialData]
        public void Field_01(bool testCSharp14)
        {
            var source1 =
@"
public interface I1
{
    int F1;
    protected static int F2;
    protected internal static int F3;
    private protected static int F4;
    int F5 = 5;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: testCSharp14 ? TestOptions.Regular14 : TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation1.VerifyEmitDiagnostics(
                // (4,9): error CS0525: Interfaces cannot contain instance fields
                //     int F1;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F1").WithLocation(4, 9),
                // (5,26): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected static int F2;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "F2").WithLocation(5, 26),
                // (6,35): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal static int F3;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "F3").WithLocation(6, 35),
                // (7,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     private protected static int F4;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "F4").WithLocation(7, 34),
                // (8,9): error CS0525: Interfaces cannot contain instance fields
                //     int F5 = 5;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F5").WithLocation(8, 9)
                );

            validate(compilation1);

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: testCSharp14 ? TestOptions.Regular14 : TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyEmitDiagnostics(
                // (4,9): error CS0525: Interfaces cannot contain instance fields
                //     int F1;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F1").WithLocation(4, 9),
                // (8,9): error CS0525: Interfaces cannot contain instance fields
                //     int F5 = 5;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainFields, "F5").WithLocation(8, 9)
                );

            validate(compilation2);

            static void validate(CSharpCompilation compilation)
            {
                var i1 = compilation.GetTypeByMetadataName("I1");
                var f1 = i1.GetMember<FieldSymbol>("F1");
                var f2 = i1.GetMember<FieldSymbol>("F2");
                var f3 = i1.GetMember<FieldSymbol>("F3");
                var f4 = i1.GetMember<FieldSymbol>("F4");

                Assert.False(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Protected, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.ProtectedOrInternal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.ProtectedAndInternal, f4.DeclaredAccessibility);
            }
        }

        [Fact]
        public void Field_02()
        {
            var source1 =
@"
public interface I1
{
    static int F1;
    public static int F2;
    internal static int F3;
    private static int F4;

    public class TestHelper
    {
        public static int F4Proxy
        {
            get => I1.F4;
            set => I1.F4 = value;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.F1 = 1;
        I1.F2 = 2;
        I1.F3 = 3;
        I1.TestHelper.F4Proxy = 4;
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}{I1.TestHelper.F4Proxy}"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<FieldSymbol>("F1");
                var f2 = i1.GetMember<FieldSymbol>("F2");
                var f3 = i1.GetMember<FieldSymbol>("F3");
                var f4 = i1.GetMember<FieldSymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Null(cctor);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1.F1 = 11;
        I1.F2 = 22;
        I1.TestHelper.F4Proxy = 44;
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.TestHelper.F4Proxy}"");
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "112244");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "112244");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,16): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static int F1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F1").WithArguments("default interface implementation", "8.0").WithLocation(4, 16),
                // (5,23): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static int F2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F2").WithArguments("default interface implementation", "8.0").WithLocation(5, 23),
                // (6,25): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     internal static int F3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F3").WithArguments("default interface implementation", "8.0").WithLocation(6, 25),
                // (7,24): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     private static int F4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F4").WithArguments("default interface implementation", "8.0").WithLocation(7, 24),
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,16): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int F1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 16),
                // (5,23): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static int F2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F2").WithArguments("static members in interfaces", "15.0").WithLocation(5, 23),
                // (6,25): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int F3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F3").WithArguments("static members in interfaces", "15.0").WithLocation(6, 25),
                // (7,24): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static int F4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F4").WithArguments("static members in interfaces", "15.0").WithLocation(7, 24)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation2 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: "112244");

            compilation3 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation3, expectedOutput: "112244");
        }

        [Fact]
        public void Field_03()
        {
            var source1 =
@"
public interface I1
{
    static readonly int F1 = 1;
    public static readonly int F2 = 2;
    internal static readonly int F3 = 3;
    private static readonly int F4 = 4;

    public class TestHelper
    {
        public static int F4Proxy
        {
            get => I1.F4;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}{I1.TestHelper.F4Proxy}"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<FieldSymbol>("F1");
                var f2 = i1.GetMember<FieldSymbol>("F2");
                var f3 = i1.GetMember<FieldSymbol>("F3");
                var f4 = i1.GetMember<FieldSymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.True(f1.IsReadOnly);
                Assert.True(f2.IsReadOnly);
                Assert.True(f3.IsReadOnly);
                Assert.True(f4.IsReadOnly);

                Assert.False(f1.IsConst);
                Assert.False(f2.IsConst);
                Assert.False(f3.IsConst);
                Assert.False(f4.IsConst);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Equal(MethodKind.StaticConstructor, cctor.MethodKind);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.TestHelper.F4Proxy}"");
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "124");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "124");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,25): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static readonly int F1 = 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F1").WithArguments("default interface implementation", "8.0").WithLocation(4, 25),
                // (5,32): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static readonly int F2 = 2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F2").WithArguments("default interface implementation", "8.0").WithLocation(5, 32),
                // (6,34): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     internal static readonly int F3 = 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F3").WithArguments("default interface implementation", "8.0").WithLocation(6, 34),
                // (7,33): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     private static readonly int F4 = 4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F4").WithArguments("default interface implementation", "8.0").WithLocation(7, 33),
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,25): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static readonly int F1 = 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 25),
                // (5,32): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static readonly int F2 = 2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F2").WithArguments("static members in interfaces", "15.0").WithLocation(5, 32),
                // (6,34): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static readonly int F3 = 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F3").WithArguments("static members in interfaces", "15.0").WithLocation(6, 34),
                // (7,33): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static readonly int F4 = 4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F4").WithArguments("static members in interfaces", "15.0").WithLocation(7, 33)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation2 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: "124");

            compilation3 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation3, expectedOutput: "124");
        }

        [Fact]
        public void Field_04()
        {
            var source1 =
@"
public interface I1
{
    const int F1 = 1;
    public const int F2 = 2;
    internal const int F3 = 3;
    private const int F4 = 4;

    public class TestHelper
    {
        public static int F4Proxy
        {
            get => I1.F4;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}{I1.TestHelper.F4Proxy}"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<FieldSymbol>("F1");
                var f2 = i1.GetMember<FieldSymbol>("F2");
                var f3 = i1.GetMember<FieldSymbol>("F3");
                var f4 = i1.GetMember<FieldSymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.True(f1.IsConst);
                Assert.True(f2.IsConst);
                Assert.True(f3.IsConst);
                Assert.True(f4.IsConst);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Null(cctor);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.TestHelper.F4Proxy}"");
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "124");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "124");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,15): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     const int F1 = 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F1").WithArguments("default interface implementation", "8.0").WithLocation(4, 15),
                // (5,22): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public const int F2 = 2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F2").WithArguments("default interface implementation", "8.0").WithLocation(5, 22),
                // (6,24): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     internal const int F3 = 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F3").WithArguments("default interface implementation", "8.0").WithLocation(6, 24),
                // (7,23): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     private const int F4 = 4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F4").WithArguments("default interface implementation", "8.0").WithLocation(7, 23),
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,15): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     const int F1 = 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 15),
                // (5,22): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public const int F2 = 2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F2").WithArguments("static members in interfaces", "15.0").WithLocation(5, 22),
                // (6,24): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal const int F3 = 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F3").WithArguments("static members in interfaces", "15.0").WithLocation(6, 24),
                // (7,23): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private const int F4 = 4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F4").WithArguments("static members in interfaces", "15.0").WithLocation(7, 23)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation2 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: "124");

            compilation3 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation3, expectedOutput: "124");
        }

        [Fact]
        public void Field_05()
        {
            var source0 =
@"
public interface I1
{
    static protected int F1;
    static protected internal int F2;
    static private protected int F3;
}
";

            var source1 =
@"

class Test1 : I1
{
    static void Main()
    {
        I1.F1 = 1;
        I1.F2 = 2;
        I1.F3 = 3;
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}"");
        Test2.Test();
    }
}

class Test2
{
    public static void Test()
    {
        I1.F2 = -2;
        System.Console.WriteLine(I1.F2);
    }
}
";
            var compilation1 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            validate(compilation1.SourceModule);

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"123
-2
",
                verify: VerifyOnMonoOrCoreClr_FailsIlVerify, symbolValidator: validate);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1.F1 = 11;
        I1.F2 = 22;
        System.Console.WriteLine($""{I1.F1}{I1.F2}"");
    }
}
";
            var references = new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() };

            foreach (var reference in references)
            {
                var compilation2 = CreateCompilation(source2, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                CompileAndVerify(compilation2, expectedOutput: ExecutionConditionUtil.IsMonoOrCoreClr ? "1122" : null, verify: VerifyOnMonoOrCoreClr_FailsIlVerify);
            }

            var source3 =
@"
class Test1
{
    static void Main()
    {
        I1.F1 = 1;
        I1.F2 = 2;
        I1.F3 = 3;
    }
}
";

            var compilation3 = CreateCompilation(source0 + source3, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics(
                // (13,12): error CS0122: 'I1.F1' is inaccessible due to its protection level
                //         I1.F1 = 1;
                Diagnostic(ErrorCode.ERR_BadAccess, "F1").WithArguments("I1.F1").WithLocation(13, 12),
                // (15,12): error CS0122: 'I1.F3' is inaccessible due to its protection level
                //         I1.F3 = 3;
                Diagnostic(ErrorCode.ERR_BadAccess, "F3").WithArguments("I1.F3").WithLocation(15, 12)
                );

            var source4 =
@"
class Test2 : I1
{
    static void Test()
    {
        I1.F3 = -3;
    }
}
";

            foreach (var reference in references)
            {
                var compilation2 = CreateCompilation(source3 + source4, new[] { reference }, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.NetCoreApp);

                compilation2.VerifyDiagnostics(
                    // (6,12): error CS0122: 'I1.F1' is inaccessible due to its protection level
                    //         I1.F1 = 1;
                    Diagnostic(ErrorCode.ERR_BadAccess, "F1").WithArguments("I1.F1").WithLocation(6, 12),
                    // (7,12): error CS0122: 'I1.F2' is inaccessible due to its protection level
                    //         I1.F2 = 2;
                    Diagnostic(ErrorCode.ERR_BadAccess, "F2").WithArguments("I1.F2").WithLocation(7, 12),
                    // (8,12): error CS0122: 'I1.F3' is inaccessible due to its protection level
                    //         I1.F3 = 3;
                    Diagnostic(ErrorCode.ERR_BadAccess, "F3").WithArguments("I1.F3").WithLocation(8, 12),
                    // (16,12): error CS0122: 'I1.F3' is inaccessible due to its protection level
                    //         I1.F3 = -3;
                    Diagnostic(ErrorCode.ERR_BadAccess, "F3").WithArguments("I1.F3").WithLocation(16, 12)
                    );
            }

            var compilation4 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,26): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static protected int F1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F1").WithArguments("default interface implementation", "8.0").WithLocation(4, 26),
                // (5,35): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static protected internal int F2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F2").WithArguments("default interface implementation", "8.0").WithLocation(5, 35),
                // (6,34): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static private protected int F3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "F3").WithArguments("default interface implementation", "8.0").WithLocation(6, 34)
                );

            validate(compilation4.SourceModule);

            void validate(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<FieldSymbol>("F1");
                var f2 = i1.GetMember<FieldSymbol>("F2");
                var f3 = i1.GetMember<FieldSymbol>("F3");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);

                Assert.Equal(Accessibility.Protected, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.ProtectedOrInternal, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.ProtectedAndInternal, f3.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Null(cctor);
            }
        }

        [Fact]
        public void Constructors_01()
        {
            var source1 =
@"
public interface I1
{
    I1(){}

    static I1() {}
}
interface I2
{
    I2();
}

";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,5): error CS0526: Interfaces cannot contain instance constructors
                //     I1(){}
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I1").WithLocation(4, 5),
                // (10,5): error CS0501: 'I2.I2()' must declare a body because it is not marked abstract, extern, or partial
                //     I2();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I2").WithArguments("I2.I2()").WithLocation(10, 5),
                // (10,5): error CS0526: Interfaces cannot contain instance constructors
                //     I2();
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I2").WithLocation(10, 5)
                );
        }

        [Fact]
        public void Constructors_02()
        {
            var source1 =
@"
interface I1
{
    static I1() {}
}
interface I2
{
    static I2() => throw null;
}
interface I3
{
    I3() {}
}
interface I4
{
    I4() => throw null;
}
interface I5
{
    I5();
}
interface I6
{
    extern static I6();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,12): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static I1() {}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "I1").WithArguments("default interface implementation", "8.0").WithLocation(4, 12),
                // (8,12): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     static I2() => throw null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "I2").WithArguments("default interface implementation", "8.0").WithLocation(8, 12),
                // (12,5): error CS0526: Interfaces cannot contain instance constructors
                //     I3() {}
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I3").WithLocation(12, 5),
                // (16,5): error CS0526: Interfaces cannot contain instance constructors
                //     I4() => throw null;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I4").WithLocation(16, 5),
                // (20,5): error CS0501: 'I5.I5()' must declare a body because it is not marked abstract, extern, or partial
                //     I5();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I5").WithArguments("I5.I5()").WithLocation(20, 5),
                // (20,5): error CS0526: Interfaces cannot contain instance constructors
                //     I5();
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I5").WithLocation(20, 5),
                // (24,19): error CS8703: The modifier 'extern' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     extern static I6();
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "I6").WithArguments("extern", "7.3", "8.0").WithLocation(24, 19),
                // (24,19): warning CS0824: Constructor 'I6.I6()' is marked external
                //     extern static I6();
                Diagnostic(ErrorCode.WRN_ExternCtorNoImplementation, "I6").WithArguments("I6.I6()").WithLocation(24, 19)
                );

            var compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                  parseOptions: TestOptions.Regular14,
                                                  targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(
                // (4,12): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static I1() {}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "I1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 12),
                // (8,12): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static I2() => throw null;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "I2").WithArguments("static members in interfaces", "15.0").WithLocation(8, 12),
                // (12,5): error CS0526: Interfaces cannot contain instance constructors
                //     I3() {}
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I3").WithLocation(12, 5),
                // (16,5): error CS0526: Interfaces cannot contain instance constructors
                //     I4() => throw null;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I4").WithLocation(16, 5),
                // (20,5): error CS0501: 'I5.I5()' must declare a body because it is not marked abstract, extern, or partial
                //     I5();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I5").WithArguments("I5.I5()").WithLocation(20, 5),
                // (20,5): error CS0526: Interfaces cannot contain instance constructors
                //     I5();
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I5").WithLocation(20, 5),
                // (24,19): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     extern static I6();
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "I6").WithArguments("static members in interfaces", "15.0").WithLocation(24, 19),
                // (24,19): warning CS0824: Constructor 'I6.I6()' is marked external
                //     extern static I6();
                Diagnostic(ErrorCode.WRN_ExternCtorNoImplementation, "I6").WithArguments("I6.I6()").WithLocation(24, 19)
                );

            var expected = new[]
            {
                // (12,5): error CS0526: Interfaces cannot contain instance constructors
                //     I3() {}
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I3").WithLocation(12, 5),
                // (16,5): error CS0526: Interfaces cannot contain instance constructors
                //     I4() => throw null;
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I4").WithLocation(16, 5),
                // (20,5): error CS0501: 'I5.I5()' must declare a body because it is not marked abstract, extern, or partial
                //     I5();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I5").WithArguments("I5.I5()").WithLocation(20, 5),
                // (20,5): error CS0526: Interfaces cannot contain instance constructors
                //     I5();
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConstructors, "I5").WithLocation(20, 5),
                // (24,19): warning CS0824: Constructor 'I6.I6()' is marked external
                //     extern static I6();
                Diagnostic(ErrorCode.WRN_ExternCtorNoImplementation, "I6").WithArguments("I6.I6()").WithLocation(24, 19)
            };

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(expected);

            compilation2 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            compilation2.VerifyDiagnostics(expected);
        }

        [Fact]
        public void Constructors_03()
        {
            var source1 =
@"
interface I1
{
    static I1(int i) {}
}
interface I2
{
    static void I2() {}
}
interface I3
{
    void I3() {}
}
interface I4
{
    public static I4() {}
}
interface I5
{
    internal static I5() {}
}
interface I6
{
    private static I6() {}
}
interface I7
{
    static I7();
}
interface I8
{
    static I8(){}

    static void M1()
    {
        I8();
    }

    void M2()
    {
        I8.I8();
    }
}
interface I9
{
    static I9() {} => throw null;
}
interface I10
{
    abstract static I10();
}
interface I11
{
    virtual static I11();
}
interface I12
{
    abstract static I12() {}
}
interface I13
{
    virtual static I13() => throw null;
}
interface I14
{
    partial static I14();
}
interface I15
{
    static partial I15();
}
interface I16
{
    partial static I16() {}
}
interface I17
{
    static partial I17() => throw null;
}
interface I18
{
    extern static I18() {}
}
interface I19
{
    static extern I19() => throw null;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,12): error CS0132: 'I1.I1(int)': a static constructor must be parameterless
                //     static I1(int i) {}
                Diagnostic(ErrorCode.ERR_StaticConstParam, "I1").WithArguments("I1.I1(int)").WithLocation(4, 12),
                // (8,17): error CS0542: 'I2': member names cannot be the same as their enclosing type
                //     static void I2() {}
                Diagnostic(ErrorCode.ERR_MemberNameSameAsType, "I2").WithArguments("I2").WithLocation(8, 17),
                // (16,19): error CS0515: 'I4.I4()': access modifiers are not allowed on static constructors
                //     public static I4() {}
                Diagnostic(ErrorCode.ERR_StaticConstructorWithAccessModifiers, "I4").WithArguments("I4.I4()").WithLocation(16, 19),
                // (20,21): error CS0515: 'I5.I5()': access modifiers are not allowed on static constructors
                //     internal static I5() {}
                Diagnostic(ErrorCode.ERR_StaticConstructorWithAccessModifiers, "I5").WithArguments("I5.I5()").WithLocation(20, 21),
                // (24,20): error CS0515: 'I6.I6()': access modifiers are not allowed on static constructors
                //     private static I6() {}
                Diagnostic(ErrorCode.ERR_StaticConstructorWithAccessModifiers, "I6").WithArguments("I6.I6()").WithLocation(24, 20),
                // (28,12): error CS0501: 'I7.I7()' must declare a body because it is not marked abstract, extern, or partial
                //     static I7();
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "I7").WithArguments("I7.I7()").WithLocation(28, 12),
                // (36,9): error CS1955: Non-invocable member 'I8' cannot be used like a method.
                //         I8();
                Diagnostic(ErrorCode.ERR_NonInvocableMemberCalled, "I8").WithArguments("I8").WithLocation(36, 9),
                // (41,12): error CS0117: 'I8' does not contain a definition for 'I8'
                //         I8.I8();
                Diagnostic(ErrorCode.ERR_NoSuchMember, "I8").WithArguments("I8", "I8").WithLocation(41, 12),
                // (46,5): error CS8057: Block bodies and expression bodies cannot both be provided.
                //     static I9() {} => throw null;
                Diagnostic(ErrorCode.ERR_BlockBodyAndExpressionBody, "static I9() {} => throw null;").WithLocation(46, 5),
                // (50,21): error CS0106: The modifier 'abstract' is not valid for this item
                //     abstract static I10();
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "I10").WithArguments("abstract").WithLocation(50, 21),
                // (54,20): error CS0106: The modifier 'virtual' is not valid for this item
                //     virtual static I11();
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "I11").WithArguments("virtual").WithLocation(54, 20),
                // (58,21): error CS0106: The modifier 'abstract' is not valid for this item
                //     abstract static I12() {}
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "I12").WithArguments("abstract").WithLocation(58, 21),
                // (62,20): error CS0106: The modifier 'virtual' is not valid for this item
                //     virtual static I13() => throw null;
                Diagnostic(ErrorCode.ERR_BadMemberFlag, "I13").WithArguments("virtual").WithLocation(62, 20),
                // (66,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', 'event', an instance constructor name, or a method or property return type.
                //     partial static I14();
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(66, 5),
                // (66,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', 'event', an instance constructor name, or a method or property return type.
                //     partial static I14();
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(66, 5),
                // (70,12): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', 'event', an instance constructor name, or a method or property return type.
                //     static partial I15();
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(70, 12),
                // (74,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', 'event', an instance constructor name, or a method or property return type.
                //     partial static I16() {}
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(74, 5),
                // (74,5): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', 'event', an instance constructor name, or a method or property return type.
                //     partial static I16() {}
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(74, 5),
                // (78,12): error CS0267: The 'partial' modifier can only appear immediately before 'class', 'record', 'struct', 'interface', 'event', an instance constructor name, or a method or property return type.
                //     static partial I17() => throw null;
                Diagnostic(ErrorCode.ERR_PartialMisplaced, "partial").WithLocation(78, 12),
                // (82,19): error CS0179: 'I18.I18()' cannot be extern and declare a body
                //     extern static I18() {}
                Diagnostic(ErrorCode.ERR_ExternHasBody, "I18").WithArguments("I18.I18()").WithLocation(82, 19),
                // (86,19): error CS0179: 'I19.I19()' cannot be extern and declare a body
                //     static extern I19() => throw null;
                Diagnostic(ErrorCode.ERR_ExternHasBody, "I19").WithArguments("I19.I19()").WithLocation(86, 19)
                );
        }

        [Fact]
        public void Constructors_04()
        {
            var source1 =
@"
interface I1
{
    static I1()
    {
        System.Console.WriteLine(""I1"");
    }

    static void Main() 
    {
        System.Console.WriteLine(""Main"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.RegularPreview);

            compilation1.VerifyDiagnostics();

            ValidateConstructor(compilation1.SourceModule);
            Assert.Empty(compilation1.GetTypeByMetadataName("I1").GetMembers("I1"));

            CompileAndVerify(compilation1, symbolValidator: ValidateConstructor, expectedOutput:
@"I1
Main
");
        }

        private static void ValidateConstructor(ModuleSymbol m)
        {
            var i1 = m.GlobalNamespace.GetTypeMember("I1");
            var cctor = i1.GetMember<MethodSymbol>(".cctor");

            Assert.False(cctor.IsAbstract);
            Assert.False(cctor.IsVirtual);
            Assert.False(cctor.IsMetadataVirtual());
            Assert.False(cctor.IsSealed);
            Assert.True(cctor.IsStatic);
            Assert.False(cctor.IsExtern);
            Assert.False(cctor.IsAsync);
            Assert.False(cctor.IsOverride);
            Assert.Equal(Accessibility.Private, cctor.DeclaredAccessibility);
            Assert.Equal(MethodKind.StaticConstructor, cctor.MethodKind);
        }

        [Fact]
        public void Constructors_05()
        {
            var source1 =
@"
interface I1
{
    static string F = ""F"";

    static void Main() 
    {
        System.Console.WriteLine(F);
        System.Console.WriteLine(""Main"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            ValidateConstructor(compilation1.SourceModule);

            CompileAndVerify(compilation1, symbolValidator: ValidateConstructor, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"F
Main
");
        }

        [Fact]
        public void Constructors_06()
        {
            var source1 =
@"
interface I1
{
    static string F = ""F"";

    static I1()
    {
        System.Console.WriteLine(F);
        System.Console.WriteLine(""I1"");
    }

    static void Main() 
    {
        System.Console.WriteLine(""Main"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            ValidateConstructor(compilation1.SourceModule);

            CompileAndVerify(compilation1, symbolValidator: ValidateConstructor, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"F
I1
Main
");
        }

        [Fact]
        public void Constructors_07()
        {
            var source1 =
@"
interface I1
{
    extern static I1();
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(
                // (4,19): warning CS0824: Constructor 'I1.I1()' is marked external
                //     extern static I1();
                Diagnostic(ErrorCode.WRN_ExternCtorNoImplementation, "I1").WithArguments("I1.I1()").WithLocation(4, 19)
                );

            var i1 = compilation1.SourceModule.GlobalNamespace.GetTypeMember("I1");
            var cctor = i1.GetMember<MethodSymbol>(".cctor");

            Assert.False(cctor.IsAbstract);
            Assert.False(cctor.IsVirtual);
            Assert.False(cctor.IsMetadataVirtual());
            Assert.False(cctor.IsSealed);
            Assert.True(cctor.IsStatic);
            Assert.True(cctor.IsExtern);
            Assert.False(cctor.IsAsync);
            Assert.False(cctor.IsOverride);
            Assert.Equal(Accessibility.Private, cctor.DeclaredAccessibility);
            Assert.Equal(MethodKind.StaticConstructor, cctor.MethodKind);

            CompileAndVerify(compilation1, symbolValidator: ValidateConstructor, verify: Verification.Skipped);
        }

        [Fact]
        public void Constructors_08()
        {
            var source1 =
@"
interface I1
{
    void I1();
}

class Test : I1
{
    void I1.I1()
    {
        System.Console.WriteLine(""Test.I1"");
    }

    static void Main() 
    {
        I1 x = new Test();
        x.I1();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: "Test.I1");
        }

        [Fact]
        public void Constructors_09()
        {
            var source1 =
@"
interface I1
{
    void I1();
    static I1()
    {
        System.Console.WriteLine(""I1..cctor"");
    }
    static void M1()
    {
        System.Console.WriteLine(""I1.M1"");
    }
}

class Test : I1
{
    void I1.I1()
    {
        System.Console.WriteLine(""Test.I1"");
    }

    static void Main() 
    {
        I1.M1();
        I1 x = new Test();
        x.I1();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"I1..cctor
I1.M1
Test.I1
");
        }

        [Fact]
        public void Constructors_10()
        {
            var source1 =
@"
interface I1
{
    void I1()
    {
        System.Console.WriteLine(""I1.I1"");
    }

    static I1()
    {
        System.Console.WriteLine(""I1..cctor"");
    }
    static void M1()
    {
        System.Console.WriteLine(""I1.M1"");
    }
}

class Test : I1
{
    static void Main() 
    {
        I1.M1();
        I1 x = new Test();
        x.I1();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"I1..cctor
I1.M1
I1.I1
",
                verify: VerifyOnMonoOrCoreClr);
        }

        [Fact]
        public void Constructors_11()
        {
            var source1 =
@"
interface I1
{
    void I1()
    {
        System.Console.WriteLine(""I1.I1"");
    }
}

class Test : I1
{
    static void Main() 
    {
        I1 x = new Test();
        x.I1();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "I1.I1", verify: VerifyOnMonoOrCoreClr);
        }

        /// <summary>
        /// Make sure runtime handles cycles in static constructors.
        /// </summary>
        [Fact]
        public void Constructors_12()
        {
            var source0 =
@"
interface I1
{
    public static int F1;
    static I1()
    {
        F1 = I2.F2;
    }
}

interface I2
{
    public static int F2;
    static I2()
    {
        F2 = I1.F1 + 1;
    }
}
";
            var source1 =
@"
class Test
{
    static void Main() 
    {
        System.Console.WriteLine(I1.F1 + I2.F2);
    }
}
";
            var compilation1 = CreateCompilation(source0 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "2");

            var source2 =
@"
class Test
{
    static void Main() 
    {
        System.Console.WriteLine(I2.F2 + I1.F1);
    }
}
";
            var compilation2 = CreateCompilation(source0 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1");
        }

        [Fact]
        public void AutoProperty_01()
        {
            var source1 =
@"
public interface I1
{
    private int F1 {get; set;}
    private int F5 {get;} = 5;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyEmitDiagnostics(
                // (4,21): error CS0501: 'I1.F1.get' must declare a body because it is not marked abstract, extern, or partial
                //     private int F1 {get; set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.F1.get").WithLocation(4, 21),
                // (4,26): error CS0501: 'I1.F1.set' must declare a body because it is not marked abstract, extern, or partial
                //     private int F1 {get; set;}
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "set").WithArguments("I1.F1.set").WithLocation(4, 26),
                // (5,17): error CS8053: Instance properties in interfaces cannot have initializers.
                //     private int F5 {get;} = 5;
                Diagnostic(ErrorCode.ERR_InstancePropertyInitializerInInterface, "F5").WithLocation(5, 17),
                // (5,21): error CS0501: 'I1.F5.get' must declare a body because it is not marked abstract, extern, or partial
                //     private int F5 {get;} = 5;
                Diagnostic(ErrorCode.ERR_ConcreteMissingBody, "get").WithArguments("I1.F5.get").WithLocation(5, 21)
                );
        }

        [Fact]
        public void AutoProperty_02()
        {
            var source1 =
@"
public interface I1
{
    static int F1 {get; set;}
    public static int F2 {get; set;}
    internal static int F3 {get; set;}
    private static int F4 {get; set;}

    public class TestHelper
    {
        public static int F4Proxy
        {
            get => I1.F4;
            set => I1.F4 = value;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.F1 = 1;
        I1.F2 = 2;
        I1.F3 = 3;
        I1.TestHelper.F4Proxy = 4;
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}{I1.TestHelper.F4Proxy}"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<PropertySymbol>("F1");
                var f2 = i1.GetMember<PropertySymbol>("F2");
                var f3 = i1.GetMember<PropertySymbol>("F3");
                var f4 = i1.GetMember<PropertySymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Null(cctor);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1.F1 = 11;
        I1.F2 = 22;
        I1.TestHelper.F4Proxy = 44;
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.TestHelper.F4Proxy}"");
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "112244");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "112244");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,16): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static int F1 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F1").WithArguments("static", "7.3", "8.0").WithLocation(4, 16),
                // (5,23): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("static", "7.3", "8.0").WithLocation(5, 23),
                // (5,23): error CS8703: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("public", "7.3", "8.0").WithLocation(5, 23),
                // (6,25): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("static", "7.3", "8.0").WithLocation(6, 25),
                // (6,25): error CS8703: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("internal", "7.3", "8.0").WithLocation(6, 25),
                // (7,24): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static int F4 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("static", "7.3", "8.0").WithLocation(7, 24),
                // (7,24): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static int F4 {get; set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("private", "7.3", "8.0").WithLocation(7, 24),
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            compilation5.VerifyDiagnostics(
                // (4,20): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int F1 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(4, 20),
                // (4,25): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int F1 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(4, 25),
                // (5,27): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static int F2 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(5, 27),
                // (5,32): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static int F2 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(5, 32),
                // (6,29): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int F3 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(6, 29),
                // (6,34): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int F3 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(6, 34),
                // (7,28): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static int F4 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(7, 28),
                // (7,33): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static int F4 {get; set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(7, 33)
                );

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            var compilation6 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation6, expectedOutput: "112244");

            var compilation7 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation7, expectedOutput: "112244");
        }

        [Fact]
        public void AutoProperty_03()
        {
            var source1 =
@"
public interface I1
{
    static int F1 {get;} = 1;
    public static int F2 {get;} = 2;
    internal static int F3 {get;} = 3;
    private static int F4 {get;} = 4;

    public class TestHelper
    {
        public static int F4Proxy
        {
            get => I1.F4;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}{I1.TestHelper.F4Proxy}"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<PropertySymbol>("F1");
                var f2 = i1.GetMember<PropertySymbol>("F2");
                var f3 = i1.GetMember<PropertySymbol>("F3");
                var f4 = i1.GetMember<PropertySymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.True(f1.IsReadOnly);
                Assert.True(f2.IsReadOnly);
                Assert.True(f3.IsReadOnly);
                Assert.True(f4.IsReadOnly);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Equal(MethodKind.StaticConstructor, cctor.MethodKind);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.TestHelper.F4Proxy}"");
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "124");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "124");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,16): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static int F1 {get;} = 1;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F1").WithArguments("static", "7.3", "8.0").WithLocation(4, 16),
                // (5,23): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get;} = 2;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("static", "7.3", "8.0").WithLocation(5, 23),
                // (5,23): error CS8703: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get;} = 2;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("public", "7.3", "8.0").WithLocation(5, 23),
                // (6,25): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get;} = 3;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("static", "7.3", "8.0").WithLocation(6, 25),
                // (6,25): error CS8703: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get;} = 3;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("internal", "7.3", "8.0").WithLocation(6, 25),
                // (7,24): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static int F4 {get;} = 4;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("static", "7.3", "8.0").WithLocation(7, 24),
                // (7,24): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static int F4 {get;} = 4;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("private", "7.3", "8.0").WithLocation(7, 24),
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,20): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int F1 {get;} = 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(4, 20),
                // (5,27): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static int F2 {get;} = 2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(5, 27),
                // (6,29): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int F3 {get;} = 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(6, 29),
                // (7,28): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static int F4 {get;} = 4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(7, 28)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            var compilation6 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation6, expectedOutput: "124");

            var compilation7 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation7, expectedOutput: "124");
        }

        [Fact]
        public void AutoProperty_04()
        {
            var source1 =
@"
public interface I1
{
    static int F1 {get; private set;}
    public static int F2 {get; private set;}
    internal static int F3 {get; private set;}

    public class TestHelper
    {
        public static void F4Proxy(int f1, int f2, int f3)
        {
            F1 = f1;
            F2 = f2;
            F3 = f3;
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.TestHelper.F4Proxy(1,2,3);
        System.Console.WriteLine($""{I1.F1}{I1.F2}{I1.F3}"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);
            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<PropertySymbol>("F1");
                var f2 = i1.GetMember<PropertySymbol>("F2");
                var f3 = i1.GetMember<PropertySymbol>("F3");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);

                Assert.Equal(Accessibility.Private, f1.SetMethod.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f2.SetMethod.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f3.SetMethod.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Null(cctor);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "123", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1.TestHelper.F4Proxy(11,22,3);
        System.Console.WriteLine($""{I1.F1}{I1.F2}"");
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1122");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1122");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (4,16): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static int F1 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F1").WithArguments("static", "7.3", "8.0").WithLocation(4, 16),
                // (4,33): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static int F1 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("private", "7.3", "8.0").WithLocation(4, 33),
                // (5,23): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("static", "7.3", "8.0").WithLocation(5, 23),
                // (5,23): error CS8703: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("public", "7.3", "8.0").WithLocation(5, 23),
                // (5,40): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static int F2 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("private", "7.3", "8.0").WithLocation(5, 40),
                // (6,25): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("static", "7.3", "8.0").WithLocation(6, 25),
                // (6,25): error CS8703: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("internal", "7.3", "8.0").WithLocation(6, 25),
                // (6,42): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static int F3 {get; private set;}
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "set").WithArguments("private", "7.3", "8.0").WithLocation(6, 42),
                // (8,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(8, 18)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,20): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int F1 {get; private set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(4, 20),
                // (4,33): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static int F1 {get; private set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(4, 33),
                // (5,27): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static int F2 {get; private set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(5, 27),
                // (5,40): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static int F2 {get; private set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(5, 40),
                // (6,29): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int F3 {get; private set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "get").WithArguments("static members in interfaces", "15.0").WithLocation(6, 29),
                // (6,42): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static int F3 {get; private set;}
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "set").WithArguments("static members in interfaces", "15.0").WithLocation(6, 42)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "123", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "123", symbolValidator: Validate1);

            var compilation6 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation6, expectedOutput: "1122");

            var compilation7 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation7, expectedOutput: "1122");
        }

        [Fact]
        public void FieldLikeEvent_01()
        {
            var source1 =
@"
public interface I1
{
    private event System.Action F1;
    private event System.Action F5 = null;
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyEmitDiagnostics(
                // (4,33): error CS0065: 'I1.F1': event property must have both add and remove accessors
                //     private event System.Action F1;
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "F1").WithArguments("I1.F1").WithLocation(4, 33),
                // (5,33): error CS0068: 'I1.F5': instance event in interface cannot have initializer
                //     private event System.Action F5 = null;
                Diagnostic(ErrorCode.ERR_InterfaceEventInitializer, "F5").WithArguments("I1.F5").WithLocation(5, 33),
                // (5,33): error CS0065: 'I1.F5': event property must have both add and remove accessors
                //     private event System.Action F5 = null;
                Diagnostic(ErrorCode.ERR_EventNeedsBothAccessors, "F5").WithArguments("I1.F5").WithLocation(5, 33),
                // (5,33): warning CS0067: The event 'I1.F5' is never used
                //     private event System.Action F5 = null;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "F5").WithArguments("I1.F5").WithLocation(5, 33),
                // (4,33): warning CS0067: The event 'I1.F1' is never used
                //     private event System.Action F1;
                Diagnostic(ErrorCode.WRN_UnreferencedEvent, "F1").WithArguments("I1.F1").WithLocation(4, 33)
                );
        }

        [Fact]
        public void FieldLikeEvent_02()
        {
            var source1 =
@"
public interface I1
{
    static event System.Action F1;
    public static event System.Action F2;
    internal static event System.Action F3;
    private static event System.Action F4;

    public class TestHelper
    {
        public static System.Action F4Proxy
        {
            set => I1.F4 += value;
        }
        public static void Raise()
        {
            F1();
            F2();
            F3?.Invoke();
            F4();
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.F1 += () => System.Console.Write(1);
        I1.F2 += () => System.Console.Write(2);
        I1.F3 += () => System.Console.Write(3);
        I1.TestHelper.F4Proxy = () => System.Console.Write(4);
        I1.TestHelper.Raise();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<EventSymbol>("F1");
                var f2 = i1.GetMember<EventSymbol>("F2");
                var f3 = i1.GetMember<EventSymbol>("F3");
                var f4 = i1.GetMember<EventSymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Null(cctor);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1.F1 += () => System.Console.Write(11);
        I1.F2 += () => System.Console.Write(22);
        I1.TestHelper.F4Proxy = () => System.Console.Write(44);
        I1.TestHelper.Raise();
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "112244");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "112244");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18),
                // (4,32): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static event System.Action F1;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F1").WithArguments("static", "7.3", "8.0").WithLocation(4, 32),
                // (5,39): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static event System.Action F2;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("static", "7.3", "8.0").WithLocation(5, 39),
                // (5,39): error CS8503: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static event System.Action F2;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("public", "7.3", "8.0").WithLocation(5, 39),
                // (6,41): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static event System.Action F3;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("static", "7.3", "8.0").WithLocation(6, 41),
                // (6,41): error CS8503: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static event System.Action F3;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("internal", "7.3", "8.0").WithLocation(6, 41),
                // (7,40): error CS8503: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static event System.Action F4;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("static", "7.3", "8.0").WithLocation(7, 40),
                // (7,40): error CS8503: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static event System.Action F4;
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("private", "7.3", "8.0").WithLocation(7, 40)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,32): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static event System.Action F1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 32),
                // (5,39): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static event System.Action F2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F2").WithArguments("static members in interfaces", "15.0").WithLocation(5, 39),
                // (6,41): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static event System.Action F3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F3").WithArguments("static members in interfaces", "15.0").WithLocation(6, 41),
                // (7,40): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static event System.Action F4;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F4").WithArguments("static members in interfaces", "15.0").WithLocation(7, 40)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation2 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: "112244");

            compilation3 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation3, expectedOutput: "112244");
        }

        [Fact]
        public void FieldLikeEvent_03()
        {
            var source1 =
@"
public interface I1
{
    static event System.Action F1 = () => System.Console.Write(1);
    public static event System.Action F2 = () => System.Console.Write(2);
    internal static event System.Action F3 = () => System.Console.Write(3);
    private static event System.Action F4 = () => System.Console.Write(4);

    public class TestHelper
    {
        public static void Raise()
        {
            F1();
            F2();
            F3();
            F4();
        }
    }
}

class Test1 : I1
{
    static void Main()
    {
        I1.TestHelper.Raise();
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            void Validate1(ModuleSymbol m)
            {
                var i1 = m.GlobalNamespace.GetTypeMember("I1");
                var f1 = i1.GetMember<EventSymbol>("F1");
                var f2 = i1.GetMember<EventSymbol>("F2");
                var f3 = i1.GetMember<EventSymbol>("F3");
                var f4 = i1.GetMember<EventSymbol>("F4");

                Assert.True(f1.IsStatic);
                Assert.True(f2.IsStatic);
                Assert.True(f3.IsStatic);
                Assert.True(f4.IsStatic);

                Assert.Equal(Accessibility.Public, f1.DeclaredAccessibility);
                Assert.Equal(Accessibility.Public, f2.DeclaredAccessibility);
                Assert.Equal(Accessibility.Internal, f3.DeclaredAccessibility);
                Assert.Equal(Accessibility.Private, f4.DeclaredAccessibility);

                var cctor = i1.GetMember<MethodSymbol>(".cctor");
                Assert.Equal(MethodKind.StaticConstructor, cctor.MethodKind);
            }

            Validate1(compilation1.SourceModule);

            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234", symbolValidator: Validate1);

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1.TestHelper.Raise();
    }
}
";

            var compilation2 = CreateCompilation(source2, new[] { compilation1.ToMetadataReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234");

            var compilation3 = CreateCompilation(source2, new[] { compilation1.EmitToImageReference() }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "1234");

            var compilation4 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(
                // (9,18): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public class TestHelper
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "TestHelper").WithArguments("default interface implementation", "8.0").WithLocation(9, 18),
                // (4,32): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     static event System.Action F1 = () => System.Console.Write(1);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F1").WithArguments("static", "7.3", "8.0").WithLocation(4, 32),
                // (5,39): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static event System.Action F2 = () => System.Console.Write(2);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("static", "7.3", "8.0").WithLocation(5, 39),
                // (5,39): error CS8703: The modifier 'public' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     public static event System.Action F2 = () => System.Console.Write(2);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F2").WithArguments("public", "7.3", "8.0").WithLocation(5, 39),
                // (6,41): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static event System.Action F3 = () => System.Console.Write(3);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("static", "7.3", "8.0").WithLocation(6, 41),
                // (6,41): error CS8703: The modifier 'internal' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     internal static event System.Action F3 = () => System.Console.Write(3);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F3").WithArguments("internal", "7.3", "8.0").WithLocation(6, 41),
                // (7,40): error CS8703: The modifier 'static' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static event System.Action F4 = () => System.Console.Write(4);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("static", "7.3", "8.0").WithLocation(7, 40),
                // (7,40): error CS8703: The modifier 'private' is not valid for this item in C# 7.3. Please use language version '8.0' or greater.
                //     private static event System.Action F4 = () => System.Console.Write(4);
                Diagnostic(ErrorCode.ERR_InvalidModifierForLanguageVersion, "F4").WithArguments("private", "7.3", "8.0").WithLocation(7, 40)
                );

            Validate1(compilation4.SourceModule);

            var compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular14, targetFramework: TargetFramework.Mscorlib461Extended);

            compilation5.VerifyDiagnostics(
                // (4,32): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     static event System.Action F1 = () => System.Console.Write(1);
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F1").WithArguments("static members in interfaces", "15.0").WithLocation(4, 32),
                // (5,39): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static event System.Action F2 = () => System.Console.Write(2);
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F2").WithArguments("static members in interfaces", "15.0").WithLocation(5, 39),
                // (6,41): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     internal static event System.Action F3 = () => System.Console.Write(3);
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F3").WithArguments("static members in interfaces", "15.0").WithLocation(6, 41),
                // (7,40): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     private static event System.Action F4 = () => System.Console.Write(4);
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "F4").WithArguments("static members in interfaces", "15.0").WithLocation(7, 40)
                );

            Validate1(compilation5.SourceModule);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation5 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMetadataImportOptions(MetadataImportOptions.All),
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            Validate1(compilation5.SourceModule);

            CompileAndVerify(compilation5, expectedOutput: "1234", symbolValidator: Validate1);

            compilation2 = CreateCompilation(source2, new[] { compilation5.ToMetadataReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation2, expectedOutput: "1234");

            compilation3 = CreateCompilation(source2, new[] { compilation5.EmitToImageReference() }, options: TestOptions.DebugExe,
                                             parseOptions: TestOptions.Regular15, targetFramework: TargetFramework.Mscorlib461Extended);

            CompileAndVerify(compilation3, expectedOutput: "1234");
        }

        [Fact]
        public void UnsupportedMemberAccess_01()
        {
            var source0 =
@"
public delegate void D0();

public interface I0
{
    static readonly int F1 = 1;
    static int P2 {get {System.Console.WriteLine(""P2""); return 2;} set {System.Console.WriteLine(""set_P2"");}}
    static void M3() {System.Console.WriteLine(""M3"");}
    static event D0 E4
    {
        add {System.Console.WriteLine(""add E4"");value();}
        remove {System.Console.WriteLine(""remove E4"");value();}
    }

    class C6
    {
        public static void M() {System.Console.WriteLine(""C6.M"");}
    }
}
";
            var source1 =
@"
public delegate void D1();

public interface I1
{
    int P20 {get {System.Console.WriteLine(""P20""); return 20;}}
    int this[int P50] {set {System.Console.WriteLine(""P50"");}}
    void M30() {System.Console.WriteLine(""M30"");}
    event D1 E40
    {
        add {System.Console.WriteLine(""add E40"");value();}
        remove {System.Console.WriteLine(""remove E40"");value();}
    }

    sealed int P200 {get {System.Console.WriteLine(""P200""); return 200;}}
    sealed int this[string P500] {set {System.Console.WriteLine(""P500"");}}
    sealed void M300() {System.Console.WriteLine(""M300"");}
    sealed event D1 E400
    {
        add {System.Console.WriteLine(""add E400"");value();}
        remove {System.Console.WriteLine(""remove E400"");value();}
    }
}

public class Test1 : I1
{
}
";
            var compilation0 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);

            compilation0.VerifyDiagnostics();

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.Net50);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source2 =
@"
class Test2
{
    static void Main()
    {
        System.Console.WriteLine(I0.F1);
        System.Console.WriteLine(I0.P2);
        I0.M3();
        I0.E4 += I0.M3;
        I0.E4 -= new D0(I0.M3);
        I0.P2 = 3;
        I0.C6.M();
    }
}
";

            var source3 =
@"
class Test3
{
    static void Main()
    {
        I1 i1 = new Test1();
        System.Console.WriteLine(i1.P20);
        i1.M30();
        i1.E40 += i1.M30;
        i1.E40 -= new D1(i1.M30);
        i1[50] = default;
    }
}
";

            var source4 =
@"
class Test4 : Test1
{
    static void Main()
    {
        I1 i1 = new Test1();
        System.Console.WriteLine(i1.P200);
        i1.M300();
        i1.E400 += i1.M300;
        i1.E400 -= new D1(i1.M300);
        i1[""500""] = default;
    }
}
";

            foreach (var refs in new[] { (comp0:compilation0.ToMetadataReference(), comp1:compilation1.ToMetadataReference()),
                                         (comp0:compilation0.EmitToImageReference(), comp1:compilation1.EmitToImageReference()) })
            {
                var compilation2 = CreateCompilation(source2, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.Net50);
                compilation2 = compilation2.AddReferences(refs.comp0);

                CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"1
P2
2
M3
add E4
M3
remove E4
M3
set_P2
C6.M
");

                var compilation3 = CreateCompilation(source3, options: TestOptions.DebugExe,
                                                     parseOptions: TestOptions.Regular,
                                                     targetFramework: TargetFramework.Net50);
                compilation3 = compilation3.AddReferences(refs.comp1);

                CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"P20
20
M30
add E40
M30
remove E40
M30
P50
");

                var compilation4 = CreateCompilation(source4, options: TestOptions.DebugExe, targetFramework: TargetFramework.Mscorlib461Extended,
                                                     parseOptions: TestOptions.Regular);
                compilation4 = compilation4.AddReferences(refs.comp1);
                Assert.False(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation4.VerifyDiagnostics(
                    // (7,34): error CS8501: Target runtime doesn't support default interface implementation.
                    //         System.Console.WriteLine(i1.P200);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.P200").WithLocation(7, 34),
                    // (8,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         i1.M300();
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.M300").WithLocation(8, 9),
                    // (9,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         i1.E400 += i1.M300;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.E400 += i1.M300").WithLocation(9, 9),
                    // (9,20): error CS8501: Target runtime doesn't support default interface implementation.
                    //         i1.E400 += i1.M300;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.M300").WithLocation(9, 20),
                    // (10,9): error CS8501: Target runtime doesn't support default interface implementation.
                    //         i1.E400 -= new D1(i1.M300);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.E400 -= new D1(i1.M300)").WithLocation(10, 9),
                    // (10,27): error CS8501: Target runtime doesn't support default interface implementation.
                    //         i1.E400 -= new D1(i1.M300);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.M300").WithLocation(10, 27),
                    // (11,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1["500"] = default;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, @"i1[""500""]").WithLocation(11, 9)
                    );
            }
        }

        [Theory]
        [CombinatorialData]
        public void UnsupportedMemberAccess_02_01(bool testCSharp14)
        {
            var source0 =
@"
public delegate void D0();

public interface I0
{
    static protected  readonly int F1 = 1;
    static protected internal int P2 {get {System.Console.WriteLine(""P2""); return 2;} set {System.Console.WriteLine(""set_P2"");}}
    static protected void M3() {System.Console.WriteLine(""M3"");}
    static protected internal event D0 E4
    {
        add {System.Console.WriteLine(""add E4"");value();}
        remove {System.Console.WriteLine(""remove E4"");value();}
    }

    protected internal class C6
    {
        public static void M() {System.Console.WriteLine(""C6.M"");}
    }

    protected class C7<T>
    {
    }

    static int P8 {protected internal get {System.Console.WriteLine(""P8""); return 8;} set {System.Console.WriteLine(""set_P8"");}}
    static int P9 {get {System.Console.WriteLine(""P9""); return 9;} protected set {System.Console.WriteLine(""set_P9"");}}
#pragma warning disable CS0067 // The event is never used
    static protected event D0 E5;
    static protected internal event D0 E6;
    static private protected event D0 E7;

    static protected internal readonly int F2 = 1;
    static private protected readonly int F3 = 1;
}

public class Test1 : I0
{
}
";
            var compilation0 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: testCSharp14 ? TestOptions.Regular14 : TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.NetCoreApp);

            compilation0.VerifyDiagnostics();

            var source2 =
@"

using static I0;

class Test2 : Test1
{
    static void Main()
    {
        System.Console.WriteLine(I0.F1);
        System.Console.WriteLine(I0.P2);
        I0.M3();
        I0.E4 += I0.M3;
        I0.E4 -= new D0(I0.M3);
        I0.P2 = 3;
        I0.C6.M();
        _ = new C7<int>();
        _ = I0.P8;
        _ = I0.P9;
        I0.P8 = 12;
        I0.P9 = 13;
    }
}
";

            foreach (var reference in new[] { compilation0.ToMetadataReference(), compilation0.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, options: TestOptions.DebugExe,
                                                     targetFramework: TargetFramework.NetCoreApp,
                                                     references: new[] { reference },
                                                     parseOptions: testCSharp14 ? TestOptions.Regular14 : TestOptions.RegularPreview);

                CompileAndVerify(compilation2, verify: Verification.Skipped, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null :
@"
1
P2
2
M3
add E4
M3
remove E4
M3
set_P2
C6.M
P8
P9
set_P8
set_P9
");
            }
        }

        [Fact]
        public void UnsupportedMemberAccess_02_02()
        {
            var source0 =
@"
public delegate void D0();

public interface I0
{
#line 1100
    static protected  readonly int F1 = 1;
#line 1200
    static protected internal int P2
    {
#line 1300
        get 
        {System.Console.WriteLine(""P2""); return 2;}
#line 1400
        set
        {System.Console.WriteLine(""set_P2"");}
    }
#line 1500
    static protected void M3()
    {System.Console.WriteLine(""M3"");}
#line 1600
    static protected internal event D0 E4
    {
#line 1700
        add
        {System.Console.WriteLine(""add E4"");value();}
#line 1800
        remove
        {System.Console.WriteLine(""remove E4"");value();}
    }
#line 1900
    protected internal class C6
    {
        public static void M() {System.Console.WriteLine(""C6.M"");}
    }

#line 2000
    protected class C7<T>
    {
    }
    static int P8
    {
#line 2100
        protected internal get
        {System.Console.WriteLine(""P8""); return 8;} 
#line 2200
        set
        {System.Console.WriteLine(""set_P8"");}
    }
    static int P9
    {
#line 2300
        get
        {System.Console.WriteLine(""P9""); return 9;}
#line 2400
        protected set
        {System.Console.WriteLine(""set_P9"");}
    }

#pragma warning disable CS0067 // The event is never used

#line 2500
    static protected event D0 E5;
#line 2600
    static protected internal event D0 E6;
#line 2700
    static private protected event D0 E7;
#line 2800
    static protected internal readonly int F2 = 1;
#line 2900
    static private protected readonly int F3 = 1;
}

public class Test1 : I0
{
}
";
            var compilation0 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.RegularPreview,
                                                 targetFramework: TargetFramework.Mscorlib461Extended);

            compilation0.VerifyDiagnostics(
                // (1100,36): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static protected  readonly int F1 = 1;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "F1").WithLocation(1100, 36),
                // (1300,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //         get 
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "get").WithLocation(1300, 9),
                // (1400,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //         set
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(1400, 9),
                // (1500,27): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static protected void M3()
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "M3").WithLocation(1500, 27),
                // (1700,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //         add
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "add").WithLocation(1700, 9),
                // (1800,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //         remove
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "remove").WithLocation(1800, 9),
                // (1900,30): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected internal class C6
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "C6").WithLocation(1900, 30),
                // (2000,21): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     protected class C7<T>
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "C7").WithLocation(2000, 21),
                // (2100,28): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //         protected internal get
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "get").WithLocation(2100, 28),
                // (2400,19): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //         protected set
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "set").WithLocation(2400, 19),
                // (2500,31): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static protected event D0 E5;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "E5").WithLocation(2500, 31),
                // (2600,40): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static protected internal event D0 E6;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "E6").WithLocation(2600, 40),
                // (2700,39): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static private protected event D0 E7;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "E7").WithLocation(2700, 39),
                // (2800,44): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static protected internal readonly int F2 = 1;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "F2").WithLocation(2800, 44),
                // (2900,43): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                //     static private protected readonly int F3 = 1;
                Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "F3").WithLocation(2900, 43)
                );

            compilation0 = CreateCompilation(source0, options: TestOptions.DebugDll,
                                             parseOptions: TestOptions.RegularPreview,
                                             targetFramework: TargetFramework.NetCoreApp);

            compilation0.VerifyDiagnostics();

            var source2 =
@"

using static I0;

class Test2 : Test1
{
    static void Main()
    {
        System.Console.WriteLine(I0.F1);
        System.Console.WriteLine(I0.P2);
        I0.M3();
        I0.E4 += I0.M3;
        I0.E4 -= new D0(I0.M3);
        I0.P2 = 3;
        I0.C6.M();
        _ = new C7<int>();
        _ = I0.P8;
        _ = I0.P9;
        I0.P8 = 12;
        I0.P9 = 13;
    }
}
";

            foreach (var reference in new[] { compilation0.ToMetadataReference(), compilation0.EmitToImageReference() })
            {
                var compilation2 = CreateCompilation(source2, options: TestOptions.DebugExe,
                                                     targetFramework: TargetFramework.Mscorlib461Extended,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.RegularPreview);

                compilation2.VerifyDiagnostics(
                    // (9,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         System.Console.WriteLine(I0.F1);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.F1").WithLocation(9, 34),
                    // (10,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         System.Console.WriteLine(I0.P2);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.P2").WithLocation(10, 34),
                    // (11,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.M3();
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.M3").WithLocation(11, 9),
                    // (12,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.E4 += I0.M3;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.E4 += I0.M3").WithLocation(12, 9),
                    // (12,18): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.E4 += I0.M3;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.M3").WithLocation(12, 18),
                    // (13,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.E4 -= new D0(I0.M3);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.E4 -= new D0(I0.M3)").WithLocation(13, 9),
                    // (13,25): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.E4 -= new D0(I0.M3);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.M3").WithLocation(13, 25),
                    // (14,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.P2 = 3;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.P2").WithLocation(14, 9),
                    // (15,12): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.C6.M();
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "C6").WithLocation(15, 12),
                    // (16,17): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         _ = new C7<int>();
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "C7<int>").WithLocation(16, 17),
                    // (17,13): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         _ = I0.P8;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.P8").WithLocation(17, 13),
                    // (20,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         I0.P9 = 13;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I0.P9").WithLocation(20, 9)
                    );
            }
        }

        [Fact]
        public void UnsupportedMemberAccess_03()
        {
            var source1 =
@"
public delegate void D1();

public interface I1
{
    protected int P20 {get {System.Console.WriteLine(""P20""); return 20;}}
    protected internal int this[int P50] {set {System.Console.WriteLine(""P50"");}}
    protected void M30() {System.Console.WriteLine(""M30"");}
    protected internal event D1 E40
    {
        add {System.Console.WriteLine(""add E40"");value();}
        remove {System.Console.WriteLine(""remove E40"");value();}
    }
    int P50 {protected get {System.Console.WriteLine(""P50""); return 50;} set {}}
    int P60 {get {System.Console.WriteLine(""P60""); return 60;} protected internal set {}}

    protected internal  sealed int P200 {get {System.Console.WriteLine(""P200""); return 200;}}
    protected sealed int this[string P500] {set {System.Console.WriteLine(""P500"");}}
    protected internal sealed void M300() {System.Console.WriteLine(""M300"");}
    protected sealed event D1 E400
    {
        add {System.Console.WriteLine(""add E400"");value();}
        remove {System.Console.WriteLine(""remove E400"");value();}
    }
    sealed int P500 {protected get {System.Console.WriteLine(""P500""); return 500;} set {}}
    sealed int P600 {get {System.Console.WriteLine(""P600""); return 600;} protected internal set {}}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source3 =
@"
interface Test3 : I1
{
    static void Main()
    {
        Test3 i1 = null;
        System.Console.WriteLine(i1.P20);
        i1.M30();
        i1.E40 += i1.M30;
        i1.E40 -= new D1(i1.M30);
        i1[50] = default;
        _ = i1.P50;
        _ = i1.P60;
        i1.P50 = 12;
        i1.P60 = 13;
    }
}
";

            var source4 =
@"
interface Test4 : I1
{
    static void Main()
    {
        Test4 i1 = null;
        System.Console.WriteLine(i1.P200);
        i1.M300();
        i1.E400 += i1.M300;
        i1.E400 -= new D1(i1.M300);
        i1[""500""] = default;
        _ = i1.P500;
        _ = i1.P600;
        i1.P500 = 12;
        i1.P600 = 13;
    }
}
";

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source3, options: TestOptions.DebugExe,
                                                     targetFramework: TargetFramework.Mscorlib461Extended,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.RegularPreview);

                compilation3.VerifyDiagnostics(
                    // (7,34): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         System.Console.WriteLine(i1.P20);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.P20").WithLocation(7, 34),
                    // (8,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1.M30();
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.M30").WithLocation(8, 9),
                    // (9,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1.E40 += i1.M30;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.E40 += i1.M30").WithLocation(9, 9),
                    // (9,19): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1.E40 += i1.M30;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.M30").WithLocation(9, 19),
                    // (10,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1.E40 -= new D1(i1.M30);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.E40 -= new D1(i1.M30)").WithLocation(10, 9),
                    // (10,26): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1.E40 -= new D1(i1.M30);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.M30").WithLocation(10, 26),
                    // (11,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1[50] = default;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1[50]").WithLocation(11, 9),
                    // (12,13): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         _ = i1.P50;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.P50").WithLocation(12, 13),
                    // (15,9): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         i1.P60 = 13;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "i1.P60").WithLocation(15, 9)
                    );

                var compilation4 = CreateCompilation(source4, options: TestOptions.DebugExe,
                                                     targetFramework: TargetFramework.Mscorlib461Extended,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.RegularPreview);

                Assert.False(compilation4.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
                compilation4.VerifyDiagnostics(
                    // (7,34): error CS8701: Target runtime doesn't support default interface implementation.
                    //         System.Console.WriteLine(i1.P200);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.P200").WithLocation(7, 34),
                    // (8,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.M300();
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.M300").WithLocation(8, 9),
                    // (9,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.E400 += i1.M300;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.E400 += i1.M300").WithLocation(9, 9),
                    // (9,20): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.E400 += i1.M300;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.M300").WithLocation(9, 20),
                    // (10,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.E400 -= new D1(i1.M300);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.E400 -= new D1(i1.M300)").WithLocation(10, 9),
                    // (10,27): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.E400 -= new D1(i1.M300);
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.M300").WithLocation(10, 27),
                    // (11,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1["500"] = default;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, @"i1[""500""]").WithLocation(11, 9),
                    // (12,13): error CS8701: Target runtime doesn't support default interface implementation.
                    //         _ = i1.P500;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.P500").WithLocation(12, 13),
                    // (13,13): error CS8701: Target runtime doesn't support default interface implementation.
                    //         _ = i1.P600;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.P600").WithLocation(13, 13),
                    // (14,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.P500 = 12;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.P500").WithLocation(14, 9),
                    // (15,9): error CS8701: Target runtime doesn't support default interface implementation.
                    //         i1.P600 = 13;
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportDefaultInterfaceImplementation, "i1.P600").WithLocation(15, 9)
                    );
            }
        }

        [Fact]
        public void UnsupportedMemberAccess_04()
        {
            var source1 =
@"
public interface I1
{
    protected interface I2 {}
    protected internal interface I3 {}
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            Assert.True(compilation1.Assembly.RuntimeSupportsDefaultInterfaceImplementation);
            compilation1.VerifyDiagnostics();

            var source3 =
@"
interface Test3 : I1
{
    class C
    {
        static void M1(I1.I2 x)
        {
            System.Console.WriteLine(""M1"");
        }

        static void M2(I1.I3 x)
        {
            System.Console.WriteLine(""M2"");
        }

        static void Main()
        {
            M1(null);
            M2(null);
        }
    }
}
";

            var source4 =
@"
class Test4 : I1
{
    interface Test5 : I1.I2
    {
    }

    interface Test6 : I1.I3
    {
    }
}
";

            foreach (var reference in new[] { compilation1.ToMetadataReference(), compilation1.EmitToImageReference() })
            {
                var compilation3 = CreateCompilation(source3, options: TestOptions.DebugExe,
                                                     targetFramework: TargetFramework.Mscorlib461Extended,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular);

                compilation3.VerifyDiagnostics(
                    // (6,27): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         static void M1(I1.I2 x)
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I2").WithLocation(6, 27),
                    // (11,27): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //         static void M2(I1.I3 x)
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I3").WithLocation(11, 27)
                    );

                var compilation4 = CreateCompilation(source4, options: TestOptions.DebugDll,
                                                     targetFramework: TargetFramework.Mscorlib461Extended,
                                                     references: new[] { reference },
                                                     parseOptions: TestOptions.Regular);

                compilation4.VerifyDiagnostics(
                    // (4,26): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //     interface Test5 : I1.I2
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I2").WithLocation(4, 26),
                    // (8,26): error CS8707: Target runtime doesn't support 'protected', 'protected internal', or 'private protected' accessibility for a member of an interface.
                    //     interface Test6 : I1.I3
                    Diagnostic(ErrorCode.ERR_RuntimeDoesNotSupportProtectedAccessForInterfaceMember, "I3").WithLocation(8, 26)
                    );
            }
        }

        [Fact]
        public void EntryPoint_01()
        {
            var source1 =
@"
public interface I1
{
    static void Main() 
    {
        System.Console.WriteLine(""I1.Main"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "I1.Main");
        }

        [Fact]
        public void EntryPoint_02()
        {
            var source1 =
@"
public interface I1
{
    static void Main() 
    {
        System.Console.WriteLine(""I1.Main"");
    }
}
public interface I2
{
    static void Main() 
    {
        System.Console.WriteLine(""I2.Main"");
    }
}
";
            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugExe.WithMainTypeName("I2"),
                                                 parseOptions: TestOptions.Regular,
                                                 targetFramework: TargetFramework.NetCoreApp);
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : "I2.Main");
        }

        [Fact]
        public void Operators_01()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator +(I1 x)
    {
        System.Console.WriteLine(""+"");
        return x;
    }

    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""-"");
        return x;
    }

    public static I1 operator !(I1 x)
    {
        System.Console.WriteLine(""!"");
        return x;
    }

    public static I1 operator ~(I1 x)
    {
        System.Console.WriteLine(""~"");
        return x;
    }

    public static I1 operator ++(I1 x)
    {
        System.Console.WriteLine(""++"");
        return x;
    }

    public static I1 operator --(I1 x)
    {
        System.Console.WriteLine(""--"");
        return x;
    }

    public static bool operator true(I1 x)
    {
        System.Console.WriteLine(""true"");
        return true;
    }

    public static bool operator false(I1 x)
    {
        System.Console.WriteLine(""false"");
        return false;
    }

    public static I1 operator +(I1 x, I1 y)
    {
        System.Console.WriteLine(""+2"");
        return x;
    }

    public static I1 operator -(I1 x, I1 y)
    {
        System.Console.WriteLine(""-2"");
        return x;
    }

    public static I1 operator *(I1 x, I1 y)
    {
        System.Console.WriteLine(""*"");
        return x;
    }

    public static I1 operator /(I1 x, I1 y)
    {
        System.Console.WriteLine(""/"");
        return x;
    }

    public static I1 operator %(I1 x, I1 y)
    {
        System.Console.WriteLine(""%"");
        return x;
    }

    public static I1 operator &(I1 x, I1 y)
    {
        System.Console.WriteLine(""&"");
        return x;
    }

    public static I1 operator |(I1 x, I1 y)
    {
        System.Console.WriteLine(""|"");
        return x;
    }

    public static I1 operator ^(I1 x, I1 y)
    {
        System.Console.WriteLine(""^"");
        return x;
    }

    public static I1 operator <<(I1 x, int y)
    {
        System.Console.WriteLine(""<<"");
        return x;
    }

    public static I1 operator >>(I1 x, int y)
    {
        System.Console.WriteLine("">>"");
        return x;
    }

    public static I1 operator >(I1 x, I1 y)
    {
        System.Console.WriteLine("">"");
        return x;
    }

    public static I1 operator <(I1 x, I1 y)
    {
        System.Console.WriteLine(""<"");
        return x;
    }

    public static I1 operator >=(I1 x, I1 y)
    {
        System.Console.WriteLine("">="");
        return x;
    }

    public static I1 operator <=(I1 x, I1 y)
    {
        System.Console.WriteLine(""<="");
        return x;
    }

    public static I1 operator >>>(I1 x, int y)
    {
        System.Console.WriteLine("">>>"");
        return x;
    }
}
";

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1 x = new Test2();
        I1 y = new Test2();

        x = +x;
        x = -x;
        x = !x;
        x = ~x;
        x = ++x;
        x = x--;

        x = x + y;
        x = x - y;
        x = x * y;
        x = x / y;
        x = x % y;
        if (x && y) { }
        x = x | y;
        x = x ^ y;
        x = x << 1;
        x = x >> 2;
        x = x > y;
        x = x < y;
        x = x >= y;
        x = x <= y;

        x = x >>> 3;
    }
}
";

            var expectedOutput =
@"
+
-
!
~
++
--
+2
-2
*
/
%
false
&
true
|
^
<<
>>
>
<
>=
<=
>>>
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr);

            var compilation6 = CreateCompilation(source1 + source2, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation6.VerifyDiagnostics(
                // (4,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator +(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "+").WithArguments("default interface implementation", "8.0").WithLocation(4, 31),
                // (10,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator -(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "-").WithArguments("default interface implementation", "8.0").WithLocation(10, 31),
                // (16,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator !(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "!").WithArguments("default interface implementation", "8.0").WithLocation(16, 31),
                // (22,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator ~(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "~").WithArguments("default interface implementation", "8.0").WithLocation(22, 31),
                // (28,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator ++(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "++").WithArguments("default interface implementation", "8.0").WithLocation(28, 31),
                // (34,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator --(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "--").WithArguments("default interface implementation", "8.0").WithLocation(34, 31),
                // (40,33): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static bool operator true(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "true").WithArguments("default interface implementation", "8.0").WithLocation(40, 33),
                // (46,33): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static bool operator false(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "false").WithArguments("default interface implementation", "8.0").WithLocation(46, 33),
                // (52,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator +(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "+").WithArguments("default interface implementation", "8.0").WithLocation(52, 31),
                // (58,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator -(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "-").WithArguments("default interface implementation", "8.0").WithLocation(58, 31),
                // (64,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator *(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "*").WithArguments("default interface implementation", "8.0").WithLocation(64, 31),
                // (70,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator /(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "/").WithArguments("default interface implementation", "8.0").WithLocation(70, 31),
                // (76,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator %(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "%").WithArguments("default interface implementation", "8.0").WithLocation(76, 31),
                // (82,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator &(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "&").WithArguments("default interface implementation", "8.0").WithLocation(82, 31),
                // (88,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator |(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "|").WithArguments("default interface implementation", "8.0").WithLocation(88, 31),
                // (94,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator ^(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "^").WithArguments("default interface implementation", "8.0").WithLocation(94, 31),
                // (100,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator <<(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "<<").WithArguments("default interface implementation", "8.0").WithLocation(100, 31),
                // (106,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator >>(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, ">>").WithArguments("default interface implementation", "8.0").WithLocation(106, 31),
                // (112,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator >(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, ">").WithArguments("default interface implementation", "8.0").WithLocation(112, 31),
                // (118,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator <(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "<").WithArguments("default interface implementation", "8.0").WithLocation(118, 31),
                // (124,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator >=(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, ">=").WithArguments("default interface implementation", "8.0").WithLocation(124, 31),
                // (130,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator <=(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "<=").WithArguments("default interface implementation", "8.0").WithLocation(130, 31),
                // (136,31): error CS8370: Feature 'unsigned right shift' is not available in C# 7.3. Please use language version 11.0 or greater.
                //     public static I1 operator >>>(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, ">>>").WithArguments("unsigned right shift", "11.0").WithLocation(136, 31),
                // (136,31): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //     public static I1 operator >>>(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, ">>>").WithArguments("default interface implementation", "8.0").WithLocation(136, 31)
                );

            var compilation61 = CreateCompilation(source1 + source2, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.Regular14);

            compilation61.VerifyDiagnostics(
                // (4,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator +(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "+").WithArguments("static members in interfaces", "15.0").WithLocation(4, 31),
                // (10,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator -(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "-").WithArguments("static members in interfaces", "15.0").WithLocation(10, 31),
                // (16,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator !(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "!").WithArguments("static members in interfaces", "15.0").WithLocation(16, 31),
                // (22,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator ~(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "~").WithArguments("static members in interfaces", "15.0").WithLocation(22, 31),
                // (28,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator ++(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "++").WithArguments("static members in interfaces", "15.0").WithLocation(28, 31),
                // (34,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator --(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "--").WithArguments("static members in interfaces", "15.0").WithLocation(34, 31),
                // (40,33): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static bool operator true(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "true").WithArguments("static members in interfaces", "15.0").WithLocation(40, 33),
                // (46,33): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static bool operator false(I1 x)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "false").WithArguments("static members in interfaces", "15.0").WithLocation(46, 33),
                // (52,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator +(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "+").WithArguments("static members in interfaces", "15.0").WithLocation(52, 31),
                // (58,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator -(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "-").WithArguments("static members in interfaces", "15.0").WithLocation(58, 31),
                // (64,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator *(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "*").WithArguments("static members in interfaces", "15.0").WithLocation(64, 31),
                // (70,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator /(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "/").WithArguments("static members in interfaces", "15.0").WithLocation(70, 31),
                // (76,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator %(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "%").WithArguments("static members in interfaces", "15.0").WithLocation(76, 31),
                // (82,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator &(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "&").WithArguments("static members in interfaces", "15.0").WithLocation(82, 31),
                // (88,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator |(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "|").WithArguments("static members in interfaces", "15.0").WithLocation(88, 31),
                // (94,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator ^(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "^").WithArguments("static members in interfaces", "15.0").WithLocation(94, 31),
                // (100,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator <<(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "<<").WithArguments("static members in interfaces", "15.0").WithLocation(100, 31),
                // (106,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator >>(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, ">>").WithArguments("static members in interfaces", "15.0").WithLocation(106, 31),
                // (112,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator >(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, ">").WithArguments("static members in interfaces", "15.0").WithLocation(112, 31),
                // (118,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator <(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "<").WithArguments("static members in interfaces", "15.0").WithLocation(118, 31),
                // (124,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator >=(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, ">=").WithArguments("static members in interfaces", "15.0").WithLocation(124, 31),
                // (130,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator <=(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, "<=").WithArguments("static members in interfaces", "15.0").WithLocation(130, 31),
                // (136,31): error CS9327: Feature 'static members in interfaces' is not available in C# 14.0. Please use language version 15.0 or greater.
                //     public static I1 operator >>>(I1 x, int y)
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion14, ">>>").WithArguments("static members in interfaces", "15.0").WithLocation(136, 31)
                );

            var compilation62 = CreateCompilation(source1 + source2, options: TestOptions.DebugDll,
                                                 targetFramework: TargetFramework.Mscorlib461Extended,
                                                 parseOptions: TestOptions.RegularPreview);

            compilation62.VerifyDiagnostics();

            compilation62 = CreateCompilation(source1 + source2, options: TestOptions.DebugDll,
                                              targetFramework: TargetFramework.Mscorlib461Extended,
                                              parseOptions: TestOptions.Regular15);

            compilation62.VerifyDiagnostics();

            var compilation7 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular7_3);

            var expected7 = new DiagnosticDescription[]
            {
                 // (9,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = +x;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "+x").WithArguments("default interface implementation", "8.0").WithLocation(9, 13),
                // (10,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = -x;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "-x").WithArguments("default interface implementation", "8.0").WithLocation(10, 13),
                // (11,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = !x;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "!x").WithArguments("default interface implementation", "8.0").WithLocation(11, 13),
                // (12,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = ~x;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "~x").WithArguments("default interface implementation", "8.0").WithLocation(12, 13),
                // (13,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = ++x;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "++x").WithArguments("default interface implementation", "8.0").WithLocation(13, 13),
                // (14,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x--;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x--").WithArguments("default interface implementation", "8.0").WithLocation(14, 13),
                // (16,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x + y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x + y").WithArguments("default interface implementation", "8.0").WithLocation(16, 13),
                // (17,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x - y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x - y").WithArguments("default interface implementation", "8.0").WithLocation(17, 13),
                // (18,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x * y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x * y").WithArguments("default interface implementation", "8.0").WithLocation(18, 13),
                // (19,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x / y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x / y").WithArguments("default interface implementation", "8.0").WithLocation(19, 13),
                // (20,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x % y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x % y").WithArguments("default interface implementation", "8.0").WithLocation(20, 13),
                // (21,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         if (x && y) { }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x && y").WithArguments("default interface implementation", "8.0").WithLocation(21, 13),
                // (21,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         if (x && y) { }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x && y").WithArguments("default interface implementation", "8.0").WithLocation(21, 13),
                // (22,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x | y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x | y").WithArguments("default interface implementation", "8.0").WithLocation(22, 13),
                // (23,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x ^ y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x ^ y").WithArguments("default interface implementation", "8.0").WithLocation(23, 13),
                // (24,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x << 1;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x << 1").WithArguments("default interface implementation", "8.0").WithLocation(24, 13),
                // (25,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x >> 2;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x >> 2").WithArguments("default interface implementation", "8.0").WithLocation(25, 13),
                // (26,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x > y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x > y").WithArguments("default interface implementation", "8.0").WithLocation(26, 13),
                // (27,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x < y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x < y").WithArguments("default interface implementation", "8.0").WithLocation(27, 13),
                // (28,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x >= y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x >= y").WithArguments("default interface implementation", "8.0").WithLocation(28, 13),
                // (29,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x <= y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x <= y").WithArguments("default interface implementation", "8.0").WithLocation(29, 13),
                // (31,13): error CS8370: Feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x >>> 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x >>> 3").WithArguments("default interface implementation", "8.0").WithLocation(31, 13),
                // (31,13): error CS8370: Feature 'unsigned right shift' is not available in C# 7.3. Please use language version 11.0 or greater.
                //         x = x >>> 3;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x >>> 3").WithArguments("unsigned right shift", "11.0").WithLocation(31, 13)
            };
            compilation7.VerifyDiagnostics(expected7);

            var compilation8 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular7_3);

            compilation8.VerifyDiagnostics(expected7);

            var source3 =
@"
class Test3 : I1
{
    static void Main()
    {
        I1 x = new Test3();
        I1 y = new Test3();
        if (x) { }
        x = x & y;
    }
}
";

            var compilation9 = CreateCompilation(source3, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular7_3);

            var expected9 = new DiagnosticDescription[]
            {
                // (8,13): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         if (x) { }
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x").WithArguments("default interface implementation", "8.0").WithLocation(8, 13),
                // (9,13): error CS8652: The feature 'default interface implementation' is not available in C# 7.3. Please use language version 8.0 or greater.
                //         x = x & y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion7_3, "x & y").WithArguments("default interface implementation", "8.0").WithLocation(9, 13)
            };
            compilation9.VerifyDiagnostics(expected9);

            var compilation10 = CreateCompilation(source3, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                  parseOptions: TestOptions.Regular7_3);

            compilation10.VerifyDiagnostics(expected9);
        }

        [Fact]
        public void Operators_02()
        {
            var source1 =
@"
public class C1
{
    public static int operator +(C1 x, I1 y)
    {
        System.Console.WriteLine(""C1.+"");
        return 0;
    }
    public static int operator -(I1 x, C1 y)
    {
        System.Console.WriteLine(""C1.-"");
        return 0;
    }
}

public class C2 : C1
{}

class Test : I1
{
    static void Main()
    {
        I1 x = new Test();
        C2 y = new C2();

        var r = y + x;
        r = x - y;
    }
}
";

            var source2 =
@"
public interface I1
{
}
";

            var source3 =
@"
public interface I1
{
    public static int operator +(C2 x, I1 y)
    {
        System.Console.WriteLine(""I1.+"");
        return 0;
    }
    public static int operator -(I1 x, C2 y)
    {
        System.Console.WriteLine(""I1.-"");
        return 0;
    }
}
";

            var expectedOutput = @"
C1.+
C1.-
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe, targetFramework: TargetFramework.NetCoreApp);
            CompileAndVerify(compilation1, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr);

            var compilation2 = CreateCompilation(source1 + source3, options: TestOptions.DebugExe, targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput, verify: VerifyOnMonoOrCoreClr);
        }

        [Fact]
        public void Operators_03()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator ==(I1 x, I1 y)
    {
        System.Console.WriteLine(""=="");
        return x;
    }

    public static I1 operator !=(I1 x, I1 y)
    {
        System.Console.WriteLine(""!="");
        return x;
    }
}
";

            var source2 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1 x = new Test2();
        I1 y = new Test2();

        x = x == y;
        x = x != y;
    }
}
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe, targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular9);
            compilation1.VerifyDiagnostics(
                // (4,31): error CS0567: Conversion, equality, or inequality operators declared in interfaces must be abstract or virtual
                //     public static I1 operator ==(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConversionOrEqualityOperators, "==").WithLocation(4, 31),
                // (10,31): error CS0567: Conversion, equality, or inequality operators declared in interfaces must be abstract or virtual
                //     public static I1 operator !=(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConversionOrEqualityOperators, "!=").WithLocation(10, 31),
                // (24,13): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         x = x == y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x == y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(24, 13),
                // (25,13): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         x = x != y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x != y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(25, 13)
                );

            CreateCompilation(source1 + source2, options: TestOptions.DebugExe, targetFramework: TargetFramework.NetCoreApp,
                              parseOptions: TestOptions.RegularPreview).VerifyDiagnostics(
                // (4,31): error CS0567: Conversion, equality, or inequality operators declared in interfaces must be abstract or virtual
                //     public static I1 operator ==(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConversionOrEqualityOperators, "==").WithLocation(4, 31),
                // (10,31): error CS0567: Conversion, equality, or inequality operators declared in interfaces must be abstract or virtual
                //     public static I1 operator !=(I1 x, I1 y)
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConversionOrEqualityOperators, "!=").WithLocation(10, 31)
                );

            CompilationReference compilationReference = compilation1.ToMetadataReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.RegularPreview, targetFramework: TargetFramework.NetCoreApp);
            compilation2.VerifyDiagnostics();

            CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                              parseOptions: TestOptions.Regular9, targetFramework: TargetFramework.NetCoreApp).VerifyDiagnostics(
                // (9,13): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         x = x == y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x == y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(9, 13),
                // (10,13): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         x = x != y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x != y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(10, 13)
                );

            var ilSource = @"
.class interface public abstract auto ansi I1
{
  .method public hidebysig specialname static 
          class I1  op_Equality(class I1 x,
                                class I1 y) cil managed
  {
    // Code size       18 (0x12)
    .maxstack  1
    .locals init (class I1 V_0)
    IL_0000:  nop
    IL_0001:  ldstr      ""==""
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ldarg.0
    IL_000d:  stloc.0
    IL_000e:  br.s       IL_0010

    IL_0010:  ldloc.0
    IL_0011:  ret
  } // end of method I1::op_Equality

  .method public hidebysig specialname static 
          class I1  op_Inequality(class I1 x,
                                  class I1 y) cil managed
  {
    // Code size       18 (0x12)
    .maxstack  1
    .locals init (class I1 V_0)
    IL_0000:  nop
    IL_0001:  ldstr      ""!=""
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ldarg.0
    IL_000d:  stloc.0
    IL_000e:  br.s       IL_0010

    IL_0010:  ldloc.0
    IL_0011:  ret
  } // end of method I1::op_Inequality

} // end of class I1
";

            var compilation3 = CreateCompilationWithIL(source2, ilSource, options: TestOptions.DebugExe, parseOptions: TestOptions.RegularPreview);
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : @"
==
!=
").VerifyDiagnostics();

            CreateCompilationWithIL(source2, ilSource, options: TestOptions.DebugExe, parseOptions: TestOptions.Regular9).VerifyDiagnostics(
                // (9,13): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         x = x == y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x == y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(9, 13),
                // (10,13): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         x = x != y;
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x != y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(10, 13)
                );

            var source3 =
@"
class Test2 : I1
{
    static void Main()
    {
        I1 x = new Test2();
        I1 y = new Test2();

        System.Console.WriteLine(x == y);
        System.Console.WriteLine(x != y);
    }
}
";
            var compilation4 = CreateCompilationWithIL(source3, ilSource, options: TestOptions.DebugExe, parseOptions: TestOptions.RegularPreview);
            compilation4.VerifyDiagnostics();
            CompileAndVerify(compilation4, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : @"
==
Test2
!=
Test2
");

            CreateCompilationWithIL(source3, ilSource, options: TestOptions.DebugExe, parseOptions: TestOptions.Regular9).VerifyDiagnostics(
                // (9,34): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         System.Console.WriteLine(x == y);
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x == y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(9, 34),
                // (10,34): error CS8773: Feature 'static abstract members in interfaces' is not available in C# 9.0. Please use language version 11.0 or greater.
                //         System.Console.WriteLine(x != y);
                Diagnostic(ErrorCode.ERR_FeatureNotAvailableInVersion9, "x != y").WithArguments("static abstract members in interfaces", "11.0").WithLocation(10, 34)
                );
        }

        [Fact]
        public void Operators_04()
        {
            var source1 =
@"
public interface I1
{
    public static implicit operator int(I1 x)
    {
        return 0;
    }
    public static explicit operator byte(I1 x)
    {
        return 0;
    }

    public static void Test(I1 x)
    {
        int y = x;
        y = (int)x;
        var z = (byte)x;
        z = x;
    }
}
";

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll, targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular);
            compilation1.VerifyDiagnostics(
                // (4,37): error CS0552: 'I1.implicit operator int(I1)': user-defined conversions to or from an interface are not allowed
                //     public static implicit operator int(I1 x)
                Diagnostic(ErrorCode.ERR_ConversionWithInterface, "int").WithArguments("I1.implicit operator int(I1)").WithLocation(4, 37),
                // (4,37): error CS0567: Conversion, equality, or inequality operators declared in interfaces must be abstract or virtual
                //     public static implicit operator int(I1 x)
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConversionOrEqualityOperators, "int").WithLocation(4, 37),
                // (8,37): error CS0552: 'I1.explicit operator byte(I1)': user-defined conversions to or from an interface are not allowed
                //     public static explicit operator byte(I1 x)
                Diagnostic(ErrorCode.ERR_ConversionWithInterface, "byte").WithArguments("I1.explicit operator byte(I1)").WithLocation(8, 37),
                // (8,37): error CS0567: Conversion, equality, or inequality operators declared in interfaces must be abstract or virtual
                //     public static explicit operator byte(I1 x)
                Diagnostic(ErrorCode.ERR_InterfacesCantContainConversionOrEqualityOperators, "byte").WithLocation(8, 37),
                // (15,17): error CS0029: Cannot implicitly convert type 'I1' to 'int'
                //         int y = x;
                Diagnostic(ErrorCode.ERR_NoImplicitConv, "x").WithArguments("I1", "int").WithLocation(15, 17),
                // (16,13): error CS0030: Cannot convert type 'I1' to 'int'
                //         y = (int)x;
                Diagnostic(ErrorCode.ERR_NoExplicitConv, "(int)x").WithArguments("I1", "int").WithLocation(16, 13),
                // (17,17): error CS0030: Cannot convert type 'I1' to 'byte'
                //         var z = (byte)x;
                Diagnostic(ErrorCode.ERR_NoExplicitConv, "(byte)x").WithArguments("I1", "byte").WithLocation(17, 17),
                // (18,13): error CS0029: Cannot implicitly convert type 'I1' to 'byte'
                //         z = x;
                Diagnostic(ErrorCode.ERR_NoImplicitConv, "x").WithArguments("I1", "byte").WithLocation(18, 13)
                );
        }

        [Fact]
        public void Operators_05()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""-"");
        return x;
    }
}

public interface I2 : I1
{}
";

            var source2 =
@"
class Test2 : I2
{
    static void Main()
    {
        I2 x = new Test2();

        I1 y = -x;
    }
}
";

            var expectedOutput =
@"
-
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe, targetFramework: TargetFramework.NetCoreApp,
                                                 parseOptions: TestOptions.Regular);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var source3 =
@"
class Test2 : I2
{
    static void Main()
    {
        Test2 x = new Test2();

        I1 y = -x;
    }
}
";

            var compilation9 = CreateCompilation(source3, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            var expected9 = new DiagnosticDescription[]
            {
                // (8,16): error CS0023: Operator '-' cannot be applied to operand of type 'Test2'
                //         I1 y = -x;
                Diagnostic(ErrorCode.ERR_BadUnaryOp, "-x").WithArguments("-", "Test2").WithLocation(8, 16)
            };
            compilation9.VerifyDiagnostics(expected9);

            var compilation10 = CreateCompilation(source3, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                  parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            compilation10.VerifyDiagnostics(expected9);
        }

        [Fact]
        public void Operators_06()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""I1.-"");
        return x;
    }
}

public interface I2 : I1
{
    public static I2 operator -(I2 x)
    {
        System.Console.WriteLine(""I2.-"");
        return x;
    }
}
";

            var source2 =
@"
class Test2 : I2
{
    static void Main()
    {
        I2 x = new Test2();
        var y = -x;
    }
}
";

            var expectedOutput =
@"
I2.-
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_07()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""I1.-"");
        return x;
    }
}

public interface I3
{
    public static I3 operator -(I3 x)
    {
        System.Console.WriteLine(""I3.-"");
        return x;
    }
}

public interface I4 : I1, I3
{
    public static I4 operator -(I4 x)
    {
        System.Console.WriteLine(""I4.-"");
        return x;
    }
}

public interface I2 : I4
{
}
";

            var source2 =
@"
class Test2 : I2
{
    static void Main()
    {
        I2 x = new Test2();
        var y = -x;
    }
}
";

            var expectedOutput =
@"
I4.-
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_08()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""I1.-"");
        return x;
    }
}

public interface I3
{
    public static I3 operator -(I3 x)
    {
        System.Console.WriteLine(""I3.-"");
        return x;
    }
}

public interface I4 : I1, I3
{
}

public interface I2 : I4
{
}
";

            var source2 =
@"
class Test2 : I2
{
    static void Main()
    {
        I2 x = new Test2();
        var y = -x;
    }
}
";

            var expected = new DiagnosticDescription[]
            {
                // (7,17): error CS9342: Operator resolution is ambiguous between the following members: 'I1.operator -(I1)' and 'I3.operator -(I3)'
                //         var y = -x;
                Diagnostic(ErrorCode.ERR_AmbigOperator, "-").WithArguments("I1.operator -(I1)", "I3.operator -(I3)").WithLocation(7, 17)
            };

            var compilation1 = CreateCompilation(source2 + source1, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(expected);

            CompilationReference compilationReference = compilation1.ToMetadataReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(expected);
        }

        [Fact]
        public void Operators_09()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""I1.-"");
        return x;
    }
}
";

            var source2 =
@"
class Test1 : Test2, I1 {}

class Test2
{
    static void Main()
    {
        Test(new Test1());
    }

    static void Test<T>(T x) where T : Test2, I1
    {
        var y = -x;
    }

    public static Test2 operator -(Test2 x)
    {
        System.Console.WriteLine(""Test2.-"");
        return x;
    }
}
";

            var expectedOutput =
@"
Test2.-
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_10()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator -(I1 x)
    {
        System.Console.WriteLine(""I1.-"");
        return x;
    }
}

public interface I3
{
    public static I3 operator -(I3 x)
    {
        System.Console.WriteLine(""I3.-"");
        return x;
    }
}

public interface I4 : I1, I3
{
    public static I4 operator -(I4 x)
    {
        System.Console.WriteLine(""I4.-"");
        return x;
    }
}

public interface I2 : I4
{
}";

            var source2 =
@"
class Test1 : Test2, I2 {}

class Test2
{
    static void Main()
    {
        Test(new Test1());
    }

    static void Test<T>(T x) where T : Test2, I2
    {
        var y = -x;
    }
}
";

            var expectedOutput =
@"
I4.-
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_11()
        {
            var source0 =
@"
public interface I1
{ }

public interface I2
{ }
";
            var source1 =
@"
public class C1
{
    public static C1 operator +(C1 x, I2 y)
    {
        System.Console.WriteLine(""C1.+1"");
        return x;
    }
    public static C1 operator +(C1 x, I1 y)
    {
        System.Console.WriteLine(""C1.+2"");
        return x;
    }
}

";

            var source2 =
@"
public interface I3 :  I1, I2
{ }
";
            var source3 =
@"
class Test2 : I3
{
    static void Main()
    {
        I3 x = new Test2();
        var y = new C1() + x;
    }
}
";

            var expected = new DiagnosticDescription[]
            {
                // (7,26): error CS9342: Operator resolution is ambiguous between the following members: 'C1.operator +(C1, I2)' and 'C1.operator +(C1, I1)'
                //         var y = new C1() + x;
                Diagnostic(ErrorCode.ERR_AmbigOperator, "+").WithArguments("C1.operator +(C1, I2)", "C1.operator +(C1, I1)").WithLocation(7, 26)
            };

            var compilation0 = CreateCompilation(source0 + source1 + source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation0.VerifyDiagnostics();

            CompilationReference compilationReference0 = compilation0.ToMetadataReference();
            MetadataReference metadataReference0 = compilation0.EmitToImageReference();

            var compilation1 = CreateCompilation(source3, new[] { compilationReference0 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(expected);

            var compilation2 = CreateCompilation(source3, new[] { metadataReference0 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(expected);

            var source4 =
@"
public interface I1
{ 
    public static C1 operator +(C1 x, I1 y)
    {
        System.Console.WriteLine(""I1.+"");
        return x;
    }
}

public interface I2
{ }
";
            var compilation3 = CreateCompilation(source4 + source1 + source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            CompilationReference compilationReference3 = compilation3.ToMetadataReference();
            MetadataReference metadataReference3 = compilation3.EmitToImageReference();

            var compilation4 = CreateCompilation(source3, new[] { compilationReference3 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(expected);

            var compilation5 = CreateCompilation(source3, new[] { metadataReference3 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation5.VerifyDiagnostics(expected);

            var source5 =
    @"
public interface I3 :  I1, I2
{ 
    public static C1 operator +(C1 x, I3 y)
    {
        System.Console.WriteLine(""I3.+"");
        return x;
    }
}
";
            var compilation6 = CreateCompilation(source4 + source1 + source5, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation6.VerifyDiagnostics();

            CompilationReference compilationReference6 = compilation6.ToMetadataReference();
            MetadataReference metadataReference6 = compilation6.EmitToImageReference();

            var compilation7 = CreateCompilation(source3, new[] { compilationReference6 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation7.VerifyDiagnostics(expected);

            var compilation8 = CreateCompilation(source3, new[] { metadataReference6 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation8.VerifyDiagnostics(expected);
        }

        [Fact]
        public void Operators_12()
        {
            var source0 =
@"
public interface I1
{ }

public interface I2
{ }
";
            var source1 =
@"
public class C1
{
    public static C1 operator +(I2 x, C1 y)
    {
        System.Console.WriteLine(""C1.+1"");
        return y;
    }
    public static C1 operator +(I1 x, C1 y)
    {
        System.Console.WriteLine(""C1.+2"");
        return y;
    }
}

";

            var source2 =
@"
public interface I3 :  I1, I2
{ }
";
            var source3 =
@"
class Test2 : I3
{
    static void Main()
    {
        I3 x = new Test2();
        var y = x + new C1();
    }
}
";

            var expected = new DiagnosticDescription[]
            {
                // (7,19): error CS9342: Operator resolution is ambiguous between the following members: 'C1.operator +(I2, C1)' and 'C1.operator +(I1, C1)'
                //         var y = x + new C1();
                Diagnostic(ErrorCode.ERR_AmbigOperator, "+").WithArguments("C1.operator +(I2, C1)", "C1.operator +(I1, C1)").WithLocation(7, 19)
            };

            var compilation0 = CreateCompilation(source0 + source1 + source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation0.VerifyDiagnostics();

            CompilationReference compilationReference0 = compilation0.ToMetadataReference();
            MetadataReference metadataReference0 = compilation0.EmitToImageReference();

            var compilation1 = CreateCompilation(source3, new[] { compilationReference0 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics(expected);

            var compilation2 = CreateCompilation(source3, new[] { metadataReference0 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(expected);

            var source4 =
@"
public interface I1
{ 
    public static C1 operator +(I1 x, C1 y)
    {
        System.Console.WriteLine(""I1.+"");
        return y;
    }
}

public interface I2
{ }
";
            var compilation3 = CreateCompilation(source4 + source1 + source2, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();

            CompilationReference compilationReference3 = compilation3.ToMetadataReference();
            MetadataReference metadataReference3 = compilation3.EmitToImageReference();

            var compilation4 = CreateCompilation(source3, new[] { compilationReference3 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation4.VerifyDiagnostics(expected);

            var compilation5 = CreateCompilation(source3, new[] { metadataReference3 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation5.VerifyDiagnostics(expected);

            var source5 =
    @"
public interface I3 :  I1, I2
{ 
    public static C1 operator +(I3 x, C1 y)
    {
        System.Console.WriteLine(""I3.+"");
        return y;
    }
}
";
            var compilation6 = CreateCompilation(source4 + source1 + source5, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation6.VerifyDiagnostics();

            CompilationReference compilationReference6 = compilation6.ToMetadataReference();
            MetadataReference metadataReference6 = compilation6.EmitToImageReference();

            var compilation7 = CreateCompilation(source3, new[] { compilationReference6 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation7.VerifyDiagnostics(expected);

            var compilation8 = CreateCompilation(source3, new[] { metadataReference6 }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation8.VerifyDiagnostics(expected);
        }

        [Fact]
        public void Operators_13()
        {
            var source1 =
@"
public interface I1
{
    public static int operator +(I1 x, I2 y)
    {
        System.Console.WriteLine(""I1.+"");
        return 1;
    }
}

public interface I2
{
    public static int operator +(I1 x, I2 y)
    {
        System.Console.WriteLine(""I2.+"");
        return 2;
    }
}
";

            var source2 =
@"
class Test2: I1, I2
{
    static void Main()
    {
        I1 x = new Test2();
        I2 y = new Test2();
        var z = x + y;
        z = y + x;
    }
}
";

            var expected = new DiagnosticDescription[]
            {
                // (8,19): error CS9342: Operator resolution is ambiguous between the following members: 'I1.operator +(I1, I2)' and 'I2.operator +(I1, I2)'
                //         var z = x + y;
                Diagnostic(ErrorCode.ERR_AmbigOperator, "+").WithArguments("I1.operator +(I1, I2)", "I2.operator +(I1, I2)").WithLocation(8, 19),
                // (9,13): error CS0019: Operator '+' cannot be applied to operands of type 'I2' and 'I1'
                //         z = y + x;
                Diagnostic(ErrorCode.ERR_BadBinaryOps, "y + x").WithArguments("+", "I2", "I1").WithLocation(9, 13)
            };

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(expected);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics(expected);
        }

        [Fact]
        public void Operators_14()
        {
            var source1 =
@"
public interface I1
{
    public static int operator +(I2 x, I1 y)
    {
        System.Console.WriteLine(""I1.+"");
        return 1;
    }
}

public interface I2 : I1
{
    public static int operator +(I2 x, I1 y)
    {
        System.Console.WriteLine(""I2.+"");
        return 2;
    }
}
";

            var source2 =
@"
class Test2 : I2
{
    static void Main()
    {
        I1 x = new Test2();
        I2 y = new Test2();
        var z = y + x;
        z = x + y;
    }
}
";

            var expected = new DiagnosticDescription[]
            {
                // (8,19): error CS9342: Operator resolution is ambiguous between the following members: 'I2.operator +(I2, I1)' and 'I1.operator +(I2, I1)'
                //         var z = y + x;
                Diagnostic(ErrorCode.ERR_AmbigOperator, "+").WithArguments("I2.operator +(I2, I1)", "I1.operator +(I2, I1)").WithLocation(8, 19),
                // (9,13): error CS0019: Operator '+' cannot be applied to operands of type 'I1' and 'I2'
                //         z = x + y;
                Diagnostic(ErrorCode.ERR_BadBinaryOps, "x + y").WithArguments("+", "I1", "I2").WithLocation(9, 13)
            };

            var compilation1 = CreateCompilation(source1, options: TestOptions.DebugDll,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics(expected);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics(expected);
        }

        [Fact]
        public void Operators_15()
        {
            var source1 =
@"
public class I1
{
    public static int operator +(I1 x, C2 y)
    {
        System.Console.WriteLine(""I1.+1"");
        return 1;
    }
    public static int operator +(C2 x, I1 y)
    {
        System.Console.WriteLine(""I1.+2"");
        return 1;
    }
}

public class I2 : I1
{
    public static int operator +(I2 x, C1 y)
    {
        System.Console.WriteLine(""I2.+1"");
        return 1;
    }
    public static int operator +(C1 x, I2 y)
    {
        System.Console.WriteLine(""I2.+2"");
        return 1;
    }
}

public class C1 { }
public class C2 : C1 { }
";

            var source2 =
@"
class Test2: I2
{
    static void Main()
    {
        I2 x = new Test2();
        C2 y = new C2();
        var z = x + y;
        z = y + x;    }
}
";

            var expectedOutput =
@"
I2.+1
I2.+2
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_16()
        {
            var source1 =
@"
public interface I1<T>
{
    public static int operator +(I1<T> x, I1<T> y)
    {
        System.Console.WriteLine(""I1.+"");
        return 1;
    }
}
";

            var source2 =
@"
class Test2: I1<object>
{
    static void Main()
    {
        I1<object> x = new Test2();
        I1<dynamic> y = new Test2();
        var z = x + y;
        z = y + x;
    }
}
";

            var expectedOutput =
@"
I1.+
I1.+
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_17()
        {
            var source1 =
@"
public interface I1
{
    public static I1 operator +(I1 x, C1 y)
    {
        System.Console.WriteLine(""I1.+1"");
        return x;
    }
    public static I1 operator +(C1 x, I1 y)
    {
        System.Console.WriteLine(""I1.+2"");
        return y;
    }
}

public interface I2 : I1
{}

public class C1{} 
";

            var source2 =
@"
class Test2 : I2
{
    static void Main()
    {
        I2 x = new Test2();
        C1 y = new C1();
        var z = x + y;
        z = y + x;
    }
}
";

            var expectedOutput =
@"
I1.+1
I1.+2
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var source3 =
@"
class Test2 : I2
{
    static void Main()
    {
        Test2 x = new Test2();
        C1 y = new C1();
        var z = x + y;
        z = y + x;
    }
}
";

            var compilation9 = CreateCompilation(source3, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            var expected9 = new DiagnosticDescription[]
            {
                // (8,17): error CS0019: Operator '+' cannot be applied to operands of type 'Test2' and 'C1'
                //         var z = x + y;
                Diagnostic(ErrorCode.ERR_BadBinaryOps, "x + y").WithArguments("+", "Test2", "C1").WithLocation(8, 17),
                // (9,13): error CS0019: Operator '+' cannot be applied to operands of type 'C1' and 'Test2'
                //         z = y + x;
                Diagnostic(ErrorCode.ERR_BadBinaryOps, "y + x").WithArguments("+", "C1", "Test2").WithLocation(9, 13)
            };
            compilation9.VerifyDiagnostics(expected9);

            var compilation10 = CreateCompilation(source3, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                  parseOptions: TestOptions.Regular7_3, targetFramework: TargetFramework.NetCoreApp);

            compilation10.VerifyDiagnostics(expected9);
        }

        [Fact]
        public void Operators_18()
        {
            var source1 =
@"
public interface I1
{
    public static int operator +(I1 x, I1 y)
    {
        System.Console.WriteLine(""I1.+"");
        return 1;
    }
}

public interface I2 : I1
{}
";

            var source2 =
@"
class Test2: I2
{
    static void Main()
    {
        I2 x = new Test2();
        I1 y = new Test2();
        var z = x + y;
        z = y + x;
        z = x + x;
        z = y + y;
    }
}
";

            var expectedOutput =
@"
I1.+
I1.+
I1.+
I1.+
";

            var compilation1 = CreateCompilation(source1 + source2, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation1.VerifyDiagnostics();
            CompileAndVerify(compilation1, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            CompilationReference compilationReference = compilation1.ToMetadataReference();
            MetadataReference metadataReference = compilation1.EmitToImageReference();

            var compilation2 = CreateCompilation(source2, new[] { compilationReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation2.VerifyDiagnostics();
            CompileAndVerify(compilation2, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);

            var compilation3 = CreateCompilation(source2, new[] { metadataReference }, options: TestOptions.DebugExe,
                                                 parseOptions: TestOptions.Regular, targetFramework: TargetFramework.NetCoreApp);

            compilation3.VerifyDiagnostics();
            CompileAndVerify(compilation3, verify: VerifyOnMonoOrCoreClr, expectedOutput: !ExecutionConditionUtil.IsMonoOrCoreClr ? null : expectedOutput);
        }

        [Fact]
        public void Operators_19()
        {
            var source1 =
@"
public interface I1<T>
{
    public static int operator +(I1<T> x, I1<T> y)
    {
        System.Console.WriteLine(""I1.+"");
        return 1;
    }
}

public interface I2<T> : I1<T>
{}
";

            var source2 =
@"
class Test2: I2<object>
{
    static void Main()
    {
        I2<object> x = new