Assert fires in EnsureMetadataVirtual

[AlekseyTs]

Assert fires in EnsureMetadataVirtual when the tests below are executed with CompilationExtensions.EnableVerifyUsedAssemblies set to true.

            public void EnsureMetadataVirtual()
            {
                // ACASEY: This assert is here to check that we're not mutating the value of IsMetadataVirtual after
                // someone has consumed it.  The best practice is to not access IsMetadataVirtual before ForceComplete
                // has been called on all SourceNamedTypeSymbols.  If it is necessary to do so, then you can pass
                // ignoreInterfaceImplementationChanges: true, but you must be conscious that seeing "false" may not
                // reflect the final, emitted modifier.
                Debug.Assert(!IsMetadataVirtualLocked);
                if ((_flags & IsMetadataVirtualBit) == 0)
                {
                    ThreadSafeFlagOperations.Set(ref _flags, IsMetadataVirtualBit);
                }
            }

Call stack:

>	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceMemberMethodSymbol.Flags.EnsureMetadataVirtual() Line 272	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceMemberMethodSymbol.EnsureMetadataVirtual() Line 569	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceMemberContainerTypeSymbol.SynthesizeInterfaceMemberImplementation(Microsoft.CodeAnalysis.CSharp.Symbols.TypeSymbol.SymbolAndDiagnostics implementingMemberAndDiagnostics, Microsoft.CodeAnalysis.CSharp.Symbol interfaceMember) Line 1852	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceMemberContainerTypeSymbol.ComputeInterfaceImplementations(Microsoft.CodeAnalysis.CSharp.BindingDiagnosticBag diagnostics, System.Threading.CancellationToken cancellationToken) Line 179	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceMemberContainerTypeSymbol.GetSynthesizedExplicitImplementations(System.Threading.CancellationToken cancellationToken) Line 52	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceMemberContainerTypeSymbol.ForceComplete(Microsoft.CodeAnalysis.SourceLocation locationOpt, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> filter, System.Threading.CancellationToken cancellationToken) Line 610	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbol.ForceCompleteMemberConditionally(Microsoft.CodeAnalysis.SourceLocation locationOpt, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> filter, Microsoft.CodeAnalysis.CSharp.Symbol member, System.Threading.CancellationToken cancellationToken) Line 964	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceNamespaceSymbol.ForceComplete(Microsoft.CodeAnalysis.SourceLocation locationOpt, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> filter, System.Threading.CancellationToken cancellationToken) Line 80	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceModuleSymbol.ForceComplete(Microsoft.CodeAnalysis.SourceLocation locationOpt, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> filter, System.Threading.CancellationToken cancellationToken) Line 272	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SourceAssemblySymbol.ForceComplete(Microsoft.CodeAnalysis.SourceLocation locationOpt, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> filter, System.Threading.CancellationToken cancellationToken) Line 929	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetSourceDeclarationDiagnostics(Microsoft.CodeAnalysis.SyntaxTree syntaxTree, Microsoft.CodeAnalysis.Text.TextSpan? filterSpanWithinTree, System.Func<System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.Diagnostic>, Microsoft.CodeAnalysis.SyntaxTree, Microsoft.CodeAnalysis.Text.TextSpan?, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.Diagnostic>> locationFilterOpt, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> symbolFilter, System.Threading.CancellationToken cancellationToken) Line 3214	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetDiagnosticsWithoutSeverityFiltering(Microsoft.CodeAnalysis.CompilationStage stage, bool includeEarlierStages, Microsoft.CodeAnalysis.CSharp.BindingDiagnosticBag builder, System.Predicate<Microsoft.CodeAnalysis.CSharp.Symbol> symbolFilter, System.Threading.CancellationToken cancellationToken) Line 2988	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetCompleteSetOfUsedAssemblies(System.Threading.CancellationToken cancellationToken) Line 71	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetCompleteSetOfUsedAssemblies.__completeTheSetOfUsedAssemblies|339_2(bool seenErrors, System.Threading.CancellationToken cancellationToken) Line 170	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetCompleteSetOfUsedAssemblies(System.Threading.CancellationToken cancellationToken) Line 86	C#
 	Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetUsedAssemblyReferences(System.Threading.CancellationToken cancellationToken) Line 25	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.VerifyUsedAssemblyReferences(System.Func<Microsoft.CodeAnalysis.CSharp.CSharpCompilation> createCompilationLambda) Line 1312	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.ValidateCompilation(System.Func<Microsoft.CodeAnalysis.CSharp.CSharpCompilation> createCompilationLambda) Line 1300	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateCompilationCore(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, string assemblyName, string sourceFileName, bool skipUsesIsNullable, Microsoft.CodeAnalysis.CSharp.MessageID? experimentalFeature, bool skipExtraValidation) Line 1275	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateEmptyCompilation(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, string assemblyName, string sourceFileName, bool skipUsesIsNullable, bool skipExtraValidation) Line 1239	C#
 	Microsoft.CodeAnalysis.CSharp.Test.Utilities.dll!Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestBase.CreateCompilation(Microsoft.CodeAnalysis.CSharp.Test.Utilities.CSharpTestSource source, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references, Microsoft.CodeAnalysis.CSharp.CSharpCompilationOptions options, Microsoft.CodeAnalysis.CSharp.CSharpParseOptions parseOptions, Roslyn.Test.Utilities.TargetFramework targetFramework, string assemblyName, string sourceFileName, bool skipUsesIsNullable) Line 1228	C#
 	Microsoft.CodeAnalysis.CSharp.Emit2.UnitTests.dll!Microsoft.CodeAnalysis.CSharp.UnitTests.Semantics.ParamsCollectionTests.Test1() Line 15795	C#

The tests:

        [Fact]
        public void Test1()
        {
            var src1 = @"
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

public struct S : IAsyncEnumerable<int>
{
    public IAsyncEnumerator<int> GetAsyncEnumerator(CancellationToken token = default) => throw null;
}
";

            var comp1 = CreateCompilation(src1, targetFramework: TargetFramework.Net80);

            var src2 = @"
using System.Threading.Tasks;

class C
{
    static async Task Main()
    {
        await foreach (var i in new S())
        {
        }
    }
}
";
            var comp2 = CreateCompilation(src2, references: [comp1.ToMetadataReference()], targetFramework: TargetFramework.Net80);
            comp2.VerifyEmitDiagnostics();
        }

        [Fact]
        public void Test2()
        {
            var src1 = @"
using System;
using System.Threading.Tasks;

public struct S2 : IAsyncDisposable
{
    public ValueTask DisposeAsync()
    {
        return ValueTask.CompletedTask;
    }
}
";

            var comp1 = CreateCompilation(src1, targetFramework: TargetFramework.Net80);

            var src2 = @"
class C
{
    static async System.Threading.Tasks.Task Main()
    {
        await using (new S2())
        {
        }

        await using (var s = new S2())
        {
        }
    }
}
";
            var comp2 = CreateCompilation(src2, references: [comp1.ToMetadataReference()], targetFramework: TargetFramework.Net80);
            comp2.VerifyEmitDiagnostics();
        }

[jaredpar]

@jcouv to assess impact of potential async issue.