Roslyn hits "unreachable" exception when processing type forward to a class containing an extension group

[tannergooding]

Version Used: 5.0.0-2.25412.102

Steps to Reproduce:

1. Create a new class library, ClassLibrary1, containing the following contents:

public static class Extensions
{
    extension(int)
    {
    }
}

2. Create another new class library, ClassLibrary2, which references the first and which contains the following contents:

[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(Extensions))]

-- Note: The original repro had a proper extension group containing members, this is a minified repro.

Expected Behavior:

The compilation should succeed.

Actual Behavior:

The compilation of ClassLibrary2 fails with the following:

  ClassLibrary1 succeeded (1.4s) → ClassLibrary1\bin\Release\net10.0\ClassLibrary1.dll
  ClassLibrary2 failed with 21 error(s) (3.2s)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error : Unhandled exception. System.InvalidOperationException: This program location is thought to be unreachable. File='/_/src/roslyn/src/Compilers/CSharp/Portable/Emitter/Model/NamedTypeSymbolAdapter.cs' Line=800
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol.Microsoft.Cci.INamedEntity.get_Name()
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.Cci.MetadataWriter.GetMetadataName(INamedTypeReference namedType, Int32 generation)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.Cci.MetadataWriter.PopulateExportedTypeTableRows()
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.Cci.MetadataWriter.PopulateTypeSystemTables(Int32[] methodBodyOffsets, Int32 mappedFieldDataStartOffset, PooledBlobBuilder& mappedFieldDataWriter, PooledBlobBuilder& resourceWriter, BlobBuilder dynamicAnalysisData, Blob& mvidFixup)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.Cci.MetadataWriter.BuildMetadataAndIL(PdbWriter nativePdbWriterOpt, BlobBuilder ilBuilder, PooledBlobBuilder& mappedFieldDataBuilder, PooledBlobBuilder& managedResourceDataBuilder, Blob& mvidFixup, Blob& mvidStringFixup)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.Cci.PeWriter.WritePeToStream(EmitContext context, CommonMessageProvider messageProvider, Func`1 getPeStream, Func`1 getPortablePdbStreamOpt, PdbWriter nativePdbWriterOpt, String pdbPathOpt, Boolean metadataOnly, Boolean isDeterministic, Boolean emitTestCoverageData, Nullable`1 privateKeyOpt, CancellationToken cancellationToken)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.Compilation.SerializePeToStream(CommonPEModuleBuilder moduleBeingBuilt, DiagnosticBag metadataDiagnostics, CommonMessageProvider messageProvider, Func`1 getPeStream, Func`1 getMetadataPeStreamOpt, Func`1 getPortablePdbStreamOpt, PdbWriter nativePdbWriterOpt, String pdbPathOpt, RebuildData rebuildData, Boolean metadataOnly, Boolean includePrivateMembers, Boolean isDeterministic, Boolean emitTestCoverageData, Nullable`1 privateKeyOpt, CancellationToken cancellationToken)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.Compilation.SerializeToPeStream(CommonPEModuleBuilder moduleBeingBuilt, EmitStreamProvider peStreamProvider, EmitStreamProvider metadataPEStreamProvider, EmitStreamProvider pdbStreamProvider, RebuildData rebuildData, Func`2 testSymWriterFactory, DiagnosticBag diagnostics, EmitOptions emitOptions, Nullable`1 privateKeyOpt, CancellationToken cancellationToken)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommonCompiler.CompileAndEmit(TouchedFileLogger touchedFilesLogger, Compilation& compilation, ImmutableArray`1 analyzers, ImmutableArray`1 generators, ImmutableArray`1 additionalTextFiles, AnalyzerConfigSet analyzerConfigSet, ImmutableArray`1 sourceFileAnalyzerConfigOptions, ImmutableArray`1 embeddedTexts, DiagnosticBag diagnostics, ErrorLogger errorLogger, CancellationToken cancellationToken, CancellationTokenSource& analyzerCts, AnalyzerDriver& analyzerDriver, Nullable`1& generatorTimingInfo)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommonCompiler.RunCore(TextWriter consoleOutput, ErrorLogger errorLogger, CancellationToken cancellationToken)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommonCompiler.Run(TextWriter consoleOutput, CancellationToken cancellationToken)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CSharp.CommandLine.Csc.<>c__DisplayClass1_0.<Run>b__0(TextWriter tw)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommandLine.ConsoleUtil.RunWithUtf8Output[T](Func`2 func)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommandLine.ConsoleUtil.RunWithUtf8Output[T](Boolean utf8Output, TextWriter textWriter, Func`2 func)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CSharp.CommandLine.Csc.Run(String[] args, BuildPaths buildPaths, TextWriter textWriter, IAnalyzerAssemblyLoader analyzerLoader)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommandLine.BuildClient.RunLocalCompilation(String[] arguments, BuildPaths buildPaths, TextWriter textWriter)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommandLine.BuildClient.RunCompilation(IEnumerable`1 originalArguments, BuildPaths buildPaths, TextWriter textWriter, String pipeName)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CommandLine.BuildClient.Run(IEnumerable`1 arguments, RequestLanguage language, CompileFunc compileFunc, CompileOnServerFunc compileOnServerFunc, ICompilerServerLogger logger)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CSharp.CommandLine.Program.MainCore(String[] args)
    D:\Users\tagoo\.nuget\packages\microsoft.net.compilers.toolset\5.0.0-2.25412.102\tasks\netcore\Microsoft.CSharp.Core.targets(84,5): error :    at Microsoft.CodeAnalysis.CSharp.CommandLine.Program.Main(String[] args)

[tannergooding]

This was found in dotnet/runtime#117975

[jcouv]

Took a quick look:
The crash is in PopulateExportedTypeTableRows. It tries to add the language-level extension block type to the exported type table and fails because extension blocks don't have names.
The logic to determine exported types is in PEModuleBuilder.GetForwardedTypes. It iterates through types listed in TypeForwardedTo and includes all the non-private children.

For extensions, I think that logic should list the metadata-level types instead (ie. grouping and marker types).

We do this kind of replacement in GetNestedTypes in NamedTypeSymbolAdapter but that only needs to deal with source extension blocks.
For the type forwarding scenario, we're dealing with PE symbols.
PENamedTypeSymbol holds some information that relates metadata-level grouping and marker types with language-level extension blocks (in PENamedTypeSymbol.ExtensionInfo) but I'm not sure if it's in a convenient shape for this purpose.

Repro:

    [Fact]
    public void TODO2()
    {
        var libSrc = """
public static class Extensions
{
    extension(int)
    {
    }
}
""";
        var libComp = CreateCompilation(libSrc);
        libComp.VerifyEmitDiagnostics();

        var src = """
[assembly: System.Runtime.CompilerServices.TypeForwardedTo(typeof(Extensions))]
""";
        var comp = CreateCompilation(src, references: [libComp.EmitToImageReference()]);
        comp.VerifyEmitDiagnostics();
    }

FYI @AlekseyTs

[333fred]

@jcouv took a look at this: maybe I'm missing something, but I'm not sure why we wouldn't just return the extension grouping name here:

roslyn/src/Compilers/CSharp/Portable/Emitter/Model/NamedTypeSymbolAdapter.cs

Line 800 in f388cb7

throw ExceptionUtilities.Unreachable();

, instead of throwing. That seems like it should make sure the grouping types get forwarded correctly?

[AlekseyTs]

I'll try to take a look shortly