Crash compiling extension static method

[stephentoub]

Version Used:
5.0.0-1.25206.1

Steps to Reproduce:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net9.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <LangVersion>Preview</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="5.0.0-1.25207.5">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
    </PackageReference>
  </ItemGroup>

</Project>

using System.Runtime.CompilerServices;

Console.WriteLine("Hello, World!");

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        extension<TSource>(IAsyncEnumerable<TSource> source)
        {
            /// <summary>
            /// Casts the elements of an <see cref="IAsyncEnumerable{Object}"/> to the specified type.
            /// </summary>
            /// <typeparam name="TResult">The type to cast the elements of source to.</typeparam>
            /// <returns>An <see cref="IAsyncEnumerable{TResult}"/> that contains each element of the source sequence cast to the <typeparamref name="TResult"/> type.</returns>
            public IAsyncEnumerable<TResult> Cast<TResult>() // satisfies the C# query-expression pattern
            {
                return Impl(source, default);

                static async IAsyncEnumerable<TResult> Impl(
                    IAsyncEnumerable<TSource?> source,
                    [EnumeratorCancellation] CancellationToken cancellationToken)
                {
                    await foreach (object? item in source.WithCancellation(cancellationToken))
                    {
                        yield return (TResult)item!;
                    }
                }
            }
        }
    }
}

Expected Behavior:
Compiles

Actual Behavior:

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.Cci.MetadataWriter.CheckNameLength(String name, INamedEntity errorEntity)
   at Microsoft.Cci.MetadataWriter.PopulateFieldTableRows()
   at Microsoft.Cci.MetadataWriter.PopulateTypeSystemTables(Int32[] methodBodyOffsets, PooledBlobBuilder& mappedFieldDataWriter, PooledBlobBuilder& resourceWriter, BlobBuilder dynamicAnalysisData, Blob& mvidFixup)
   at Microsoft.Cci.MetadataWriter.BuildMetadataAndIL(PdbWriter nativePdbWriterOpt, BlobBuilder ilBuilder, PooledBlobBuilder& mappedFieldDataBuilder, PooledBlobBuilder& managedResourceDataBuilder, Blob& mvidFixup, Blob& mvidStringFixup)
   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)
   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)
   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)
   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)
   at Microsoft.CodeAnalysis.CommonCompiler.RunCore(TextWriter consoleOutput, ErrorLogger errorLogger, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CommonCompiler.Run(TextWriter consoleOutput, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.CSharp.CommandLine.Csc.<>c__DisplayClass1_0.<Run>b__0(TextWriter tw)
   at Microsoft.CodeAnalysis.CommandLine.ConsoleUtil.RunWithUtf8Output[T](Func`2 func)
   at Microsoft.CodeAnalysis.CSharp.CommandLine.Csc.Run(String[] args, BuildPaths buildPaths, TextWriter textWriter, IAnalyzerAssemblyLoader analyzerLoader)
   at Microsoft.CodeAnalysis.CommandLine.BuildClient.RunLocalCompilation(String[] arguments, BuildPaths buildPaths, TextWriter textWriter)
   at Microsoft.CodeAnalysis.CommandLine.BuildClient.RunCompilation(IEnumerable`1 originalArguments, BuildPaths buildPaths, TextWriter textWriter, String pipeName)
   at Microsoft.CodeAnalysis.CommandLine.BuildClient.Run(IEnumerable`1 arguments, RequestLanguage language, CompileFunc compileFunc, CompileOnServerFunc compileOnServerFunc, ICompilerServerLogger logger)
   at Microsoft.CodeAnalysis.CSharp.CommandLine.Program.MainCore(String[] args)
   at Microsoft.CodeAnalysis.CSharp.CommandLine.Program.Main(String[] args)

[stephentoub]

cc: @jaredpar, @jcouv