Wrong error when compiling code containing IEnumerable<Span<T>>

[aannenko]

Version Used:
9.0.100

Steps to Reproduce:

1. dotnet new console
2. Replace contents of the Program.cs with this:

foreach (var item in IntsAsStrings(5))
    Console.WriteLine(item.ToString());

// intentionally invalid code
static IEnumerable<ReadOnlySpan<char>> IntsAsStrings(int count)
{
    var chars = new char[11];
    for (var i = 0; i < count; i++)
    {
        i.TryFormat(chars, out var charsWritten);
        yield return chars.AsSpan(0, charsWritten);
    }
}

3. Try compiling it.

Diagnostic Id:

Severity	Code	Description	Project	File	Line	Suppression State
Error (active)	MSB6006	"csc.exe" exited with code -2146232797.	test	C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Current\Bin\Roslyn\Microsoft.CSharp.Core.targets	89

Expected Behavior:
Like in .NET 8, we get an appropriate error:

CS9244 The type 'ReadOnlySpan' may not be a ref struct or a type parameter allowing ref structs in order to use it as parameter 'T' in the generic type or method 'IEnumerable'

Actual Behavior:
csc.exe tries and fails to compile the above code.

dotnet build produces the following output:

PS D:\Code\test> dotnet build
Restore complete (0.2s)
  test failed with 58 error(s) (8.0s)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error : Process terminated. System.InvalidOperationException: Unexpected value 'NoConversion' of type 'Microsoft.CodeAnalysis.CSharp.ConversionKind'
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitConversion(BoundConversion conversion)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitConversionExpression(BoundConversion conversion, Boolean used)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitExpressionCoreWithStackGuard(BoundExpression expression, Boolean used)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitExpression(BoundExpression expression, Boolean used)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitReturnStatement(BoundReturnStatement boundReturnStatement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(ImmutableArray`1 statements)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitUninstrumentedBlock(BoundBlock block)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.GenerateImpl()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.GenerateMethodBody(PEModuleBuilder moduleBuilder, MethodSymbol method, Int32 methodOrdinal, BoundStatement block, ImmutableArray`1 lambdaDebugInfo, ImmutableArray`1 orderedLambdaRuntimeRudeEdits, ImmutableArray`1 closureDebugInfo, ImmutableArray`1 stateMachineStateDebugInfos, StateMachineTypeSymbol stateMachineTypeOpt, VariableSlotAllocator variableSlotAllocatorOpt, BindingDiagnosticBag diagnostics, DebugDocumentProvider debugDocumentProvider, ImportChain importChainOpt, Boolean emittingPdb, ImmutableArray`1 codeCoverageSpans, AsyncForwardEntryPoint entryPointOpt)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileSynthesizedMethods(TypeCompilationState compilationState)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Environment.FailFast(System.Runtime.CompilerServices.StackCrawlMarkHandle, System.String, System.Runtime.CompilerServices.ObjectHandleOnStack, System.String)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Environment.FailFast(System.Threading.StackCrawlMark ByRef, System.String, System.Exception, System.String)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Environment.FailFast(System.String, System.Exception)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity, Boolean)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.ErrorReporting.FatalError.ReportAndPropagateUnlessCanceled(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler+<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Runtime.EH.FindFirstPassHandler(System.Object, UInt32, System.Runtime.StackFrameIterator ByRef, UInt32 ByRef, Byte* ByRef)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Runtime.EH.DispatchEx(System.Runtime.StackFrameIterator ByRef, ExInfo ByRef)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Runtime.EH.RhThrowEx(System.Object, ExInfo ByRef)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitConversion(Microsoft.CodeAnalysis.CSharp.BoundConversion)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitConversionExpression(Microsoft.CodeAnalysis.CSharp.BoundConversion, Boolean)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitExpressionCoreWithStackGuard(Microsoft.CodeAnalysis.CSharp.BoundExpression, Boolean)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitExpression(Microsoft.CodeAnalysis.CSharp.BoundExpression, Boolean)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitReturnStatement(Microsoft.CodeAnalysis.CSharp.BoundReturnStatement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(Microsoft.CodeAnalysis.CSharp.BoundStatement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.CSharp.BoundStatement>)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitUninstrumentedBlock(Microsoft.CodeAnalysis.CSharp.BoundBlock)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(Microsoft.CodeAnalysis.CSharp.BoundStatement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.GenerateImpl()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.GenerateMethodBody(Microsoft.CodeAnalysis.CSharp.Emit.PEModuleBuilder, Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol, Int32, Microsoft.CodeAnalysis.CSharp.BoundStatement, System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.Emit.EncLambdaInfo>, System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.CodeGen.LambdaRuntimeRudeEditInfo>, System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.Emit.EncClosureInfo>, System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.CodeGen.StateMachineStateDebugInfo>, Microsoft.CodeAnalysis.CSharp.StateMachineTypeSymbol, Microsoft.CodeAnalysis.CodeGen.VariableSlotAllocator, Microsoft.CodeAnalysis.CSharp.BindingDiagnosticBag, Microsoft.CodeAnalysis.CodeGen.DebugDocumentProvider, Microsoft.CodeAnalysis.CSharp.ImportChain, Boolean, System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.CodeGen.SourceSpan>, AsyncForwardEntryPoint)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileSynthesizedMethods(Microsoft.CodeAnalysis.CSharp.TypeCompilationState)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler+<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(System.Threading.Thread, System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef, System.Threading.Thread)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Threading.ThreadPoolWorkQueue.Dispatch()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at System.Threading.PortableThreadPool+WorkerThread.WorkerThreadStart()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error : System.InvalidOperationException: Unexpected value 'NoConversion' of type 'Microsoft.CodeAnalysis.CSharp.ConversionKind'
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitConversion(BoundConversion conversion)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitConversionExpression(BoundConversion conversion, Boolean used)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitExpressionCoreWithStackGuard(BoundExpression expression, Boolean used)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitExpression(BoundExpression expression, Boolean used)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitReturnStatement(BoundReturnStatement boundReturnStatement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(ImmutableArray`1 statements)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitUninstrumentedBlock(BoundBlock block)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.GenerateImpl()
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.GenerateMethodBody(PEModuleBuilder moduleBuilder, MethodSymbol method, Int32 methodOrdinal, BoundStatement block, ImmutableArray`1 lambdaDebugInfo, ImmutableArray`1 orderedLambdaRuntimeRudeEdits, ImmutableArray`1 closureDebugInfo, ImmutableArray`1 stateMachineStateDebugInfos, StateMachineTypeSymbol stateMachineTypeOpt, VariableSlotAllocator variableSlotAllocatorOpt, BindingDiagnosticBag diagnostics, DebugDocumentProvider debugDocumentProvider, ImportChain importChainOpt, Boolean emittingPdb, ImmutableArray`1 codeCoverageSpans, AsyncForwardEntryPoint entryPointOpt)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileSynthesizedMethods(TypeCompilationState compilationState)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType)
    C:\Program Files\dotnet\sdk\9.0.100\Roslyn\Microsoft.CSharp.Core.targets(89,5): error :    at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()

[jjonescz]

Looks like a duplicate of #75569.

[aannenko]

Yes, thanks. Should I close this one then?