Roslyn crashed in generic when assigned await expression into ref returned locals

[egortech]

Version Used: Compiler version: '4.12.0-3.24473.3 (5ef52ae)'. Language version: latest (13.0).

Steps to Reproduce:

https://godbolt.org/z/eM96s7rKo

https://godbolt.org/z/PG8fcnP8q

Expected Behavior: Error CS8178.

Actual Behavior:

csc crash:

Process terminated. System.InvalidCastException: Unable to cast object of type 'Microsoft.CodeAnalysis.CSharp.Symbols.TypeSubstitutedLocalSymbol' to type 'Microsoft.CodeAnalysis.CSharp.Symbols.SynthesizedLocal'.

Full StackTrace

Process terminated. System.InvalidCastException: Unable to cast object of type 'Microsoft.CodeAnalysis.CSharp.Symbols.TypeSubstitutedLocalSymbol' to type 'Microsoft.CodeAnalysis.CSharp.Symbols.SynthesizedLocal'.
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.VisitAssignmentOperator(BoundAssignmentOperator node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriterWithStackGuard.VisitExpressionWithoutStackGuard(BoundExpression node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeVisitor.VisitExpressionWithStackGuard(BoundExpression node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeVisitor.VisitExpressionWithStackGuard(Int32& recursionDepth, BoundExpression node)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.VisitExpressionStatement(BoundExpressionStatement node)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.DoVisitList[T](ImmutableArray`1 list)
   at Microsoft.CodeAnalysis.CSharp.Symbols.MethodToClassRewriter.VisitBlock(BoundBlock node, Boolean removeInstrumentation)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.PossibleIteratorScope(ImmutableArray`1 locals, Func`1 wrapped)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.VisitSequencePoint(BoundSequencePoint node)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.DoVisitList[T](ImmutableArray`1 list)
   at Microsoft.CodeAnalysis.CSharp.Symbols.MethodToClassRewriter.VisitBlock(BoundBlock node, Boolean removeInstrumentation)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.PossibleIteratorScope(ImmutableArray`1 locals, Func`1 wrapped)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.VisitBody(BoundStatement body)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.GenerateMoveNext(BoundStatement body, MethodSymbol moveNextMethod)
   at Microsoft.CodeAnalysis.CSharp.AsyncRewriter.GenerateMethodImplementations()
   at Microsoft.CodeAnalysis.CSharp.StateMachineRewriter.Rewrite()
   at Microsoft.CodeAnalysis.CSharp.AsyncRewriter.Rewrite(BoundStatement bodyWithAwaitLifted, MethodSymbol method, Int32 methodOrdinal, ArrayBuilder`1 stateMachineStateDebugInfoBuilder, VariableSlotAllocator slotAllocatorOpt, TypeCompilationState compilationState, BindingDiagnosticBag diagnostics, AsyncStateMachine& stateMachineType)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileSynthesizedMethods(TypeCompilationState compilationState)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()
   at System.Environment.FailFast(System.Runtime.CompilerServices.StackCrawlMarkHandle, System.String, System.Runtime.CompilerServices.ObjectHandleOnStack, System.String)
   at System.Environment.FailFast(System.Threading.StackCrawlMark ByRef, System.String, System.Exception, System.String)
   at System.Environment.FailFast(System.String, System.Exception)
   at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity, Boolean)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.ReportAndPropagateUnlessCanceled(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler+<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()
   at System.Runtime.EH.FindFirstPassHandler(System.Object, UInt32, System.Runtime.StackFrameIterator ByRef, UInt32 ByRef, Byte* ByRef)
   at System.Runtime.EH.DispatchEx(System.Runtime.StackFrameIterator ByRef, ExInfo ByRef)
   at System.Runtime.EH.RhThrowEx(System.Object, ExInfo ByRef)
   at System.Runtime.CompilerServices.CastHelpers.ChkCast_Helper(Void*, System.Object)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.VisitAssignmentOperator(Microsoft.CodeAnalysis.CSharp.BoundAssignmentOperator)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriterWithStackGuard.VisitExpressionWithoutStackGuard(Microsoft.CodeAnalysis.CSharp.BoundExpression)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeVisitor.VisitExpressionWithStackGuard(Microsoft.CodeAnalysis.CSharp.BoundExpression)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeVisitor.VisitExpressionWithStackGuard(Int32 ByRef, Microsoft.CodeAnalysis.CSharp.BoundExpression)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(Microsoft.CodeAnalysis.CSharp.BoundNode)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.VisitExpressionStatement(Microsoft.CodeAnalysis.CSharp.BoundExpressionStatement)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(Microsoft.CodeAnalysis.CSharp.BoundNode)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.DoVisitList[[System.__Canon, System.Private.CoreLib, Version=9.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]](System.Collections.Immutable.ImmutableArray`1<System.__Canon>)
   at Microsoft.CodeAnalysis.CSharp.Symbols.MethodToClassRewriter.VisitBlock(Microsoft.CodeAnalysis.CSharp.BoundBlock, Boolean)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.PossibleIteratorScope(System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.CSharp.Symbols.LocalSymbol>, System.Func`1<Microsoft.CodeAnalysis.CSharp.BoundStatement>)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(Microsoft.CodeAnalysis.CSharp.BoundNode)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.VisitSequencePoint(Microsoft.CodeAnalysis.CSharp.BoundSequencePoint)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(Microsoft.CodeAnalysis.CSharp.BoundNode)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.DoVisitList[[System.__Canon, System.Private.CoreLib, Version=9.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]](System.Collections.Immutable.ImmutableArray`1<System.__Canon>)
   at Microsoft.CodeAnalysis.CSharp.Symbols.MethodToClassRewriter.VisitBlock(Microsoft.CodeAnalysis.CSharp.BoundBlock, Boolean)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.PossibleIteratorScope(System.Collections.Immutable.ImmutableArray`1<Microsoft.CodeAnalysis.CSharp.Symbols.LocalSymbol>, System.Func`1<Microsoft.CodeAnalysis.CSharp.BoundStatement>)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(Microsoft.CodeAnalysis.CSharp.BoundNode)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.VisitBody(Microsoft.CodeAnalysis.CSharp.BoundStatement)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.GenerateMoveNext(Microsoft.CodeAnalysis.CSharp.BoundStatement, Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol)
   at Microsoft.CodeAnalysis.CSharp.AsyncRewriter.GenerateMethodImplementations()
   at Microsoft.CodeAnalysis.CSharp.StateMachineRewriter.Rewrite()
   at Microsoft.CodeAnalysis.CSharp.AsyncRewriter.Rewrite(Microsoft.CodeAnalysis.CSharp.BoundStatement, Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol, Int32, Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder`1<Microsoft.CodeAnalysis.CodeGen.StateMachineStateDebugInfo>, Microsoft.CodeAnalysis.CodeGen.VariableSlotAllocator, Microsoft.CodeAnalysis.CSharp.TypeCompilationState, Microsoft.CodeAnalysis.CSharp.BindingDiagnosticBag, Microsoft.CodeAnalysis.CSharp.AsyncStateMachine ByRef)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileSynthesizedMethods(Microsoft.CodeAnalysis.CSharp.TypeCompilationState)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler+<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()
   at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(System.Threading.Thread, System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef, System.Threading.Thread)
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading.PortableThreadPool+WorkerThread.WorkerThreadStart()
System.InvalidCastException: Unable to cast object of type 'Microsoft.CodeAnalysis.CSharp.Symbols.TypeSubstitutedLocalSymbol' to type 'Microsoft.CodeAnalysis.CSharp.Symbols.SynthesizedLocal'.
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.VisitAssignmentOperator(BoundAssignmentOperator node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriterWithStackGuard.VisitExpressionWithoutStackGuard(BoundExpression node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeVisitor.VisitExpressionWithStackGuard(BoundExpression node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeVisitor.VisitExpressionWithStackGuard(Int32& recursionDepth, BoundExpression node)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.VisitExpressionStatement(BoundExpressionStatement node)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.DoVisitList[T](ImmutableArray`1 list)
   at Microsoft.CodeAnalysis.CSharp.Symbols.MethodToClassRewriter.VisitBlock(BoundBlock node, Boolean removeInstrumentation)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.PossibleIteratorScope(ImmutableArray`1 locals, Func`1 wrapped)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.VisitSequencePoint(BoundSequencePoint node)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.BoundTreeRewriter.DoVisitList[T](ImmutableArray`1 list)
   at Microsoft.CodeAnalysis.CSharp.Symbols.MethodToClassRewriter.VisitBlock(BoundBlock node, Boolean removeInstrumentation)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.PossibleIteratorScope(ImmutableArray`1 locals, Func`1 wrapped)
   at Microsoft.CodeAnalysis.CSharp.MethodToStateMachineRewriter.Visit(BoundNode node)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.VisitBody(BoundStatement body)
   at Microsoft.CodeAnalysis.CSharp.AsyncMethodToStateMachineRewriter.GenerateMoveNext(BoundStatement body, MethodSymbol moveNextMethod)
   at Microsoft.CodeAnalysis.CSharp.AsyncRewriter.GenerateMethodImplementations()
   at Microsoft.CodeAnalysis.CSharp.StateMachineRewriter.Rewrite()
   at Microsoft.CodeAnalysis.CSharp.AsyncRewriter.Rewrite(BoundStatement bodyWithAwaitLifted, MethodSymbol method, Int32 methodOrdinal, ArrayBuilder`1 stateMachineStateDebugInfoBuilder, VariableSlotAllocator slotAllocatorOpt, TypeCompilationState compilationState, BindingDiagnosticBag diagnostics, AsyncStateMachine& stateMachineType)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileSynthesizedMethods(TypeCompilationState compilationState)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType)
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0()Program terminated with signal: SIGSEGV
Compiler returned: 139

Looks like affected all .NET version from 6.0 to 9.0

[AlekseyTs]

@egortech Please provide complete repro steps with relevant source code snippets. A link to an external resource is not a replacement, the resource might go offline temporarily or permanently. A privacy policy of the resource might not be acceptable due to one reason or the other. Etc. The issue should be self-contained.

[egortech]

@AlekseyTs Sure, without problems:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

public sealed class Foo<T>
{
	private readonly T[] _values = new T[10];

	public ref T this[int type] => ref _values[type];
}

public static class Foo
{
	public static async Task<Foo<TResult>> Bar<TResult>()
	{
		var res = new Foo<TResult>();

		var taskGroup = new List<KeyValuePair<int, Task<TResult>>>();

		await Task.WhenAll(taskGroup.Select(async kv =>
		{
			res[0] = await kv.Value;
		}));

		return res;
	}
}

using System.Threading.Tasks;

public sealed class Foo<T>
{
    private T _t;
    public ref T Get() => ref _t;
}

public static class Foo
{
    public static void Bar<T>()
    {
        var res = new Foo<T?>();

        async Task M()
        {
            res.Get() = await Task.FromResult(default(T?));
        }
    }
}

This code snipped in any .NET version crashed csc