Compiler crashing when using !! (Bang-Bang-Operator) in async methods

[Narumikazuchi]

Describe the bug

I'm using the latest preview of C#11 in some of my code. While using the new Bang-Bang-operator for null-checks I encountered an error with the build not completing due to an error with the compiler. After some time I found the source to be my async methods, which use the Bang-Bang-operator for some of their parameters.

To Reproduce

Opt-into preview features on .NET 6, use language version preview and create an async method which takes at least on parameter that is adorned with the Bang-Bang-operator. Upon trying to build, the build should fail.

Exceptions (if any)

MSB6006 "csc.exe" exited with code -2146232797.

StackTrace:
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : Process terminated. System.ArgumentException: An item with the same key has already been added. Key: ? [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Collections.Generic.Dictionary2.TryInsert(TKey key, TValue value, InsertionBehavior behavior) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Collections.Generic.Dictionary2.Add(TKey key, TValue value) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CodeGen.LocalSlotManager.DeclareLocal(ITypeReference type, ILocalSymbolInternal symbol, String name, SynthesizedLocalKind kind, LocalDebugId id, LocalVariableAttributes pdbAttributes, LocalSlotConstraints constraints, ImmutableArray1 dynamicTransformFlags, ImmutableArray1 tupleElementNames, Boolean isSlotReusable) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.DefineLocal(LocalSymbol local, SyntaxNode syntaxNode) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(BoundBlock block) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(ImmutableArray1 statements) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(BoundBlock block) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(ImmutableArray1 statements) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(BoundBlock block) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatementList(BoundStatementList list) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.GenerateImpl() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.GenerateMethodBody(PEModuleBuilder moduleBuilder, MethodSymbol method, Int32 methodOrdinal, BoundStatement block, ImmutableArray1 lambdaDebugInfo, ImmutableArray1 closureDebugInfo, StateMachineTypeSymbol stateMachineTypeOpt, VariableSlotAllocator variableSlotAllocatorOpt, BindingDiagnosticBag diagnostics, DebugDocumentProvider debugDocumentProvider, ImportChain importChainOpt, Boolean emittingPdb, Boolean emitTestCoverageData, ImmutableArray1 dynamicAnalysisSpans, AsyncForwardEntryPoint entryPointOpt) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethod(MethodSymbol methodSymbol, Int32 methodOrdinal, ProcessedFieldInitializers& processedInitializers, SynthesizedSubmissionFields previousSubmissionFields, TypeCompilationState compilationState) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Environment.FailFast(System.String, System.Exception) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.FailFast+<>c.<.cctor>b__5_0(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity, Boolean) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity, Boolean) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.ErrorReporting.FatalError.ReportAndPropagateUnlessCanceled(System.Exception, Microsoft.CodeAnalysis.ErrorReporting.ErrorSeverity) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler+<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException[[System.__Canon, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]](System.__Canon) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Collections.Generic.Dictionary2[[System.__Canon, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.__Canon, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].TryInsert(System.__Canon, System.__Canon, System.Collections.Generic.InsertionBehavior) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Collections.Generic.Dictionary2[[System.__Canon, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.__Canon, System.Private.CoreLib, Version=6.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].Add(System.__Canon, System.__Canon) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CodeGen.LocalSlotManager.DeclareLocal(Microsoft.Cci.ITypeReference, Microsoft.CodeAnalysis.Symbols.ILocalSymbolInternal, System.String, Microsoft.CodeAnalysis.SynthesizedLocalKind, Microsoft.CodeAnalysis.CodeGen.LocalDebugId, System.Reflection.Metadata.LocalVariableAttributes, Microsoft.CodeAnalysis.LocalSlotConstraints, System.Collections.Immutable.ImmutableArray1, System.Collections.Immutable.ImmutableArray1<System.String>, Boolean) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.DefineLocal(Microsoft.CodeAnalysis.CSharp.Symbols.LocalSymbol, Microsoft.CodeAnalysis.SyntaxNode) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(Microsoft.CodeAnalysis.CSharp.BoundBlock) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(Microsoft.CodeAnalysis.CSharp.BoundStatement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(System.Collections.Immutable.ImmutableArray1<Microsoft.CodeAnalysis.CSharp.BoundStatement>) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(Microsoft.CodeAnalysis.CSharp.BoundBlock) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(Microsoft.CodeAnalysis.CSharp.BoundStatement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(System.Collections.Immutable.ImmutableArray1<Microsoft.CodeAnalysis.CSharp.BoundStatement>) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(Microsoft.CodeAnalysis.CSharp.BoundBlock) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(Microsoft.CodeAnalysis.CSharp.BoundStatement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatementList(Microsoft.CodeAnalysis.CSharp.BoundStatementList) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(Microsoft.CodeAnalysis.CSharp.BoundStatement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.GenerateImpl() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): 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.ImmutableArray1<Microsoft.CodeAnalysis.CodeGen.LambdaDebugInfo>, System.Collections.Immutable.ImmutableArray1<Microsoft.CodeAnalysis.CodeGen.ClosureDebugInfo>, Microsoft.CodeAnalysis.CSharp.StateMachineTypeSymbol, Microsoft.CodeAnalysis.CodeGen.VariableSlotAllocator, Microsoft.CodeAnalysis.CSharp.BindingDiagnosticBag, Microsoft.CodeAnalysis.CodeGen.DebugDocumentProvider, Microsoft.CodeAnalysis.CSharp.ImportChain, Boolean, Boolean, System.Collections.Immutable.ImmutableArray1<Microsoft.CodeAnalysis.CodeGen.SourceSpan>, AsyncForwardEntryPoint) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethod(Microsoft.CodeAnalysis.CSharp.Symbols.MethodSymbol, Int32, ProcessedFieldInitializers ByRef, Microsoft.CodeAnalysis.CSharp.SynthesizedSubmissionFields, Microsoft.CodeAnalysis.CSharp.TypeCompilationState) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(Microsoft.CodeAnalysis.CSharp.Symbols.NamedTypeSymbol) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler+<>c__DisplayClass25_0.b__0() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Roslyn.Utilities.UICultureUtilities+<>c__DisplayClass5_0.b__0() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.Tasks.Task.InnerInvoke() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.Tasks.Task+<>c.<.cctor>b__272_0(System.Object) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.ExecutionContext.RunFromThreadPoolDispatchLoop(System.Threading.Thread, System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef, System.Threading.Thread) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.Tasks.Task.ExecuteEntryUnsafe(System.Threading.Thread) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.ThreadPoolWorkQueue.Dispatch() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.PortableThreadPool+WorkerThread.WorkerThreadStart() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Threading.Thread.StartCallback() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : System.ArgumentException: An item with the same key has already been added. Key: ? [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Collections.Generic.Dictionary2.TryInsert(TKey key, TValue value, InsertionBehavior behavior) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at System.Collections.Generic.Dictionary2.Add(TKey key, TValue value) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CodeGen.LocalSlotManager.DeclareLocal(ITypeReference type, ILocalSymbolInternal symbol, String name, SynthesizedLocalKind kind, LocalDebugId id, LocalVariableAttributes pdbAttributes, LocalSlotConstraints constraints, ImmutableArray1 dynamicTransformFlags, ImmutableArray1 tupleElementNames, Boolean isSlotReusable) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.DefineLocal(LocalSymbol local, SyntaxNode syntaxNode) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(BoundBlock block) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(ImmutableArray1 statements) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(BoundBlock block) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj] C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatements(ImmutableArray1 statements) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitBlock(BoundBlock block) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatementList(BoundStatementList list) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.EmitStatement(BoundStatement statement) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.CodeGen.CodeGenerator.GenerateImpl() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.GenerateMethodBody(PEModuleBuilder moduleBuilder, MethodSymbol method, Int32 methodOrdinal, BoundStatement block, ImmutableArray1 lambdaDebugInfo, ImmutableArray1 closureDebugInfo, StateMachineTypeSymbol stateMachineTypeOpt, VariableSlotAllocator variableSlotAllocatorOpt, BindingDiagnosticBag diagnostics, DebugDocumentProvider debugDocumentProvider, ImportChain importChainOpt, Boolean emittingPdb, Boolean emitTestCoverageData, ImmutableArray`1 dynamicAnalysisSpans, AsyncForwardEntryPoint entryPointOpt) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethod(MethodSymbol methodSymbol, Int32 methodOrdinal, ProcessedFieldInitializers& processedInitializers, SynthesizedSubmissionFields previousSubmissionFields, TypeCompilationState compilationState) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType) [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]
C:\Program Files\dotnet\sdk\6.0.202\Roslyn\Microsoft.CSharp.Core.targets(75,5): error : at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.b__0() [D:\Data\Software Development\Projects\EquinoxDb\Narumikazuchi.EquinoxDb\Narumikazuchi.EquinoxDb.csproj]

Further technical details

Running on Visual Studio 2022 17.4.1

dotnet --info:
.NET SDK (reflecting any global.json):
Version: 6.0.202
Commit: f8a5561

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18363
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\6.0.202\

Host (useful for support):
Version: 6.0.4
Commit: be98e88c76

.NET SDKs installed:
6.0.202 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 3.1.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.24 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.24 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[am11]

This operator is being removed from C# 11. See https://devblogs.microsoft.com/dotnet/csharp-11-preview-updates/#remove-parameter-null-checking-from-c-11 and dotnet/runtime#68178.

The crash you encountered in async method might be unexpected (if this feature is revived in the future).
cc @stephentoub

[stephentoub]

cc: @RikkiGibson

[baronfel]

Since !! was removed from C# for the .NET 7 release and is going under further development, I am going to close this issue for now. Thank you for the report!