`EnsureCSharpSymbolOrNull` throws in some IDE features

Hit in latest main, but likely a bug that has been since long and affects multiple IDE features. For example, https://github.com/dotnet/roslyn/issues/37333 was previously opened.

I don't have a minimal repro but I have insights as to what's going on.

First, this is the callstack I'm hitting this in:

Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.Symbols.SymbolExtensions.EnsureCSharpSymbolOrNull(Microsoft.CodeAnalysis.ISymbol symbol, string paramName) Line 378
	at /_/src/Compilers/CSharp/Portable/Symbols/SymbolExtensions.cs(378)
Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.ClassifyConversion(Microsoft.CodeAnalysis.ITypeSymbol source, Microsoft.CodeAnalysis.ITypeSymbol destination) Line 2412
	at /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs(2412)
Microsoft.CodeAnalysis.CSharp.dll!Microsoft.CodeAnalysis.CSharp.CSharpCompilation.ClassifyCommonConversion(Microsoft.CodeAnalysis.ITypeSymbol source, Microsoft.CodeAnalysis.ITypeSymbol destination) Line 2431
	at /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs(2431)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<System.__Canon>.ImplementInterfaceGenerator.CanForwardToConflictingMethod.AnonymousMethod__1((Microsoft.CodeAnalysis.IParameterSymbol p1, Microsoft.CodeAnalysis.IParameterSymbol p2) t) Line 82
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator_Method.cs(82)
System.Core.dll!System.Linq.Enumerable.Any<(Microsoft.CodeAnalysis.IParameterSymbol, Microsoft.CodeAnalysis.IParameterSymbol)>(System.Collections.Generic.IEnumerable<(Microsoft.CodeAnalysis.IParameterSymbol, Microsoft.CodeAnalysis.IParameterSymbol)> source, System.Func<(Microsoft.CodeAnalysis.IParameterSymbol, Microsoft.CodeAnalysis.IParameterSymbol), bool> predicate) Line 3148
	at System.Linq\Enumerable.cs(3148)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<Microsoft.CodeAnalysis.CSharp.Syntax.TypeDeclarationSyntax>.ImplementInterfaceGenerator.CanForwardToConflictingMethod(Microsoft.CodeAnalysis.Compilation compilation, Microsoft.CodeAnalysis.IMethodSymbol method, Microsoft.CodeAnalysis.IMethodSymbol conflictingMethod) Line 81
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator_Method.cs(81)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<Microsoft.CodeAnalysis.CSharp.Syntax.TypeDeclarationSyntax>.ImplementInterfaceGenerator.GenerateStatement(Microsoft.CodeAnalysis.Compilation compilation, Microsoft.CodeAnalysis.IMethodSymbol updatedMethod, Microsoft.CodeAnalysis.IMethodSymbol conflictingMethod) Line 53
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator_Method.cs(53)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<Microsoft.CodeAnalysis.CSharp.Syntax.TypeDeclarationSyntax>.ImplementInterfaceGenerator.GenerateMethod(Microsoft.CodeAnalysis.Compilation compilation, Microsoft.CodeAnalysis.IMethodSymbol method, Microsoft.CodeAnalysis.IMethodSymbol conflictingMethod, Microsoft.CodeAnalysis.Accessibility accessibility, Microsoft.CodeAnalysis.Editing.DeclarationModifiers modifiers, bool generateAbstractly, bool useExplicitInterfaceSymbol, string memberName) Line 36
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator_Method.cs(36)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<System.__Canon>.ImplementInterfaceGenerator.GenerateMembers(Microsoft.CodeAnalysis.Compilation compilation, Microsoft.CodeAnalysis.ISymbol member, Microsoft.CodeAnalysis.ISymbol conflictingMember, string memberName, bool generateInvisibly, bool generateAbstractly, bool addNew, bool addUnsafe, Microsoft.CodeAnalysis.ImplementType.ImplementTypePropertyGenerationBehavior propertyGenerationBehavior) Line 299
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator.cs(299)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<System.__Canon>.ImplementInterfaceGenerator.GenerateMembers(Microsoft.CodeAnalysis.Compilation compilation, Microsoft.CodeAnalysis.ParseOptions options, Microsoft.CodeAnalysis.ISymbol member, Microsoft.CodeAnalysis.PooledObjects.ArrayBuilder<Microsoft.CodeAnalysis.ISymbol> implementedVisibleMembers, Microsoft.CodeAnalysis.ImplementType.ImplementTypePropertyGenerationBehavior propertyGenerationBehavior) Line 217
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator.cs(217)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<Microsoft.CodeAnalysis.CSharp.Syntax.TypeDeclarationSyntax>.ImplementInterfaceGenerator.GenerateMembers(Microsoft.CodeAnalysis.Compilation compilation, Microsoft.CodeAnalysis.ParseOptions options, System.Collections.Immutable.ImmutableArray<(Microsoft.CodeAnalysis.INamedTypeSymbol type, System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.ISymbol> members)> unimplementedMembers, Microsoft.CodeAnalysis.ImplementType.ImplementTypePropertyGenerationBehavior propertyGenerationBehavior) Line 127
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator.cs(127)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<Microsoft.CodeAnalysis.CSharp.Syntax.TypeDeclarationSyntax>.ImplementInterfaceGenerator.ImplementInterfaceAsync(System.Collections.Immutable.ImmutableArray<(Microsoft.CodeAnalysis.INamedTypeSymbol type, System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.ISymbol> members)> unimplementedMembers, System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.ISymbol> extraMembers, System.Threading.CancellationToken cancellationToken) Line 98
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator.cs(98)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<System.__Canon>.ImplementInterfaceGenerator.ImplementInterfaceAsync(System.Threading.CancellationToken cancellationToken) Line 64
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/ImplementInterfaceGenerator.cs(64)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<Microsoft.CodeAnalysis.CSharp.Syntax.TypeDeclarationSyntax>.ImplementInterfaceAsync(Microsoft.CodeAnalysis.Document document, Microsoft.CodeAnalysis.ImplementInterface.ImplementInterfaceInfo info, Microsoft.CodeAnalysis.ImplementType.ImplementTypeOptions options, Microsoft.CodeAnalysis.ImplementInterface.ImplementInterfaceConfiguration configuration, System.Threading.CancellationToken cancellationToken) Line 116
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/AbstractImplementInterfaceService.cs(116)
Microsoft.CodeAnalysis.Features.dll!Microsoft.CodeAnalysis.ImplementInterface.AbstractImplementInterfaceService<System.__Canon>.GetCodeActionsAsync.AnonymousMethod__0(System.Threading.CancellationToken cancellationToken) Line 165
	at /_/src/Analyzers/Core/CodeFixes/ImplementInterface/AbstractImplementInterfaceService.cs(165)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.Create.AnonymousMethod__0(System.IProgress<Microsoft.CodeAnalysis.CodeAnalysisProgress> _, System.Threading.CancellationToken c) Line 487
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(487)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.DocumentChangeAction.GetChangedDocumentAsync(System.IProgress<Microsoft.CodeAnalysis.CodeAnalysisProgress> progress, System.Threading.CancellationToken cancellationToken) Line 694
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(694)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.GetChangedSolutionAsync(System.IProgress<Microsoft.CodeAnalysis.CodeAnalysisProgress> progress, System.Threading.CancellationToken cancellationToken) Line 356
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(356)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.ComputeOperationsAsync(System.IProgress<Microsoft.CodeAnalysis.CodeAnalysisProgress> progress, System.Threading.CancellationToken cancellationToken) Line 310
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(310)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.ComputePreviewOperationsAsync(System.Threading.CancellationToken cancellationToken) Line 324
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(324)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.DocumentChangeAction.ComputePreviewOperationsAsync(System.Threading.CancellationToken cancellationToken) Line 687
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(687)
Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.CodeActions.CodeAction.GetPreviewOperationsAsync(Microsoft.CodeAnalysis.Solution originalSolution, System.Threading.CancellationToken cancellationToken) Line 268
	at /_/src/Workspaces/Core/Portable/CodeActions/CodeAction.cs(268)
Microsoft.CodeAnalysis.EditorFeatures.dll!Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.EditorSuggestedAction.GetPreviewOperationsAsync(System.Threading.CancellationToken cancellationToken) Line 81
	at /_/src/EditorFeatures/Core/Suggestions/SuggestedActions/EditorSuggestedAction.cs(81)
[Resuming Async Method]
mscorlib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Line 980
	at f:\dd\ndp\clr\src\BCL\system\threading\executioncontext.cs(980)
mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state, bool preserveSyncCtx) Line 927
	at f:\dd\ndp\clr\src\BCL\system\threading\executioncontext.cs(927)
mscorlib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.MoveNextRunner.Run() Line 1070
	at f:\dd\ndp\clr\src\BCL\system\runtime\compilerservices\AsyncMethodBuilder.cs(1070)
mscorlib.dll!System.Runtime.CompilerServices.AsyncMethodBuilderCore.OutputAsyncCausalityEvents.AnonymousMethod__0() Line 975
	at f:\dd\ndp\clr\src\BCL\system\runtime\compilerservices\AsyncMethodBuilder.cs(975)
mscorlib.dll!System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem() Line 1246
	at f:\dd\ndp\clr\src\BCL\system\threading\threadpool.cs(1246)
mscorlib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch() Line 819
	at f:\dd\ndp\clr\src\BCL\system\threading\threadpool.cs(819)
[Async Call Stack]
[Async] Microsoft.CodeAnalysis.EditorFeatures.dll!Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.EditorSuggestedAction.GetPreviewResultAsync(System.Threading.CancellationToken cancellationToken) Line 214
	at /_/src/EditorFeatures/Core/Suggestions/SuggestedActions/EditorSuggestedAction.cs(214)
[Async] Microsoft.CodeAnalysis.EditorFeatures.dll!Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.EditorSuggestedActionWithNestedFlavors.GetPreviewAsync.AnonymousMethod__0(System.Threading.CancellationToken cancellationToken) Line 175
	at /_/src/EditorFeatures/Core/Suggestions/SuggestedActions/EditorSuggestedActionWithNestedFlavors.cs(175)
[Async] Microsoft.CodeAnalysis.Workspaces.dll!Microsoft.CodeAnalysis.Extensions.IExtensionManagerExtensions.PerformFunctionAsync<System.Collections.Generic.IReadOnlyList<object>>(Microsoft.CodeAnalysis.Extensions.IExtensionManager extensionManager, object extension, System.Func<System.Threading.CancellationToken, System.Threading.Tasks.Task<System.Collections.Generic.IReadOnlyList<object>>> function, System.Collections.Generic.IReadOnlyList<object> defaultValue, System.Threading.CancellationToken cancellationToken) Line 80
	at /_/src/Workspaces/Core/Portable/ExtensionManager/IExtensionManagerExtensions.cs(80)
[Async] Microsoft.CodeAnalysis.EditorFeatures.dll!Microsoft.CodeAnalysis.Editor.Implementation.Suggestions.EditorSuggestedActionWithNestedFlavors.GetPreviewAsync(System.Threading.CancellationToken cancellationToken) Line 171
	at /_/src/EditorFeatures/Core/Suggestions/SuggestedActions/EditorSuggestedActionWithNestedFlavors.cs(171)
[Async] Microsoft.VisualStudio.Platform.VSEditor.dll!Microsoft.VisualStudio.Text.Utilities.GuardedOperations.CallExtensionPointAsync<object>(object errorSource, System.Func<System.Threading.Tasks.Task<object>> asyncCall, object valueOnThrow) Line 654
	at D:\a\_work\1\s\src\Editor\Text\Util\TextDataUtil\GuardedOperations.cs(654)
[Async] Microsoft.VisualStudio.Platform.VSEditor.dll!Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulbSession.GetPreviewAsync(Microsoft.VisualStudio.Language.Intellisense.ISuggestedAction action, System.Threading.CancellationToken showPreviewCancellationToken) Line 532
	at D:\a\_work\1\s\src\Editor\Language\Impl\Intellisense\LightBulb\LightBulbSession.cs(532)
[Async] Microsoft.VisualStudio.Platform.VSEditor.dll!Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulbMenuItem.GetOrCreatePreviewAsync(System.Func<System.Threading.Tasks.Task<object>> previewCreator, System.Threading.CancellationToken cancellationToken) Line 755
	at D:\a\_work\1\s\src\Editor\Language\Impl\Intellisense\LightBulb\DefaultPresenter\LightBulbMenuItem.cs(755)
[Async] Microsoft.VisualStudio.Platform.VSEditor.dll!Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulbMenuItem.InitializeFlavoredAsync(Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulb lightBulbControl, System.Threading.CancellationToken cancellationToken) Line 273
	at D:\a\_work\1\s\src\Editor\Language\Impl\Intellisense\LightBulb\DefaultPresenter\LightBulbMenuItem.cs(273)
[Async] Microsoft.VisualStudio.Platform.VSEditor.dll!Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulbMenuItem.InitializeAsync(System.Threading.CancellationToken cancellationToken) Line 166
	at D:\a\_work\1\s\src\Editor\Language\Impl\Intellisense\LightBulb\DefaultPresenter\LightBulbMenuItem.cs(166)
[Async] Microsoft.VisualStudio.Platform.VSEditor.dll!Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulb.OnMenuItemGotKeyboardFocus(Microsoft.VisualStudio.Language.Intellisense.ISuggestedAction action, System.Windows.Input.KeyboardFocusChangedEventArgs e, Microsoft.VisualStudio.Language.Intellisense.Implementation.LightBulbMenuItem menuItem) Line 390
	at D:\a\_work\1\s\src\Editor\Language\Impl\Intellisense\LightBulb\DefaultPresenter\LightBulb.xaml.cs(390)

The symbol in question is CodeGenerationConstructedNamedTypeSymbol. This is IDE-specific implementation that the compiler isn't aware of and cannot deal with as of today.

The interesting stack frame looks like the following to me:

The bad symbol is likely coming from either EnsureNonConflictingNames or RemoveInaccessibleAttributesAndAttributesOfTypes.

Start with EnsureCSharpSymbolOrNull in src/Compilers/CSharp/Portable/Symbols/SymbolExtensions.cs and trace the call path through CSharpCompilation.cs into ImplementInterfaceGenerator_Method.cs. Compare the behavior with the context in issue #37333, then investigate a minimal reproduction for the reported IDE feature failure. Done means the affected IDE feature no longer throws, with regression coverage for the failure if an appropriate test location is identified.