at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at System.Threading.Tasks.Parallel.ForWorker[TLocal](Int32 fromInclusive, Int32 toExclusive, ParallelOptions parallelOptions, Action`1 body, Action`2 bodyWithState, Func`4 bodyWithLocal, Func`1 localInit, Action`1 localFinally)
   at System.Threading.Tasks.Parallel.For(Int32 fromInclusive, Int32 toExclusive, ParallelOptions parallelOptions, Action`1 body)
   at Roslyn.Utilities.RoslynParallel.For(Int32 fromInclusive, Int32 toExclusive, Action`1 body, CancellationToken cancellationToken) in /_/src/Compilers/Core/Portable/InternalUtilities/RoslynParallel.cs:line 23
   at Microsoft.CodeAnalysis.CSharp.Symbols.SourceNamespaceSymbol.ForceComplete(SourceLocation locationOpt, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamespaceSymbol_Completion.cs:line 56
   at Microsoft.CodeAnalysis.CSharp.Symbols.SourceModuleSymbol.ForceComplete(SourceLocation locationOpt, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Symbols/Source/SourceModuleSymbol.cs:line 267
   at Microsoft.CodeAnalysis.CSharp.Symbols.SourceAssemblySymbol.ForceComplete(SourceLocation locationOpt, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Symbols/Source/SourceAssemblySymbol.cs:line 913
   at Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetSourceDeclarationDiagnostics(SyntaxTree syntaxTree, Nullable`1 filterSpanWithinTree, Func`4 locationFilterOpt, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs:line 2951
   at Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetDiagnosticsWithoutFiltering(CompilationStage stage, Boolean includeEarlierStages, BindingDiagnosticBag builder, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs:line 2721
   at Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetDiagnostics(CompilationStage stage, Boolean includeEarlierStages, DiagnosticBag diagnostics, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs:line 2647
   at Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetDiagnostics(CompilationStage stage, Boolean includeEarlierStages, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs:line 2639
   at Microsoft.CodeAnalysis.CSharp.CSharpCompilation.GetDiagnostics(CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs:line 2633
   at Microsoft.CodeAnalysis.DiagnosticExtensions.VerifyDiagnostics[TCompilation](TCompilation c, DiagnosticDescription[] expected) in /_/src/Compilers/Test/Core/Diagnostics/DiagnosticExtensions.cs:line 103
   at Microsoft.CodeAnalysis.CSharp.UnitTests.Semantics.NullableReferenceTypesTests.NotNullWhenFalse_WithNullLiteral() in /_/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs:line 35693
----- Inner Stack Trace -----
   at Microsoft.CodeAnalysis.ThrowingTraceListener.Fail(String message, String detailMessage) in /_/src/Compilers/Test/Core/ThrowingTraceListener.cs:line 26
   at System.Diagnostics.TraceListener.Fail(String message)
   at System.Diagnostics.TraceInternal.Fail(String message)
   at System.Diagnostics.Debug.Assert(Boolean condition)
   at Microsoft.CodeAnalysis.CustomAttributesBag`1.get_IsEarlyDecodedWellKnownAttributeDataComputed() in /_/src/Compilers/Core/Portable/Symbols/Attributes/CustomAttributesBag.cs:line 179
   at Microsoft.CodeAnalysis.CSharp.Symbols.SourceNamedTypeSymbol.GetEarlyDecodedWellKnownAttributeData() in /_/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol.cs:line 855
   at Microsoft.CodeAnalysis.CSharp.Symbols.SourceNamedTypeSymbol.GetAppliedCon```  


From https://github.com/dotnet/roslyn/blob/MainSnapshot/src/Compilers/Core/Portable/Symbols/Attributes/CustomAttributesBag.cs#L179:

            // If late attributes are complete, early attributes must also be complete
            Debug.Assert(!IsPartComplete(CustomAttributeBagCompletionPart.DecodedWellKnownAttributeData) || earlyComplete);