JsonSourceGenerator can fail with global using aliases

[ltrzesniewski]

Description

Under some particular conditions, JsonSourceGenerator can fail with the following warning:

CSC : warning CS8785: Generator 'JsonSourceGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'InvalidOperationException' with message 'Unreachable'.

Reproduction Steps

Build the following project:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net6.0</TargetFramework>
    <LangVersion>13.0</LangVersion>
  </PropertyGroup>
</Project>

global using Foo = object;

Expected behavior

I do realize that net6.0 + C# 13 is not officially supported, but I didn't expect a source generator to fail. I expected this to compile successfully without warnings.

Actual behavior

This compiles successfully but JsonSourceGenerator fails.

The stack trace in the binlog is:

System.InvalidOperationException: Unreachable
at Microsoft.CodeAnalysis.DotnetRuntime.Extensions.RoslynExtensions.GetUnqualifiedName(NameSyntax name)
at Microsoft.CodeAnalysis.DotnetRuntime.Extensions.CSharpSyntaxHelper.AddAliases(SyntaxList`1 usings, ValueListBuilder`1& aliases, Boolean global)
at Microsoft.CodeAnalysis.DotnetRuntime.Extensions.CSharpSyntaxHelper.AddAliases(SyntaxNode node, ValueListBuilder`1& aliases, Boolean global)
at Microsoft.CodeAnalysis.DotnetRuntime.Extensions.SyntaxValueProviderExtensions.<ForAttributeWithSimpleName>g__getGlobalAliasesInCompilationUnit|5_7(SyntaxNode compilationUnit)
at Microsoft.CodeAnalysis.TransformNode`2.UpdateStateTable(Builder builder, NodeStateTable`1 previousTable, CancellationToken cancellationToken)

Regression?

I don't know. I was introducing this global using alias.

Known Workarounds

• using System.Object instead of object in the alias
• targeting a different framework version than net6.0

I suspect the generator doesn't handle keywords in global using aliases.

Configuration

Tested on Windows 11 x64 and Linux x64 (Ubuntu WSL) with the .NET 9 SDK:

.NET SDK:
Version: 9.0.100
Commit: 59db016f11
Workload version: 9.0.100-manifests.c6f19616
MSBuild version: 17.12.7+5b8665660

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22631
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\9.0.100\

Other information

I stumbled upon this by writing the following code in a multi-targeted project:

#if NET9_0_OR_GREATER
global using Lock = System.Threading.Lock;
#else
global using Lock = object;
#endif

Using System.Object fixed the issue.

[ltrzesniewski]

Looks like this thing is missing a PredefinedTypeSyntax branch:

runtime/src/libraries/Common/src/Roslyn/GetBestTypeByMetadataName.cs

Lines 178 to 185 in a25130b

	public static SimpleNameSyntax GetUnqualifiedName(this NameSyntax name)
	=> name switch
	{
	AliasQualifiedNameSyntax alias => alias.Name,
	QualifiedNameSyntax qualified => qualified.Right,
	SimpleNameSyntax simple => simple,
	_ => throw new InvalidOperationException("Unreachable"),
	};

and this probably also applies there:

runtime/src/tools/illink/src/ILLink.RoslynAnalyzer/CompilationExtensions.cs

Lines 156 to 162 in a25130b

	public static SimpleNameSyntax GetUnqualifiedName (this NameSyntax name)
	=> name switch {
	AliasQualifiedNameSyntax alias => alias.Name,
	QualifiedNameSyntax qualified => qualified.Right,
	SimpleNameSyntax simple => simple,
	_ => throw new InvalidOperationException ("Unreachable"),
	};

(I guess NameSyntax should be changed to TypeSyntax, but you get the idea)

[MichalPetryka]

I do realize that net6.0 + C# 13 is not officially supported, but I didn't expect a source generator to fail. I expected this to compile successfully without warnings.

.NET 6 is out of support now so unless the issue also reproduces on .NET 8 or .NET Framework 4.8, it won't be actioned on.

[ltrzesniewski]

Hmm actually I didn't realize it's the source generator from .NET 6 which gets executed during the build:

So I guess you're right: I missed the deadline by a few days... 😬

Anyway I compared v6.0.36 to v9.0.0, the GetUnqualifiedName method is the same, but AddAlias has the following change:

diff --git a/src/libraries/Common/src/Roslyn/CSharpSyntaxHelper.cs b/src/libraries/Common/src/Roslyn/CSharpSyntaxHelper.cs
index f7ab0d494a5..4638d138e24 100644
--- a/src/libraries/Common/src/Roslyn/CSharpSyntaxHelper.cs
+++ b/src/libraries/Common/src/Roslyn/CSharpSyntaxHelper.cs
@@ -93,6 +93,11 @@ private static void AddAliases(SyntaxList<UsingDirectiveSyntax> usings, ref Valu
                 if (global != usingDirective.GlobalKeyword.Kind() is SyntaxKind.GlobalKeyword)
                     continue;

+                // We only care about aliases from one name to another name.  e.g. `using X = A.B.C;`  That's because
+                // the caller is only interested in finding a fully-qualified-metadata-name to an attribute.
+                if (usingDirective.Name is null)
+                    continue;
+
                 var aliasName = usingDirective.Alias.Name.Identifier.ValueText;
                 var symbolName = usingDirective.Name.GetUnqualifiedName().Identifier.ValueText;
                 aliases.Append((aliasName, symbolName));

So I haven't really debugged this, but I suppose GetUnqualifiedName actually received a null name parameter, and we reached the unreachable switch case because of that. 😅

So this is basically fixed by #96048, thanks @CyrusNajmabadi 🙂