Inconsistent handling of file-local types in generic constraints causes path exception in Source Generator

[KARPED1EM]

Issue Description

When using file-local types as generic type parameters in Source Generator implementation, Roslyn fails to properly validate the usage and results in runtime path exception, despite the code being syntactically valid.

Reproduction Code

Basic structure showing the issue:

public abstract class SyntaxGeneratorCollectionBase<T> : IIncrementalGenerator
{
    protected abstract T? ExtractDataFromContext(GeneratorSyntaxContext context, CancellationToken _);
    protected abstract void GenerateFromData(SourceProductionContext spc, ImmutableArray<T?> dataCollection);
}

[Generator(LanguageNames.CSharp)]
file sealed class ServiceExtensionsGenerator : SyntaxGeneratorCollectionBase<ViewData>
{
    protected override ViewData? ExtractDataFromContext(GeneratorSyntaxContext context, CancellationToken _) 
        => /* implementation */;

    protected override void GenerateFromData(SourceProductionContext spc, ImmutableArray<ViewData?> dataCollection) 
        => /* implementation */;
}

file sealed record ViewData(string Name);

Current Behavior

1. Code compiles successfully despite potential invalidity of file-local type usage
2. At runtime, source generator fails with:

System.ArgumentException: Illegal characters in path.
   at System.IO.Path.CheckInvalidPathChars(String path, Boolean checkAdditional)
   at Microsoft.CodeAnalysis.GeneratorDriver.ParseAdditionalSources(...)

Expected Behavior

Roslyn should either:

• Properly validate and reject invalid file-local type usage in generic constraints at compile time
• Or handle file-local types correctly without throwing path exceptions

Environment

• Latest Roslyn/MSBuild toolchain
• Reproducible in both Visual Studio and Rider
• Source Generator compilation context