fix: add support for [GenerateGenericTest] on generic classes (#4431)

[thomhurst]

Summary

Fixes #4431 - Adds support for [GenerateGenericTest] attribute on generic test classes.

Changes:

• Fix generic class handling in reflection mode by supporting [GenerateGenericTest] attribute
• Fix init-only property setters for generic types (UnsafeAccessor doesn't work with open generics)

Details

ReflectionTestDataCollector.cs

Handle [GenerateGenericTest] attributes that explicitly specify type arguments for generic test classes. Previously, the reflection mode only worked with data sources to determine generic type arguments.

TestMetadataGenerator.cs

Use reflection-based setter for init-only properties on generic types instead of UnsafeAccessor, which doesn't work with open generic types.

Test plan

• Added test files in TUnit.TestProject/Bugs/4431/ demonstrating generic class scenarios
• Run existing test suite to ensure no regressions

Known limitation

There is still an issue with generic methods (not classes) that have both [GenerateGenericTest] AND [MethodDataSource] - these tests may not be discovered correctly. A separate issue will be filed for this case.

🤖 Generated with Claude Code

[thomhurst]

Summary

Adds support for [GenerateGenericTest] attribute on generic test classes in both source-gen and reflection modes, fixing issue #4431.

Critical Issues

1. AOT Compatibility Violation - Generic Type Init-Only Property Setters

Location: TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs:1538-1549

The PR introduces runtime reflection in source-generated code for generic types with init-only properties. This code uses GetType().GetField() without DynamicallyAccessedMembers annotations, which will be problematic with Native AOT and IL trimming.

Problems:

1. Violates CLAUDE.md Rule Or assertion conditions #5 (AOT Compatible)
2. Relies on compiler implementation details (backing field naming)
3. Silent failure with ?. operator

Suggested fix: Generate a clear exception for generic types with init-only properties and data sources, directing users to use regular properties or constructors instead.

2. Missing Test Coverage

The PR description shows unchecked test plan item. Please run the full test suite to verify no regressions.

3. Snapshot Tests Not Run

Per CLAUDE.md Rule #2, changes to source generator output require running snapshot tests in TUnit.Core.SourceGenerator.Tests.

Suggestions

1. The new cartesian product logic for class + method generic combinations looks correct and is properly mirrored between source-gen and reflection modes.
2. Consider caching ExtractTypeArgumentSets() results to avoid repeated attribute iteration.

Verdict

⚠️ REQUEST CHANGES - Critical AOT compatibility issue must be addressed before merge.

[thomhurst]

Summary

Adds support for [GenerateGenericTest] attribute on generic test classes, enabling both source-gen and reflection modes to discover generic tests with explicit type arguments.

Critical Issues

None found ✅

Suggestions

1. Consider Performance Optimization - Cartesian Product Computation

The new cartesian product logic for class + method generic combinations is implemented correctly, but there's repeated attribute iteration. Consider caching ExtractTypeArgumentSets() results if the same attributes are being processed multiple times during generation.

2. Documentation for Init-Only Property Limitation

The PR correctly identifies and handles the limitation that init-only properties don't work with generic types (since UnsafeAccessor doesn't support open generics). The error message is clear and helpful. No action needed, but this is good defensive programming.

3. Snapshot Test Verification

Several new .verified.txt files were added (6,500+ lines of generated code). The generated code appears correct based on spot checks:

• Generic metadata structure with ConcreteInstantiations dictionary is properly formed
• Both TypeA and TypeB instantiations are generated
• InstanceFactory correctly handles type arguments

Assuming these were generated by running the snapshot tests as required by CLAUDE.md Rule #2. ✅

Previous Review Status

The previous review comment (from PR author) raised concerns about:

1. AOT Compatibility - This has been RESOLVED. The new code explicitly throws a clear exception for init-only properties on generic types rather than using problematic runtime reflection.
2. Missing Test Coverage - Test plan item still shows as unchecked in PR description.
3. Snapshot Tests - RESOLVED - Multiple snapshot files were added/updated.

The critical AOT issue has been properly addressed by avoiding the problematic runtime reflection path entirely.

Verdict

✅ APPROVE - No critical issues. The implementation correctly:

• Handles dual-mode requirement (both source-gen and reflection)
• Properly generates cartesian products for class+method generic combinations
• Avoids AOT incompatibility by explicitly rejecting init-only properties on generic types
• Updates snapshot tests as required

The "Known limitation" mentioned in the PR description about generic methods with both [GenerateGenericTest] AND [MethodDataSource] is acknowledged and will be tracked separately.