[Copilot]

A type reached only through typeof is marked relevant to variant casting, and its marked interface implementations are kept. Its uninstantiated base type was not marked, so the base lost its interface implementations and explicit implementation methods (e.g. IEnumerable.GetEnumerator) while the derived type kept the same interfaces. The trimmed derived type then fails to load with TypeLoadException.

abstract class Base<T> : IEnumerable<T> { /* explicit IEnumerable.GetEnumerator */ }
interface IDerived<T> : IEnumerable<T> { }
class Derived<T> : Base<T>, IDerived<T> { }

var t = typeof(Derived<>);   // nothing instantiates Base<T> or Derived<T>

• MarkStep.MarkRelevantToVariantCasting
  • Also marks the type's base types as relevant to variant casting. A type's interface set includes the interfaces from its base types, so this follows the same rule.
  • The walk stops at the first base type that is already marked, since its own bases are then marked too.
  • _typeMapHandler.ProcessType is still only invoked for the type itself, so type-map behavior for base types is unchanged.
• Test
  • Added Inheritance.Interfaces/OnReferenceType/UninstantiatedGenericBaseWithInterfacesAndDerivedInterface.cs. It mirrors the repro with custom interfaces and an explicit implementation on Base<T>.
  • Without the fix, Base<T> loses its interfaces and the explicit method while Derived<T> keeps them.

Behavior change: base types of any reflection-visible (typeof/ldtoken) type now keep their marked interface implementations even when never instantiated. This costs a small amount of size and avoids the load failure.

• Fixes ILLink strips interfaces of an uninstantiated generic base class while the derived class keeps them, causing TypeLoadException #135913

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[jtschuster]

@copilot ilcompiler tests are failing. Make sure they pass before pushing.

[Copilot]

@copilot ilcompiler tests are failing. Make sure they pass before pushing.

Fixed in ae2dad5. The failing linux-x64 checked CLR_Tools_Tests job was ILCompiler.Trimming.Tests on Generics.VariantCasting: NativeAOT doesn't keep the unconstructed UninstantiatedBaseWithInterfaces<T>, its interfaces, or the unreachable interface methods. I scoped those expectations to By = Tool.Trimmer. I built clr.aot and libs.sfx locally and ran ILCompiler.Trimming.Tests for the Generics and Reflection suites; everything passes except Reflection.TypeUsedViaReflection, which needs ilasm (not built in my environment). Mono.Linker.Tests Generics and Reflection also pass (71/71).

[sbomer]

LGTM, thank you!