[naot] [Runtime async] Support for covariant override of Task -> Task<T>

[eduardo-vp]

Mirrors work in #125900 for Native AOT.

Fixes #126685.

[Copilot]

Pull request overview

Mirrors the CoreCLR runtime-async fix for covariant overrides (Task base overridden by Task<T> derived) into the NativeAOT type system by introducing a return-dropping async thunk and wiring it into virtual/interface resolution, and re-enables the previously NativeAOT-disabled test.

Changes:

• Add ReturnDroppingAsyncThunk and a corresponding IL stub emitter that calls the T-returning async variant and discards the result.
• Update NativeAOT async-aware virtual/interface method resolution to select the return-dropping thunk when the async slot is void-returning but the resolved override is T-returning.
• Re-enable the covariant-return async test project for NativeAOT.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/tests/async/covariant-return/covariant-returns.csproj	Re-enables building the test under NativeAOT.
src/coreclr/vm/asyncthunks.cpp	Adds a comment noting parity with the managed type system emitter.
src/coreclr/tools/Common/TypeSystem/IL/Stubs/AsyncThunks.cs	Adds IL emission for the return-dropping async thunk.
src/coreclr/tools/Common/TypeSystem/IL/NativeAotILProvider.cs	Routes ReturnDroppingAsyncThunk to the new IL emitter.
src/coreclr/tools/Common/Compiler/CompilerTypeSystemContext.Async.cs	Wraps resolved async variants with a return-dropping thunk for Task -> Task<T> async-slot mismatches; adds caching.
src/coreclr/tools/Common/Compiler/AsyncMethodVariant.Mangling.cs	Adds prefix mangling support for ReturnDroppingAsyncThunk.
src/coreclr/tools/Common/Compiler/AsyncMethodVariant.cs	Introduces ReturnDroppingAsyncThunk type and updates async-thunk classification helpers.

[Copilot]

EmitReturnDroppingThunk says it matches the VM and the comment notes CALLVIRT, but this stub currently emits ILOpcode.call (line 424). call bypasses virtual dispatch and doesn't match CoreCLR's EmitCALLVIRT; use callvirt for the instance case (and only use call if the thunk can be static).

Suggested change

	codestream.Emit(ILOpcode.call, emitter.NewToken(asyncVariantTarget));
	ILOpcode callOpcode = asyncVariantTarget.Signature.IsStatic ? ILOpcode.call : ILOpcode.callvirt;
	codestream.Emit(callOpcode, emitter.NewToken(asyncVariantTarget));

[Copilot]

Argument loading in EmitReturnDroppingThunk unconditionally does EmitLdArg(0) and then loops over sig.Length, which is correct for instance methods but will push one extra arg for static signatures. Either assert this thunk is never static, or mirror EmitAsyncMethodThunk and only load this when !sig.IsStatic.

Suggested change

	codestream.EmitLdArg(localArg++);
	if (!sig.IsStatic)
	{
	codestream.EmitLdArg(localArg++);
	}

[Copilot]

ReturnDroppingAsyncThunk is used as the resolved target for async-variant virtual calls, but it isn't covered by IsAsyncCall (currently based on IsAsyncVariant() / IsAsync). That risks the thunk being compiled/called without CORINFO_CALLCONV_ASYNCCALL in non-runtime-async scenarios, causing an ABI mismatch. Ensure async-call detection includes IsReturnDroppingAsyncThunk() (or similar).

[dotnet-policy-service]

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