Add AsyncVariantLookup::ReturnDroppingThunk for parallel RDT lookup in FindOrCreateAssociatedMethodDesc

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The assert !pPrimaryMD->IsReturnDroppingThunk() in FindOrCreateAssociatedMethodDesc fires for generic virtual methods with covariant return, when the primary MethodDesc is itself a ReturnDroppingThunk (RDT) and we need to look up a parallel RDT that differs only in something other than async kind (e.g. generic arguments). The instantiated-method-desc hash table also keyed on a single bool isAsyncVariant, which cannot distinguish a regular async variant from an RDT (both have IsAsyncVariantMethod() == true).

Changes

• AsyncVariantLookup::ReturnDroppingThunk enum value (method.hpp) — matched only by methods where IsReturnDroppingThunk() is true. Existing Async continues to match only regular async variants (not RDTs).

• MethodDesc::GetAsyncVariantLookup() helper — returns the lookup value matching a method's own async kind (Ordinary/Async/ReturnDroppingThunk).

• Convenience overload of FindOrCreateAssociatedMethodDesc — replaces the _ASSERTE(!pPrimaryMD->IsReturnDroppingThunk()) with a 3-way selection of the variant lookup based on the primary MD:

  AsyncVariantLookup variantLookup = pPrimaryMD->IsReturnDroppingThunk() ? AsyncVariantLookup::ReturnDroppingThunk :
                                     pPrimaryMD->IsAsyncVariantMethod()  ? AsyncVariantLookup::Async :
                                                                           AsyncVariantLookup::Ordinary;

• Hash table API widened — InstMethodHashTable::FindMethodDesc and InstantiatedMethodDesc::FindLoadedInstantiatedMethodDesc now take AsyncVariantLookup instead of bool isAsyncVariant; ContainsMethodDesc passes pMD->GetAsyncVariantLookup(). All 9 call sites in genmeth.cpp updated. This is required because a bool key cannot disambiguate regular-async from RDT entries that hash to the same bucket — without it, GetAsyncVariant() on an RDT can return the RDT itself.

Notes

• The original assertion no longer fires; GetAsyncVariant() on an RDT now correctly returns its parallel regular-async variant.
• Enabling TestGenericVirtualMethod on CoreCLR surfaces a separate, deeper bug in shared-canonical GVM dispatch (recursive re-entry into the RDT, stack overflow). That is out of scope for this change set, so the existing [ActiveIssue(...)] attribute on the test is intentionally left in place. The enum/lookup changes here are self-contained prerequisites that remove the misleading assert and unblock further investigation.

Note

This PR description and the changes were generated with assistance from GitHub Copilot.

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Pull request overview

This PR updates CoreCLR’s async-variant method lookup so that ReturnDroppingThunk (RDT) methods can be distinguished from “regular” async variants during FindOrCreateAssociatedMethodDesc / instantiated-method-desc hashtable lookups, preventing incorrect matches when multiple async-kind variants share the same method identity apart from async kind.

Changes:

• Adds AsyncVariantLookup::ReturnDroppingThunk plus MethodDesc::GetAsyncVariantLookup() and updates matching logic (MatchesAsyncVariantLookup) to disambiguate regular async variants vs RDTs.
• Widens instantiated-method-desc lookup APIs from bool isAsyncVariant to AsyncVariantLookup and updates call sites accordingly.
• Modifies the covariant-return async test by removing an ActiveIssue skip annotation on TestGenericVirtualMethod.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/tests/async/covariant-return/covariant-returns.cs	Removes the ActiveIssue gate for TestGenericVirtualMethod, re-enabling the test.
src/coreclr/vm/method.hpp	Introduces AsyncVariantLookup::ReturnDroppingThunk, adds GetAsyncVariantLookup(), and refines async-kind matching logic.
src/coreclr/vm/instmethhash.h	Updates InstMethodHashTable::FindMethodDesc signature to accept AsyncVariantLookup (and adds a forward declaration).
src/coreclr/vm/instmethhash.cpp	Uses MatchesAsyncVariantLookup + GetAsyncVariantLookup() to filter and validate hashtable entries.
src/coreclr/vm/genmeth.cpp	Plumbs AsyncVariantLookup through instantiated method desc lookups/creation paths.