Add AsyncVariantLookup::ReturnDroppingThunk for parallel RDT lookup in FindOrCreateAssociatedMethodDesc

[Copilot]

Fixes: #127197

Basically, allow parallel lookups for ReturnDroppingThunk as the need may arise if covariant override is a GVM that needs to lookup for a specialized RDT during GVM resolution.

Initial assumption (and assert) that noone will ever need to lookup for ReturnDroppingThunk is only partially correct.
We will not lookup for an RDT starting from other kind of variant, that part is correct, but we may need to lookup for an RDT, starting from a prototype RDT if something else needs changing - like generic substitution.

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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.

Note

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

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[Copilot]

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.

[VSadov]

@copilot resolve the merge conflicts in this pull request

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved. The single conflict was in src/coreclr/vm/genmeth.cpp — NewInstantiatedMethodDesc was refactored in main to move the lock acquisition to the top of the function (collapsing two FindLoadedInstantiatedMethodDesc calls into one). The auto-merge had already correctly applied our GetAsyncVariantLookup() change to that remaining call; the only conflict marker was in the else if (getWrappedCode) setup block, which was resolved by taking main's two-line version (SetupSharedMethodInstantiation + assert). All other PR changes (method.hpp, instmethhash.h, instmethhash.cpp, test files) merged cleanly.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.