Fix TypeLoadException in .NET 11 Preview 7 with DotNext threading

[Copilot]

Fixes a TypeLoadException ("does not have an implementation") thrown when loading a concrete type whose base hierarchy contains an abstract covariant Task → Task<T> override (the DotNext.Threading failure reported in the issue).

Root cause

For a covariant Task → Task<T> virtual override, MethodTableBuilder::EnumerateClassMethods synthesizes a third MethodDesc — a return-dropping thunk — whose void-returning signature MethodImpl-overrides the base's void-returning async variant. All three MethodDescs share the same metadata token, and MethodDesc::GetAttrs() reads attributes straight from metadata. When the covariant override is declared abstract, the synthesized thunk therefore also reports mdAbstract, even though EmitReturnDroppingThunk always emits a body (CALLVIRT to the T-returning async variant + POP + RET).

A concrete derived type's own Task<T> override matches the abstract declaration exactly, so no MethodImpl and no additional variant is created for it — nothing ever implements the inherited "abstract" thunk slot, and VerifyVirtualMethodsImplemented fails the type load.

Fix

MethodDesc::GetAttrs() now masks off mdAbstract for return-dropping thunks. Patching the single choke point keeps the VM, JIT (CORINFO_FLG_ABSTRACT), MayHaveNativeCode()/MayHavePrecode() and default-interface-method resolution consistent. Async variants are filtered out of reflection, so the adjusted attributes are not observable from managed code.

• Analyze the issue and identify the root cause
• Baseline build (./build.sh clr+libs -lc release -rc checked) + Core_Root layout
• Reproduce the failure with a standalone repro (abstract covariant override fails, non-abstract passes)
• Minimal VM fix in MethodDesc::GetAttrs()
• Add regression test to src/tests/async/covariant-return/covariant-returns.cs (abstract covariant method override + abstract covariant property override, the get_Task shape from the issue)
• Verify new tests fail without the fix (both throw the reported TypeLoadException)
• Verify new tests pass with the fix and no async tests regress (144 passed / 0 failed)

Not verified: only linux-x64 Checked was built/tested; no test trees outside src/tests/async were run; parallel_validation produced no signal (code-review binary missing in the environment, CodeQL skipped both cpp and csharp for database size). The NativeAOT/crossgen2 path models this differently (CompilerTypeSystemContext.Async.cs wraps the resolved implementation in a ReturnDroppingAsyncThunk) and appears unaffected, but was reasoned about rather than tested; interface/DIM variants of this shape are likewise untested.

Resolves #132971

Note

This pull request description was generated by GitHub Copilot.

• Fixes .NET 11 Preview 7 Unexpectedly Throws TypeLoadException #132971

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[VSadov]

@copilot - does NativeAOT needs a separate fix for the same issue/test?

[VSadov]

@copilot - the original repro mentions _ = typeof(DotNext.Threading.AsyncAutoResetEvent).Assembly.GetTypes();

can we have an additional repro scenario with a shape similar to that?
(make sure it fails on base and works with the fix)

[Copilot]

Copilot review overview

🔵 Needs a closer look

Interface/default-interface-method coverage or justification remains unresolved.

Review effort: Lite
Findings: None

What changed in this PR

Fixes a CoreCLR TypeLoadException involving abstract covariant Task overrides.

Changes:

• Masks mdAbstract on return-dropping thunks.
• Adds method and property regression tests.

File	Summary
src/​tests/​async/​covariant-return/​covariant-returns.cs	Adds abstract covariant override coverage.
src/​coreclr/​vm/​method.cpp	Corrects synthesized thunk attributes.

[Copilot]

Copilot review overview

🔵 Needs a closer look

CoreCLR dispatch behavior and the reported assembly-enumeration path remain insufficiently covered.

Review effort: Balanced
Findings: None

[Copilot]

@copilot - does NativeAOT needs a separate fix for the same issue/test?

No separate NativeAOT fix is needed. NativeAOT resolves the async slot to the concrete override first and then wraps that resolved implementation in ReturnDroppingAsyncThunk, rather than inheriting CoreCLR’s synthesized abstract thunk slot. I also built and ran the full covariant-return test as a standalone NativeAOT executable; it passed with exit code 100.

@copilot - the original repro mentions _ = typeof(DotNext.Threading.AsyncAutoResetEvent).Assembly.GetTypes();

can we have an additional repro scenario with a shape similar to that?
...

Added typeof(Program).Assembly.GetTypes() coverage in commit 468102c. The filtered test fails without the VM fix with ReflectionTypeLoadException/TypeLoadException (“Method 'M1' ... does not have an implementation”) and passes with the fix. The full test also passes under CoreCLR and NativeAOT.

[Copilot]

Copilot review overview

🔵 Needs a closer look

A critical interface thunk layout issue remains unresolved and requires additional coverage.

Review effort: Balanced
Findings: 1

Open (1)

• Allocate native slots for abstract interface return-dropping thunks · New

[Copilot]

Copilot review overview

🔵 Needs a closer look

The unresolved moderate consistency concern and limited validation warrant final human review.

Review effort: Balanced
Findings: 1

Open (1)

• Allocate native slots for abstract interface return-dropping thunks

[Copilot]

Copilot review overview

🔵 Needs a closer look

The low-level runtime change had limited platform and scenario validation.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Allocate native slots for abstract interface return-dropping thunks

[VSadov]

@jakobbotsch - could you take a look at this PR

[VSadov]

/backport to release/11.0

[github-actions]

Started backporting to release/11.0 (link to workflow run)