[release/11.0] Fix TypeLoadException in .NET 11 Preview 7 with DotNext threading - #134685
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…34460) 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. - [x] Analyze the issue and identify the root cause - [x] Baseline build (`./build.sh clr+libs -lc release -rc checked`) + Core_Root layout - [x] Reproduce the failure with a standalone repro (abstract covariant override fails, non-abstract passes) - [x] Minimal VM fix in `MethodDesc::GetAttrs()` - [x] 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) - [x] Verify new tests fail without the fix (both throw the reported `TypeLoadException`) - [x] 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. <!-- START COPILOT CODING AGENT SUFFIX --> - Fixes #132971 --------- Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: VSadov <8218165+VSadov@users.noreply.github.com>
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Backport of #134460 to release/11.0
/cc @VSadov @copilot
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IMPORTANT: If this backport is for a servicing release, please verify that:
release/X.0-staging, notrelease/X.0.release/X.0(no-stagingsuffix).Package authoring no longer needed in .NET 9
IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.