Fix TypeLoadException when async Task<T> overrides Task

[Copilot]

Description

Runtime throws TypeLoadException for valid C# code where async Task<T> overrides Task. This breaks applications like DevBetterWeb on SDK 11.0.100-preview.1.26066.111.

public abstract class Base {
    public abstract Task HandleAsync();
}

public class Derived : Base {
    // Valid C# - compiles but runtime rejects
    public override async Task<string> HandleAsync() {
        await Task.Yield();
        return string.Empty;
    }
}

Async methods generate variant methods with unwrapped return types (string vs Task<string>) for the async calling convention. The covariant return type validator treats these synthetic methods as invalid overrides.

Changes

• src/coreclr/vm/class.cpp: Skip async variant methods in ValidateMethodsWithCovariantReturnTypes. Check is placed early (right after resolving the derived class method) to avoid unnecessary work resolving the parent method when validation will be skipped. Only checks the derived class method since async variant methods should only override other async variant methods.
• src/tests/.../AsyncCovariantReturn.cs: Regression test that validates type loading succeeds without throwing TypeLoadException. Test uses void return type following standard test pattern for tests without conditional logic.

Pattern matches existing async variant handling in comdelegate.cpp:1046 and dispatchinfo.cpp:2585.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[jkotas]

[error]src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/AsyncCovariantReturn.cs(13,23): error XUW1002: Tests should not unconditionally return 100. Convert to a void return. [/__w/1/s/src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/AsyncCovariantReturn.csproj]

@copilot

[Copilot]

[error]src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/AsyncCovariantReturn.cs(13,23): error XUW1002: Tests should not unconditionally return 100. Convert to a void return. [/__w/1...

Done in e719797. Converted TestEntryPoint to void return since it only validates type loading and doesn't need conditional exit codes.

[Copilot]

Pull request overview

Fixes a CoreCLR type-loading failure where runtime-generated async-variant methods (with unwrapped return types) were incorrectly included in covariant return type validation, causing TypeLoadException for otherwise valid overrides.

Changes:

• Skip async-variant methods during ValidateMethodsWithCovariantReturnTypes to avoid validating synthetic signatures.
• Add a regression test project covering an async Task<T> override of a Task virtual method.
• Add test source that forces creation/loading of the relevant override shape.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/coreclr/vm/class.cpp	Skips covariant return validation for async-variant methods to prevent false TypeLoadException.
src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/AsyncCovariantReturn.csproj	Adds a new test project for the regression scenario.
src/tests/Loader/classloader/MethodImpl/CovariantReturns/UnitTest/AsyncCovariantReturn.cs	Adds a regression test ensuring type loading succeeds for the async/covariant-return scenario.

[MichalStrehovsky]

Why does this require process isolation?

[MichalStrehovsky]

Do we have a plan to add actual test coverage for covariant returns? The test here inherits whatever is the global runtime-async setting, but I believe it might be useful to have real coverage under tests/async that tests: actually calling the method, pairs of "base is runtime-async, override is not" and inverted.

This test has a minimal value because it surgically only checks type load and nothing else. We don't get much bang for our RequiresProcessIsolation buck.