APICompat does not catch inconsistent return types of overidden member in multi-targeting library

[ericstj]

Describe the bug

If I change the return type a normal non-virtual method, APICompat will catch this and raise a diagnostic. Even if the new return type is covariant.

If instead the method is virtual and overridden with different return types using https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/covariant-returns we will not flag the difference.

This creates a problem when another derived type exists in an assembly which doesn't multi-target in the same way.

To Reproduce

See https://github.com/ericstj/sample-code/tree/covariantReturn

The application will throw a TypeLoadException when run, but apicompat against the library does not catch this.

Exceptions (if any)

TypeLoadException.

Further technical details

It looks to me like the metadata for covariant returns marks the derived type as new. I can reproduce a similar bug without covariant returns feature if I make the derived method new virtual and then override that in the netstandard library. APICompat will not flag this and it will cause a runtime error since the most-derived member (in the netstandard library) will return a non-covariant type with the more specific base assembly.

[dotnet-policy-service]

@dotnet/area-infrastructure-libraries a new issue has been filed in the ApiCompat area, please triage

[ericstj]

Here's the conversation around this issue : microsoft/agent-framework#798 (comment)