"cannot implement" interface error when using delegating java.util.List via @link in JDK 21

[rsmckinney]

The following class will cause a compile error: "MyList cannot implement reversed()..."

public class MyList<E> implements List<E> {
  @link List<E> l = new ArrayList<>();
}

Note, error is nondeterministic. It happens maybe every other build.

After some digging, the error is a result of how delegation classes are generated wrt implementing interfaces. The order of the interfaces that are implemented is nondeterministic from build to build because javac Type classes don't override hashcode, so it's based on default hashcode. But the real problem is that delegation code does not take into account covariant return types in some cases, which can lead to "cannot implement" compile errors.

[rsmckinney]

Fixed with release 2023.1.23