Refinement of parameter types in overridden methods and polymorphic types

Here is an example of code that I would expect to type-check, but it issues an override.param.invalid error.

import org.checkerframework.checker.nullness.qual.*;

public class RefineOverrideShort {

  class Super<T> {
    void m6(@Nullable String s) { }
  }

  class Sub<T> extends Super<T> {
    @Override
    void m6(@PolyNull String s) { }
  }

}

From a client's point of view, the subclass can be used in any situation that the superclass can be: there are no superclass calls that are illegal in the subclass. Therefore, I believe that the overriding should be permitted.

There are more test cases in file checker-framework/checker/tests/nullness/RefineOverride.java.

Start with checker-framework/checker/tests/nullness/RefineOverride.java and run the existing nullness tests to reproduce the override.param.invalid diagnostics for the provided RefineOverrideShort example. Trace the checker logic that validates parameter annotations in overridden methods, then add or adjust tests so the reported refinement and polymorphic cases match the intended type-checking behavior.