Do not pull Supertype-Cast to instanceof - #577

When the cast type expression is a supertype of the instanceof type, the recipe should back off. The code semantics could change otherwise, because more instances can potentially pass the check. Fixes GH-572.

What's changed?

Type casts after instanceof checks which use a different type than the one used in the instanceof check itself will now be ignored since changing the instanceof type might alter the code semantics.

What's your motivation?

Issue #572

Anything in particular you'd like reviewers to focus on?

The isCheckedCastCompatible might be extended even further to take generic type variables into account. Cases like these could also be migrated, since the type variables are the same:

// java 11
class A<T> {
   void test(List<T> o) {
       if (o instanceof ArrayList<?>) {
           ((ArrayList<T>) o).trimToSize();
       }
   }
}

// java 16+ where the reifiable type-restriction was lifted
class A<T> {
   void test(List<T> o) {
       if (o instanceof ArrayList<T> list) {
           list.trimToSize();
       }
   }
}

The isCheckedCastCompatible might be a duplicate of existing code, but I didn't find any publicly accessible methods which perform such checks ☹️

Have you considered any alternatives or workarounds?

In the first try I just flipped the type TypeUtils#isAssignableTo parameters in the registerTypeCast method, but then subtype casts were pulled up to the instanceof check, resulting in the same problem of changed semantics.

Checklist

• [✅] I've added unit tests to cover both positive and negative cases
• [✅] I've read and applied the recipe conventions and best practices
• [❌] I've used the IntelliJ IDEA auto-formatter on affected files, since the formatting differs a bit from the existing code when using the IntelliJ formatter