Subclassing annotation.ClassfileAnnotation ...

class Foo extends annotation.ClassfileAnnotation

... emits the following, misleading warning:

Implementation restriction: subclassing Classfile does not make your
annotation visible at runtime. If that is what you want,
you must write the annotation class in Java.

A) Um yes? ... That's exactly as intended. That's why the class is called ClassfileAnnotation and not RuntimeAnnotation.
B) The class is called ClassfileAnnotation, not Classfile.

If we had a non-functional RuntimeAnnotation somewhere in the library, the warning would make sense, but like this, it doesn't.