The following shows that javac is inconsistent in determining
which overriders are allowed:

class Box<T extends Number> {}

class T1 { // works
    static class A {
        void f(Box<?> box) {}
    }
    static class B extends A {
        void f(Box<? extends Object> box) {}
    }
}

class T2 { // works
    static class A {
        void f(Box<?> box) {}
    }
    static class B extends A {
        void f(Box<? extends Number> box) {}
    }
}

class T3 { // works
    static class A {
        void f(Box<? extends Object> box) {}
    }
    static class B extends A {
        void f(Box<?> box) {}
    }
}

class T4 { // works
    static class A {
        void f(Box<? extends Number> box) {}
    }
    static class B extends A {
        void f(Box<?> box) {}
    }
}

class T5 { // fails
    static class A {
        void f(Box<? extends Object> box) {}
    }
    static class B extends A {
        void f(Box<? extends Number> box) {}
    }
}