according to JSR14, the following should be allowed, but javac doesn't allow it.

class WildcardFBoundCheck {
    interface I4<T> {}

    static class C4<X extends I4<Y>, Y extends I4<X>> {}

    WildcardFBoundCheck()
    {
        C4<I4<?>,?> x2;  // <<pass>>
    }
}