The new subst scheme breaks an idiom used in java.lang.ref. An isolated test case follows. This is perfectly typesafe, yet rejected, with

enqueue(? extends Ref<? extends java.lang.Object>) in Q<? super T> cannot be applied to (Ref<T>)
        this.queue.enqueue(this);
        ^



class Q<T> {
    void enqueue(Ref<? extends T> r) {
    }
}

class Ref<T> {
    Q<? super T> queue;

    void enqueue() {
        this.queue.enqueue(this);
    }
}