This will not compile:

    void test(Box<? extends Box<? extends Number>> b) {
        Number n = b.map(Box::get).get();
    }

    interface Func<S,T> { T apply(S arg); }

    interface Box<T> {
        T get();
        <R> Box<R> map(Func<T,R> f);
    }

error: incompatible types: inferred type does not conform to upper bound(s)
        Number n = b.map(Box::get).get();
                        ^
    inferred: ? extends Number
    upper bound(s): Object

If I remove the wildcards, there is no error.

Perhaps javac is failing to capture the return type of Box.get?