Wildcard instantiated List does not accept value

I don’t get why it’s not okay to put a String into the List below:

        List<? extends Comparable<String>> test = new ArrayList<String>();
        String string = "A";
        test.add(string); // why is this not ok? in fact "String" should be a subype of "? extends Comparable<String>

Can anyone please explain where the problem is? I know that there is no benefit in having a wildcard at this point because we can’t inherit from "String", but I want to understand what’s happening an why the compiler is complaining about it.

>Solution :

Consider this:

class X implements Comparable<String> {
    @Override
    public int compareTo(String o) {
        return 0;
    }
}
List<? extends Comparable<String>> test = new ArrayList<X>();

Does it make sense to add a String to test?