========== $ cat -n T.java
     1 class T {
     2 void f(java.util.List<? super Integer> l) {
     3 l.add(new Integer(3));
     4 }
     5 }
========== $ newjavac -source 1.5 T.java
T.java:3: reference to add is ambiguous, both method add(E) in java.util.Collection<=? super java.lang.Integer> and method add(E) in java.util.List<? super java.lang.Integer> match
        l.add(new Integer(3));
         ^
1 error
========== $