$ cat -n A.java
     1 import java.util.*;
     2 class Comparators {
     3 public static <C extends Comparable<? super C>> Comparator<C> naturalOrder() {
     4 return null;
     5 }
     6 public static Comparator<Integer> c() {
     7 return naturalOrder();
     8 }
     9 }
$ /usr/local/jdk6/bin/javac A.java
A.java:7: incompatible types; no instance(s) of type variable(s) C exist so that java.util.Comparator<C> conforms to java.util.Comparator<java.lang.Integer>
found : <C>java.util.Comparator <C>
required: java.util.Comparator<java.lang.Integer>
    return naturalOrder();
                       ^
1 error
$