generics: problems with proposed signature for Collections.unmodifiableCollectio

==============================> cat -n T.java
     1 import java.util.*;
     2
     3 class Collections2 {
     4 // proposed signature for Collections.unmodifiableCollection
     5 public static <T, S extends T> Collection<T>
     6 umc(Collection<S> c) {
     7 return null;
     8 }
     9 }
    10
    11 class Test {
    12 public static void main(String[] args) {
    13 Collection<Integer> c1 = null;
    14 Collection<Number> c2 = Collections2.umc(c1);
    15 }
    16 }
==============================> jsr14javac -gj T.java
T.java:14: umc<T, S extends T>(java.util.Collection<S>) in Collections2 cannot be applied to (java.util.Collection<java.lang.Integer>)
        Collection<Number> c2 = Collections2.umc(c1);
                                            ^
1 error
==============================>