generics: incorrect unchecked exception

$ cat -n T.java
     1 // from the library
     2 class Comparator<T> {}
     3
     4 // from the library
     5 class TreeSet<T> {
     6 public TreeSet(Comparator<T> c) {}
     7 }
     8
     9 // in non-generic user code
    10 public class T {
    11 void f() {
    12 Comparator HASH_COMPARATOR = null;
    13 TreeSet set = new TreeSet(HASH_COMPARATOR);
    14 }
    15 }
$ newjavac -warnunchecked T.java
T.java:13: warning: unchecked call to TreeSet(Comparator<T>) as a member of the raw type TreeSet
        TreeSet set = new TreeSet(HASH_COMPARATOR);
                      ^
1 warning
$