Created for https://github.com/openjdk/jdk/pull/11679

   NavigableMap m = Collections.synchronizedNavigableMap(new TreeMap());
   NavigableMap m2 = m.subMap(foo, true, bar, false);
       ...
   Set s2 = m2.keySet(); // Needn't be in synchronized block
       ...
   synchronized (m) { // Synchronizing on m, not m2 or s2!
       Iterator i = s.iterator(); // Must be in synchronized block
       while (i.hasNext())
           foo(i.next());
   }

Should be
       Iterator i = s2.iterator();