This program compiles with b71,
but fails to compile with earlier mustang builds

import java.util.*;

public class rr {
    public static void main(String[] args) throws Throwable {
List<?> x1 = new ArrayList<Integer>();
List<?> x2 = new ArrayList<Integer>();
x1.addAll(x2);
    }
}

The expected error is:

rr.java:8: cannot find symbol
symbol : method addAll(java.util.List<capture of ?>)
location: interface java.util.List<capture of ?>
x1.addAll(x2);
^
1 error

(although the error message could be improved)