Capture conversion implementation breaks a cast test

The following test case, from
    test/tools/javac/generics/wildcards/pos/CastTest.java line 122
fails under the recent capture conversion implementation:

     1 interface DA<T> { }
     2 interface DB<T> extends DA<T> { }
     3 interface DC<T> extends DA<Integer> { }
     4
     5 class Main {
     6 Object o = (DC<?>) (DA<?>) null; // <<pass>>
     7 }