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  }