boolean b = (3 == (Object)null);
  compiles because javac performs a boxing conversion followed by a
  widening reference conversion. This isn't currently a valid combination
  under casting conversion (as required by 15.21.3) but 6526446 will
  expand casting conversion and it will become valid.

However, this does not compile, and should:

boolean b = ((Object)null == 3);  // incomparable types: java.lang.Object and int