Heh, thanks for the confirmation, but I just remembered: we still have a similiar rule in place, so the behaviour is not just implementation defined in C++11, but still undefined as in C++03:
Quote Originally Posted by C++11 Clause 1.9 Paragraph 15 (part)
If a side effect on a scalar object is unsequenced relative to either another side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.