To do star star x = f(y), star x and f(y) each need to be evaluated. You'd think the right-hand side would always be evaluated before the left-hand, but that's not always possible (what should "arr[i] = i++" do? I stole this from wikipedia btw). In the above case, f(y) has this kind of side effect that change x, which is undefined.

Rather than changing the macro, this could be fixed easily by putting the assignment into its own function. Then the argument (f(y)) must be fully evaluated before star x.

Edit: I have no idea how to escape asterisk on HN :(