I re-read C99, and the reason this is allowed is not because of §6.5.2.2 p10, but rather §6.5.16 p4 as far as I can tell. Though it's weird to me that the expression (x = 2) can ever be evaluated as anything but 2.

Also, it's not illegal C, it's undefined C and compilers are only allowed to do whatever they want within the undefined area (which is larger than I expected.)