You simply wouldn't be able to teach it that move. It wouldn't let you delete any of the HM moves, so your only option is to cancel your Pokemon learning the move.
This doesn't mean the Pokemon loses the ability to learn that move though; if you take a Heart Scale to the move relearner in Dendemille Town they will give you a list of your Pokemon's level up moves for you to teach to your Pokemon. Of course, make sure you've deleted one of the HM moves before you talk to the move relearner.