A garment bag is clearly over the carry-on size limit. Having said that, if it's just a standard cover for tux / dress (such as what they give you in the store when you buy the tux / dress) as opposed to a true garment bag I don't see why AC would protest, particularly when you tell them what's inside and why you're carrying on instead of checking.

Way to go on the Paris wedding by the way! Out of curiosity, it is much more effort to arrange a wedding in Paris than in Canada? Or is it pretty much just the same thing with a bit of time zone and currency exchange added in?