This question comes from today's statement by Tusk that "a short extension will be possible but to be conditional on a positive vote on the withdrawal agreement in the House of Commons." This makes me wonder why an extension of any length would be necessary if the withdrawal agreement (WA) gets approved prior to Brexit day? It is my understanding that at this point some legislation would not be able to pass in time. What legislation are we talking about? What would go wrong if we reach 29 March 2019 with an approved WA but without an agreed extension?
I am not asking about the likeliness of this scenario happening. I only hope that it is realistic enough to influence the thinking of politicians today and that as such this question doesn't fall under absurd hypotheticals, which would be off topic.