Excerpt from https://www.theglobeandmail.com/politics/article-china-decries-usmca-provisions-governing-free-trade-talks-with-non/:
...University of Ottawa international affairs professor Roland Paris, who served briefly as Prime Minister Justin Trudeau’s foreign-policy adviser, expressed astonishment Canada would give the United States the ability to vet a bilateral free-trade deal with China.
“It is unusual to give another country a formal role in vetting a trade negotiation that they are not directly involved in,” he said in an interview. “There may be good reasons for us not to want to pursue a free-trade agreement with China, but that should be our decision entirely. It shouldn’t be based on a formal vetting of trade negotiations by a third country.”
However, Mr. Paris said in practice he would expect any Canadian government to be consulting with Washington on the outlines of a trade agreement that could affect U.S. interests, but it is the “formalization of the role that I find unusual.”