That's a very good question. I knew you would be the one to ask it, given the last few committee meetings.
It's simple. A free trade agreement can be an exception to the general principles of international trade only if it meets certain conditions of form and substance. We can skip the form, since the substance is more important.
First and foremost, you have to reach a free trade agreement that covers and liberalizes the bulk of trade between states. In practical terms, the WTO has never really defined what “the bulk of trade” means exactly, but it is understood to be about 90% of the trade between different countries. That means you couldn't have a free trade agreement liberalizing aluminum, steel, sugar, or even bananas. We've seen examples of this in the past at the WTO.
We couldn't reach a sectoral agreement on government procurement alone. If we were to reopen negotiations on government procurement with the United States, it would absolutely have to be done within the framework of the agreement we have at the moment, which is CUSMA.