I mention it among the unresolved questions at the bottom. I think this question more properly belongs to the area of subtyping/Coercible/whatever we want to do there, and the specific syntax we decide to use for it can be considered a free parameter of this proposal.

That said, the unresolved questions, from my end, basically just amount to coming up with two words.  (But e.g. @nick29581 objected to having variance annotations at all, so I dunno.)