So the syntax being proposed here is quite different than a generic typealias. It is more like a shorthand for a protocol type and a where clause to be expanded at the point of use (even if you decide not to allow it to be used as a type of a value).
(emphasis added)
I agree with this interpretation -- I think what is being asked for is a way to bundle up constraints and refer to them by some shorthand, without any additional exists quantifier. (And this is what I was referring to in my original comment about Haskell's constraint aliases and Rust's trait aliases).