SE-0346: Lightweight same-type requirements for primary associated types

[Slava_Pestov]

@Tino

Therefore, my preference would be to delay this proposal until there is a final decision about labeled generic parameters and generic protocols.

@John_McCall writes in https://forums.swift.org/t/pitch-2-light-weight-same-type-requirement-syntax/55081/180:

@John_McCall

There is zero interest on the Core Team in reserving P<Int> for “generic protocols”. I can say that with confidence because we’ve had multiple conversations about it.