SE-0244: Opaque Result Types

[Joe_Groff]

As written, nothing is inferred; it doesn't yet propose a way to describe constraints on the returned type. Your notation would be one way to do so; other approaches are being discussed in this thread:

Protocol<.AssocType == T> shorthand for combined protocol and associated type constraints without naming the constrained type

Swift's notation for generic constraints generally requires naming the things being constrained; you say that a particular generic parameter conforms to a protocol by naming the generic parameter and the protocol it conforms to, like <C: Collection>, and you put further constraints on its associated types by naming them in a where clause, like C.Element: Equatable. However, opaque result types, generalized existentials, and other conceivable future language features need a way to describe constr…