SE-0244: Opaque Result Types (reopened)

[Moximillian (Mox)]

@Joe_Groff

The @_opaqueReturnTypeOf attribute, and the mangled representation of opaque types, currently uses an ordinal like this, yeah. If we adopt the dual-generic-signature notation, like:

func makeAnimals() -> <T: Animal, U: Animal> (T, U) { ... }

then maybe you could use the names too, like #returnType(of: makeAnimals).T.

As was mentioned in Improving the UI of generics - #46 by Joe_Groff thread, I'd much prefer using syntax of

func makeAnimals() -> (some Animal T, some Animal U) { ... }

for naming the types, applying to both some and any types. And in both parameter and return type positions.