Status of generalized existentials

[dan-zheng]

@andybezaire

Is there any interest in reviving this?

I would love to take my: [...] and make it be:

type alias AnyPublisher<I, O> = Publisher where .Input = I, .Output = O

Hi Andy! It seems like you're interested in a language feature I'd call "typealiases with extra syntactic generic requirements", which is a bit different from "generalized existentials".

Check out this thread:

Generic typealiases with additional constraints?

Right, but it would behave like an existential type when used in a generic constraint. If I wrote this today typealias Foo<T> = T where T : Sequence, T.Element == Int Then it is not valid to write func foo<T : Foo>(_: T) {} But presumably, you could do that if you instead wrote typealias Foo = Sequence where .Element == Int 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 th…

For interested compiler hackers, I would say this feature is a medium t-shirt size.