[Pitch] Introduce existential `any`

[DevAndArtist (Adrian Meister)]

@jrose

I’m extremely in favor of this, thank you for making it happen! I want to offer one suggestion: allow any P.Type . That’s easier to type, obviously, but I’ll argue there’s a semantic reason as well. This type serves two purposes: it’s both the result of type(of: someExistentialValue) , and it’s the type that has the operations marked static in the protocol. In that sense, today’s P.Type is an existential type as well, not merely the metatype for an existential type. An API that has a parameter of type P.Type probably has arguments that look like NSObject.self , not the type of an existing instance. In another language, P.Type could be a protocol that NSObject.Type conforms to. That means a hypothetical any (P.Type) is a reasonable type, which also happens to be the same type, in Swift, as (any P).Type . Which works out nicely!

Now, if only I could convince myself the same applies to any P? …

@Joe_Groff

With the any modifier, there’s an opportunity to revise the current meta type syntax to be more obvious. P.Protocol , the type of P.self , would be (any P).Type , whereas the type of all T.self where T: P would be any P.Type if we say that P.Type is a generic constraint for a meta type conforming to P.

I'm also going to bump the ideas regarding the issues with the current metatype spelling. There are meta types which are static and the result of Type.self and there are meta type existentials. The current state of the proposal pitch does not cover this whole area, which can be improved with the introduction of the any keyword.

---

Without derailing the current topic from the any keyword, in the UX related thread we also briefly discussed the idea of another keyword named meta to improve metatypes. Here's a short explanation of the types it would cover compared against the any syntax:

typealias Meta<T> = meta T

P.Protocol == (any P).Type == Meta<any P> == Meta<P> == meta P
    P.Type ==   any P.Type == any Meta<P> == any meta P
              ~~~~~~~~~~~~

I'm not asking for an additional introduction of that keyword, I just mention it to further clarify what any keyword can already cover and why.

This also further emphasis why the any keyword should support not only existentials (e.g. protocol conformances) but sub-typing (e.g. inheritance) as well!