[swift-evolution] [Pitch] Require Any for existentials

[Haravikk]

> On 23 Aug 2016, at 23:14, Adrian Zubarev via swift-evolution <swift-evolution at swift.org> wrote:
> 
> The basic design is fine, but I wouldn't want to add more noise to my code. We could keep (label: Protocol) as a shorthand form for (label: Any<Protocol>) similar to Optionals.
> 
> What's the benifite of this anywasys? What exactly will this solve?!

Currently we can only use a protocol as if it were a type when it has no associated types, existentials will eliminate that restriction, allowing us to use a protocol Foo where we would currently either have to use AnyFoo or a very verbose generic constraint. I think the point of this proposal is to ask whether we should be able to use protocols as if they were concrete types, or whether it would be better to require Any<> to clarify what's really happening.

Personally I'm on the fence; I don't particularly mind being able to use a protocol name as if it were a concrete type, but I do generally prefer for things to be explicit wherever confusion may be caused. I think that requiring Any<> could be a good thing, as it clarifies that what you're interacting with is not a concrete type, and highlights that it may optimise differently (I'm not 100% on the details of how existentials actually work).

So I'd say I'm a tentative +0.5; in terms of migration I don't think this is all that source breaking, as it's very easy to add a fixit for (possibly even an automatic one?) and it only currently applies to non-generic protocols anyway, which seems relatively niche.