This is not supported right now but it is within the realm of possibility of
things that we can support.
The restriction on using protocols as types is artificial — it was put in place
to avoid confusing users. So it is a matter of tweaking the logic which
diagnosed unsupported protocol types to somehow check for fully-constrained
associated types instead.
Slava
> On Aug 27, 2017, at 2:54 PM, Jarod Long via swift-dev <swift-dev@swift.org>
> wrote:
>
> Apologies for any terminology that I'm not using correctly, but I'm wondering
> if there's any way to make this work in Swift 4:
>
> ```
> protocol P1 {
> associatedtype Thing
> func makeThing() -> Thing
> }
>
> protocol P2: P1 where Thing == String {}
>
> func test(_ p2: P2) { // Can only use P2 as a generic constraint even though
> P2.Thing should be known to be String
> print(p2.makeThing())
> }
> ```
>
> I want P2 to conform to P1 and make its associated type concrete so that it
> can be used directly. If this isn't possible right now, are there any plans
> for something like this to be added in the future? I've looked through the
> generics manifesto, but I didn't see anything that seemed to address this
> issue.
>
> Thanks!
>
> Jarod
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