[swift-evolution] protocol can only be used as a generic constraint because it has Self or associated type requirements

[Dave Abrahams]

> On Dec 15, 2015, at 10:02 AM, Thorsten Seitz <tseitz42 at icloud.com> wrote:
> 
> Shouldn’t we just narrow the type of protocols with associated types and/or Self types when the protocol is used as a type?
> 


Then you have the “P doesn’t conform to P” problem, which is too crazy to live.  A recent internal discussion led to the idea that we should have notation for the existentially-viable subset of a protocol, so you could always make Array<existential P> but not necessarily Array<P> for an arbitrary protocol P.  Just one way to address this use-case, of course.

-Dave



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