[swift-evolution] [Completing Generics] Arbitrary requirements in protocols

[Jacob Bandes-Storch]

Doug wrote this in the Completing Generics manifesto, under "Minor
extensions":


> **Arbitrary requirements in protocols*



Currently, a new protocol can inherit from other protocols, introduce new
> associated types, and add new conformance constraints to associated types
> (by redeclaring an associated type from an inherited protocol). However,
> one cannot express more general constraints. Building on the example from
> “Recursive protocol constraints”, we really want the element type of a
> Sequence’s SubSequence to be the same as the element type of the Sequence,
> e.g.,
>


    protocol Sequence {
>         associatedtype Iterator : IteratorProtocol
>         …
>         associatedtype SubSequence : Sequence where
> SubSequence.Iterator.Element == Iterator.Element
>     }
>


+1.

To make it into Swift 3, would this feature require a proposal of its own?
How feasible would it be to implement on top of the current system?

Jacob
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