[swift-evolution] Typealiases in protocols and protocol extensions

[David Hart]

Hello,

I’ve come again with another proposal directly from the Generics Manifesto. Please let me know if it needs any modifications before sending the pull request.

Typealiases in protocols and protocol extensions

Proposal: SE-XXXX <https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md>
Authors: David Hart <https://github.com/hartbit>, Doug Gregor <https://github.com/DougGregor>
Status: TBD
Review manager: TBD
 <https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#introduction>Introduction

This proposal is from the Generics Manifesto <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md> and brings the typealias keyword back into protocols for type aliasing.

 <https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#motivation>Motivation

In Swift versions prior to 2.2, the typelias keyword was used outside of protocols to declare type aliases and in protocols to declare associated types. Since SE-0011 <https://github.com/apple/swift-evolution/blob/master/proposals/0011-replace-typealias-associated.md> and Swift 2.2, associated type now use the associatedtype keyword and typelias is available for implementing true associated type aliases. 

 <https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#proposed-solution>Proposed solution

The solution allows the creation of associated type aliases. Here is an example from the standard library:

protocol Sequence {
  associatedtype Iterator : IteratorProtocol
  typealias Element = Iterator.Element
}
The example above shows how this simplifies referencing indirect associated types:

func sum<T: Sequence where T.Element == Int>(sequence: T) -> Int {
    return sequence.reduce(0, combine: +)
}
 <https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#detailed-design>Detailed design

The following grammar rules needs to be added:

protocol-member-declaration → protocol-typealias-declaration

protocol-typealias-declaration → typealias-declaration

 <https://github.com/hartbit/swift-evolution/blob/typealiases-in-protocols/proposals/XXXX-typealiases-in-protocols.md#impact-on-existing-code>Impact on existing code

This will have no impact on existing code, but will probably require improving the Fix-It that was created for migrating typealias to associatedtype in Swift 2.2.
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