Open Radar

 
A protocol that attempts to define a typealias that is a function using the protocol type constraints does not work.

protocol Emotional {
    typealias Sad
    typealias Happy
    
    typealias F = (state: Sad) -> Happy
    typealias G = (state: Int) -> Int
    typealias H = (hi: Sad, bye: Happy)
    
    var funcs: [F] { get }
    var emotes: [G] { get }
    var faces: [H] { get }
}

And the conforming type:

struct Emote : Emotional {
    let funcs: [(state: Int) -> Int] = [{ $0 }]
    let emotes: [(state: Int) -> Int] = [{ $0 }]
    let faces: [(hi: Int, bye: Int)] = [(hi: 0, bye: 1)]
}

let e = Emote()
e.funcs[0](state: 1)     // 1
e.emotes[0](state: 12)   // 12
e.faces[0]               // (.0 0, .1 1)

This won’t compile; Emote does not conform to Emotional.

However, changing the protocol to this is fine:

protocol Emotional {
    typealias Sad
    typealias Happy
    
    typealias G = (state: Int) -> Int
    typealias H = (hi: Sad, bye: Happy)
    
    var funcs: [(state: Sad) -> Happy] { get }
    var emotes: [G] { get }
    var faces: [H] { get }
}