なぜなのか #CodePiece

CodePiece.swift

protocol P: Hashable {}

func ==(lhs: S, rhs: S) -> Bool { return lhs.name == rhs.name }
struct S: P {
    var name = "struct"
    var hashValue: Int { get{ return name.hash } }
}

let setS: Set<S> = [S()]
let setP: Set<P> = [S()] // Protocol 'P' can only be used as a generic constraint because it has Self or assosiated type requirements