genericFunctions.swift

import Foundation

protocol R {
    associatedtype V

    func hoge() -> V
}

struct R1: R {
    typealias V = Int

    func hoge() -> V {
        return 100
    }
}

struct R2: R {
    typealias V = Void

    func hoge() -> V {}
}

struct S<T> {
    let value: T
}

struct C {}

struct Session {
    func response<T: R>(subject: T) -> S<T.V> where T.V: Any {
        return S(value: subject.hoge())
    }

    func response<T: R>(subject: T) -> C where T.V == Void {
        return C()
    }
}

let r1 = R1()
let r2 = R2()

let session = Session()
session.response(subject: r1) // -> S<Int>
session.response(subject: r2) // -> C