swift-dynamic-cond-conf.swift

/*
ビルド & 実行

# a.swiftmoduleの生成
$ swiftc -emit-module a.swift

# liba.dylibの生成
$ swiftc -emit-library a.swift 

# b.swiftの実行
$ swift -I . -L. -la b.swift
*/

// --------------------------------
// a.swift
// --------------------------------

public protocol P {}

public struct S : P {}
open class A<T> {
    public init() {}
}

extension A : P where T : P {
}

open class C {
    public init() {}
}

public func test1(_ x: Any) {
    if x is P {
        print("type of x (\(type(of: x))) conforms P")
    } else {
        print("type of x (\(type(of: x))) does not conforms P")
    }
}

// --------------------------------
// b.swift
// --------------------------------

import a

//public class B<U> : A<Int> {}

// b.swift:5:15: error: conflicting conformance of 'B<U>' to protocol 'P'; there cannot be more than one conformance, even with different conditional bounds
// 親クラスAがすでにPのcondconfを持っているから駄目らしい
//extension B : P where U : P {}

class D<U> : C {}

extension D : P where U : P {}

let a1 = A<Int>()
test1(a1) // not conform

let a2 = A<S>()
test1(a2) // conditionally conform

let d1 = D<Int>()
test1(d1) // not conform

let d2 = D<S>()
test1(d2) // conditionally conform