covariance.swift

protocol A {}
struct B: A{}

let tuple1: (A, Int) = (B(), 1)
let tuple2: (B, Int) = (B(), 1)
// let tuple3: (A, Int) = tuple2 // 共変っぽいけどだめ。

let closure1: (A, Int) -> () = { _ in }
let closure2: (B, Int) -> () = { _ in }
let closure3: (B, Int) -> () = closure1 // 反変。いける。

let tupleClosure1: ((A, Int), Int) -> () = { _ in }
let tupleClosure2: ((B, Int), Int) -> () = { _ in }
let tupleClosure3: ((B, Int), Int) -> () = tupleClosure1 // Tupleの共変サポートされてないのにここはいけるのか

class MyG<T> {
    var genericsClosure: (T, Int) -> () = { _ in }
}

let g = MyG<(B, Int)>()
g.genericsClosure = tupleClosure1 // Generics型でも動いてる