Covariant Error

gistfile1.swift

import Foundation

class A0 { }
class A1: A0 { }
class A2: A1 { }

class Array<X> {
    func get() -> X { fatalError() }
    func set(x: X) -> () { fatalError() }
}

let a0 = A0()
let a1 = A1()
let a2 = A2()

let c = Array<A1>()

let c0: Array<A0> = c            // Compile error
let c1: Array<A1> = c
let c2: Array<A2> = c            // Compile error

// c0.set(a0)
// c2.get()    => A0 is now treated as A2

// c0.get() => c.get() as A0      => (x: A1) as A0   o
// c1.get() => c.get() as A1      => (x: A1) as A1   o
// c2.get() => c.get() as A2      => (x: A1) as A2   ???

// c0.set(a0)          =>  A0 is treated as A1 in c0.set
// c1.set(a1)          =>  ok
// c2.set(a2)          =>  ok