variance.swift

class Animal {}
class Cat: Animal {}

let cats: Array<Cat> = [Cat()]
let animals: Array<Animal> = cats // OK

let cat: Optional<Cat> = .some(Cat())
let animal: Optional<Animal> = cat // OK

struct Box<Value> {
	var value: Value
}

let catBox: Box<Cat> = Box(value: Cat())
let animalBox: Box<Animal> = catBox // error: cannot convert value of type 'Box<Cat>' to specified type 'Box<Animal>'