covariant.swift

class Fruit {}
class Apple: Fruit {}
class Orange: Fruit {}

class Basket<T> where T: Fruit
{
    var items: [T] = []

    func add(_ item: T) {
        items.append(item)
    }
}

func addItem<T>(_ basket: Basket<T>, _ item: T) where T: Fruit
{
    basket.add(item)
}

func logType(fruit: Fruit) {
    print(type(of: fruit))
}

var basket = Basket<Fruit>()

addItem(basket, Apple())
addItem(basket, Orange())

basket.items.forEach(logType)