Covariance.swift

class Product {
    func calculateShippingCost() -> BaseShippingStrategy {
        return BaseShippingStrategy()
    }
}

class PhoneProduct: Product {
    // An instance of a subclass can always be substituted for an instance of its superclass.
    override func calculateShippingCost() -> WorldShippingStrategy {
        return WorldShippingStrategy()
    }
}