Covariance / Contravariance

[young (rtSwift)]

How are `Array` and `Value` covariant

I just noticed that Array and Value are covariant, but general generic placeholders are invariant: class Base {} class Derived: Base {} struct Foo<Value> {} // Ok let array: Array<Base> = Array<Derived>() // Fail let foo: Foo<Base> = Foo<Derived>() You can even down cast [Base] to [Derived] using as! with appropriate values. Is this a special behaviour, or is there a way to annotate it?

Covariance and Contravariance in Swift