Mike Ash: Covariance is when subtypes are accepted. Overridden read-only properties are covariant. Contravariance is when supertypes are accepted. The parameters of overridden methods are contravariant. Invariance is when neither supertypes nor subtypes are accepted. Swift generics are invariant. Rick: Swift generics are normally invariant, but the Swift standard library collection types — even though … Continue reading Covariance and Contravariance