Parametric polymorphism gets you subtyping for function types even if you don't have it in general. `forall a . a -> a` is a subtype of `T -> T` for any particu

Parametric polymorphism gets you subtyping for function types even if you don't have it in general. `forall a . a -> a` is a subtype of `T -> T` for any particular `T`, for instance. And if you don't have subtyping or polymorphism, then talking about "leaving out" co/contravariance doesn't really make sense: there's nothing left to vary.