co-/contra-variance is a fact about the underlying data structure, not an aberration of the type system. You need to reason about it even in an untyped language

co-/contra-variance is a fact about the underlying data structure, not an aberration of the type system. You need to reason about it even in an untyped language. The number of hours I've spent debugging Python/PHP code written by developers who don't understand co-/contra-variance...