Your example is identical to the case I covered in the second paragraph of my comment. ie your code compiles because it is legal to assign a `Cat[]` to an `Animal[]`, as I said.

When you mentioned "covariant param passing" I assumed you were referring to the long-lasting issue about callbacks, hence my original comment mentioning that that issue has been fixed. It's not useful to differentiate between "assigning a value to a variable of a certain type" and "passing a value to a function parameter of a certain type" because those are identical.