[Shacklz]

From my point of view, this is one of those cases where structural typing just doesn't work all that well when used for OOP.

Typescript does give you a solution to this problem, namely that you use generics to constrain the parameters of your method:

    let append_animals = <T extends Animal>(animals: T[], animal: T) => animals.push(animal)

Your example now gives the expected error.

TypeScript does indeed have its quirks, but most of them do not really matter for real-life purposes or can easily be worked around like in the example above.