[Arnavion]

The covariant param passing was fixed some time ago with the (now on-by-default) `strictFunctionTypes` option. Eg `function f(cb: (_: Animal) => void) { cb(new Dog()); }` can no longer be used as `function takes_cat(_: Cat) { } f(takes_cat);`

Assignment is still there though (ie you can still assign a `Cat[]` to an `Animal[]` and push a `new Dog()` into it).

[kevinb7]

I believe that `strictFunctionTypes` only applies to callbacks. TypeScript is okay with the following code even with that option turned on:

  class Animal {}
  class Cat extends Animal { meow() {} }
  class Dog extends Animal { woof() {} }

  function foo(animals: Animal[]) {
    animals.push(new Dog)
  }

  const cats: Cat[] = [new Cat];

  foo(cats);

https://www.typescriptlang.org/play/#code/MYGwhgzhAECCB2BLAt...

[Arnavion]

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.