Not sure if this has ever been discussed before, but here we go: just recently I had wanted to extend the List companion with a factory method, but noticed List (and most or all other collection classes) don't define a companion object, so there's nothing you can extend. Of course it's completely possible to just place these extensions elsewhere and import them as static methods, but for the sake of discoverability,

List

etc. is what people would first try to find a method on in order to work with lists. That said, do you see value in defining empty companions for these classes as a hook for standard library extensions?