Method call with nullable and not nullable Collection

Question

Is it possible in Kotlin to write a function which can be called with nullable and not nullable collections? I think about something like this:

fun <C: MutableCollection<out String>> f(c: C): C {
    // ...
}

Not that I need to write it like this because I have a return value of type C. Also note the out keyword but even with it I can't call f(mutableListOf<String?>) but f(mutableListOf<String>) works fine. What do I have to change here or isn't that possible in Kotlin? With arrays this would work just fine...

Answer 1

fun <C: MutableCollection<out String?>> f(c: C): C {
    return c;
}

fun main(args: Array<String>) {
    println(f(mutableListOf("hello")))
    println(f(mutableListOf<String?>(null)))
}