Why does this produce a warning: ```open class Sup...

# getting-started

[JP Sugarbroad]

Why does this produce a warning:

open class Super

class Container<T: Super>

fun test(thing: Any) {
    if (thing is Container<*>) {
        val thing2 = thing as Container<Super>
    }
}

But this does not?

open class Super

class Container<T: Super>

fun test(thing: Any) {
    if (thing is Container<out Super>) {
        val thing2 = thing as Container<Super>
    }
}

Shouldn't * be equivalent to out Super in this case?

[loke]

I'm more surprised that the second doesn't show a warning. You're doing is on an erased type.

[loke]

Ah, it's because you are checking for exactly the toplevel type for Container.

[JP Sugarbroad]

Seems like a soundness hole to me.

[Sam]

That is weird...

fun main() {
  test(mutableListOf<String>())
}

fun test(list: Any) {
  if (list is MutableList<out Any?>) {
    val list2 = list as MutableList<Any?>
    list2.add(null)
  }
}

Definitely feels like there's a warning missing here.

[JP Sugarbroad]

Does it happen on K1?

[JP Sugarbroad]

I can't test right now.

[Sam]

Oh good shout, it does indeed produce an unchecked cast warning when K2 mode is disabled

[Sam]

I guess it's a K2 bug then? You could create a YouTrack issue for it, or let me know if you would like me to create one

[JP Sugarbroad]

I'll do it. Thanks for checking!

[JP Sugarbroad]

KT-77444 K2: Casting from List to List should produce a warning

[Sam]

Nice 👍. Would it be better to explicitly mention MutableList in the description rather than just List? A List<T> is always a List<out T> so I suspect it wouldn't exhibit the bug.

[JP Sugarbroad]

Oh oops

[JP Sugarbroad]

Fixed!