sealed class User {
    object Unauthenticated: User()
    sealed class Authenticated: User() {
        object Client: Authenticated()
        object Owner: Authenticated()
    }
}

fun getUser(): User {
    return User.Authenticated.Client
}

fun main() {
    when (val user: User = getUser()) {
        is User.Unauthenticated -> { println("Not Authenticated!") }
        is User.Authenticated -> {
            when (user) {
                is User.Authenticated.Client -> { println("Hello Client!") }
                is User.Authenticated.Owner -> { println("Hello Owner!") }
            }
        }
    }
}

The second

when

expression shows the warning:

I can add to the

when

expression

user as Authenticated

but then it is marked as "unnecessary":

I found two issues related to this problem, one is from 2 years ago https://youtrack.jetbrains.com/issue/KT-29087, and the other is from 4 months ago https://youtrack.jetbrains.com/issue/KT-41305. Both are unassigned so I guess all we can do is wait and ignore the warning