Ковариантный ящик и Nothing

sealed interface Res<out T>
data class Ok<T>(val v: T) : Res<T>
data object Fail : Res<Nothing>

fun <T> Res<T>.orElse(d: T): T = when (this) {
    is Ok -> v
    Fail -> d
}

fun main() {
    val a: Res<String> = Ok("x")
    val b: Res<String> = Fail
    val c: Res<Any> = a
    println(a.orElse("default"))
    println(b.orElse("default"))
    println(c)
}