fun isNull(x: Unit?) = x == null

fun isNullGeneric<T : Any>(x: T?) = x == null

fun deepIsNull0(x: Unit?) = isNull(x)
fun deepIsNull(x: Unit?) = deepIsNull0(x)

fun box(): String {
    if (!isNull(null)) return "Fail 1"

    val x: Unit? = null
    if (!isNull(x)) return "Fail 2"

    val y = x
    if (!isNullGeneric(y)) return "Fail 3"

    if (!deepIsNull((null : Unit?) ?: null)) return "Fail 4"

    return "OK"
}