Kotlin: Using enums with when

You don't need when in this particular use-case.

Since your goal is to get the name of the enum element having a specific value x, you can iterate over the elements of PaymentStatus like that and pick the matching element using firstOrNull:

fun getStatusWithValue(x: Int) = PaymentStatus.values().firstOrNull {
     it.value == x
}?.toString()

println(getStatusWithValue(2)) // "UNPAID"

Calling toString() on an enum element will return its name.

Edit: Since you don't want the code to compile when a new PaymentStatus is added, you can just use an exhaustive when:

fun paymentStatusNumToString(x: Int): String {
  val status = PaymentStatus.values().first { it.value == x }

  // when must be exhaustive here, because we don't use an else branch
  return when(status) {
    PaymentStatus.PAID -> "PAID" // you could use status.toString() here too
    PaymentStatus.UNPAID -> "UNPAID"
  }
}

If you need to check a value you can do something like this:

fun f(x: Int) {
    val foo = when (x) {
        PaymentStatus.PAID.value -> "PAID"
        PaymentStatus.UNPAID.value -> "UNPAID"

        else -> throw IllegalStateException()
    }
}

Or you can create factory method create in the companion object of enum class:

enum class PaymentStatus(val value: Int) {
    PAID(1),
    UNPAID(2);

    companion object {
        fun create(x: Int): PaymentStatus {
            return when (x) {
                1 -> PAID
                2 -> UNPAID
                else -> throw IllegalStateException()
            }
        }
    }
}

fun f(x: Int) {
    val foo = when (PaymentStatus.create(x)) {
        PaymentStatus.PAID -> "PAID"
        PaymentStatus.UNPAID -> "UNPAID"
    }
}