Recursive datatypes in Kotlin

Recursive

sealed class NaturalNum

object Zero: NaturalNum()

class PlusOne<N: NaturalNum>(): NaturalNum()

fun num(zero: Zero) = 0

fun num(one: PlusOne<Zero>) = 1

@JvmName("two")
fun num(two: PlusOne<PlusOne<Zero>>) = 2

fun main(args: Array<String>) {
    val a: PlusOne<Zero> = PlusOne()
    val b: PlusOne<PlusOne<Zero>> = PlusOne()
    
    println(num(Zero)) // 0
    println(num(a))    // 1
    println(num(b))    // 2
    
    val c: PlusOne<PlusOne<PlusOne<Zero>>> = PlusOne()

    // println(num(c)) **compilation error**
    
    // fun <N: NaturalNum> thing(a: PlusOne<N>) = num(a) **compilation error**
}

The functionality is limited to cases where the generics are known during compilation, see line 26.