Kotlin, remove duplicates from Linkedlist : smart cast is impossible - kotlin

Question

I implemented a LinkedList in kotlin and wrote a method to remove duplicates from it:
class Node (value:Int) {
var value = value
var next:Node? = null
fun addNodeToTail(value:Int){
var node = this
while (node.next != null) {
node = node.next
}
val newNode= Node(value)
node.next= newNode
}
fun removeDuplicates (){
val set = HashSet<Int>()
var node = this
set.add(node.value)
while(node.next != null){
if (set.contains(node.next?.value)){
node.next= node.next?.next
}else{
set.add(node.next.value)
node= node.next
}
}
}
}
In the last two lines:
set.add(node.next.value)
node= node.next
(and in the addNodeToTail method), the compiler says that smart cast is impossible because of complex expression. I have to add non-null asserted call (!!).
I want to understand why this solution is not accepted, although the while expression checks that node.next is not null. And I want to know if there is a better solution than using non-null asserted call (!!).
Thank you for your help

Answer 1

Pawel technically answered in the comment.
Basically smart casts are not always possible. In particular, if you define a mutable var of nullable type that is technically accessible by multiple threads, the compiler cannot guarantee that the value stays the same between the null check and the usage. That's why you get this error "smart cast impossible".
A common way of dealing with the problem is to store the value in a local val variable, to guarantee that this value will not change, and allow the compiler to smart cast it.
In your case though, it's not ideal because the while has to check the actual node's value every time. So you'll have to assert that the value is not null at some point, either with !! or with an elvis (?:) and an error() or throw.
I would personally go for:
while (node.next != null) {
val nextNode = node.next ?: throw ConcurrentModificationException()
if (set.contains(nextNode.value)) {
node.next = nextNode.next
} else {
set.add(nextNode.value)
node = nextNode
}
}