Why DOESN’T Kotlin smart cast work, even when both are `val`s

[dalewking]

@gopalakshintala

In the below code, both the variables are val s, but the smart cast doesn’t work even after the null check. Why?

fun SLLNode?.sumListWith(node: SLLNode?, carry: Int = 0): SLLNode? =
    when {
        this == null && node == null -> if (carry == 0) null else SLLNode(carry)
        this == null -> node.also { it!!.value += carry } // Smart cast doesn't work here.
        node == null -> this.also { value += carry } // Works here.
        else -> {
            ...
        }
    }

I ran across this and was going to reply some optimization to that code, but it occurred to me that the real issue is an incomplete design. In reality you should be using immutable data structures, not mutable ones, but I will stick with mutable.

What you need is this method:

operator fun SLLNode?.plusAssign(operand: Int): SLLNode? =
    when {
        operand == 0 -> this
        else -> this?.also { value += operand } ?: SLLNode(operand)
    }

Then your original code just becomes:

fun SLLNode?.sumListWith(node: SLLNode?, carry: Int = 0): SLLNode? =
    when {
        this == null -> node += carry
        node == null -> this += carry
        else -> {
            ...
        }
    }