Why does the smart-casting not work here? And how would you solve it?

fun main() {
    var value: Any? = "hello"

    if (value != null) {
        for (i in 1..10) {
            value = nonNullFoo(value) // smart casting fails
        }
    }
}

fun nonNullFoo(obj: Any): Any {
    return "World"
}

I believe it fails because

value

might have been changed and the compiler can't guarantee it wasn't

value

is only every assigned the result of

nonNullFoo

which is

Any

the compiler just can't see that far

I believe that's exactly it

thanks

You could use

value?.let {...}

I think

let

will mask the fact that

value

can never be null. It will give the obvious possibility that it is null sometimes

The compiler looks further, actually. Who knows if

value

is being changed by another thread somewhere else in the code? Or even from some included dependency library? Contracts is the way to solve this.

how could

value

be changed by another thread when it's a local variable?

Value is local, other threads cant touch the reference

Admittedly! Don't know why I missed that, I just answered with the autopilot turned on, since this question arises from time to time, and mostly with code where a different thread actually may touch the

value

... I decompiled the code to take a look at the generated bytecode, and am now just as stumped as you are. My only explanation for this is that the compiler must be as blind as I was, and/or didn't dare insert a

Contract

all by itself.

@poohbar I just wrote the same code in a scratch file and the smart cast works just as intended. This should only fail when dealing with

var

properties, what version of Kotlin are you using ?