can anyone help with my question on <KT-54309>?

# compiler

[Alex Schiff]

can anyone help with my question on KT-54309?

[dmitriy.novozhilov]

Answered

[Alex Schiff]

Thanks! I do get an error when trying to invoke the method itself:

[dmitriy.novozhilov]

Exactly, there is an error because it is unsafe
Consider the following example (which is more or less identical to yours)

class Box<T>(var x: T)

fun test(b: Box<*>) {
    val a: Any? = b.x
    b.x = "hello" // unsafe, error
}

fun main() {
    val box = Box(1)
    test(box)
}

Inside function test you don't know which exact Box was passed inside
So the type of b is actually Box<Captured(*)>
Captured(*) means that it is some type which can be passed outside. It may be Int, it may be String, it maybe anything.
So when you try to read b.x it will return this Captured(*), which getting approximated to Any? (as there are no bounds which reduces the set of possible types for it)

But if Captured(*) stands in input position (like in setter method or when you just assign something to property of such type), there is actually no any reasonable type for it
You can't assign String to b.x, as at call site you can pass Box<Int>
You can't assign Int to b.x, as at call site you can pass Box<String>

[Alex Schiff]

Hmm. So is there any way to invoke this method from kotlin? or is it not possible?

[Alex Schiff]

in 2.0.0

[dmitriy.novozhilov]

And KT-54309 stands for the same situation form my example above, but when Box is not kotlin class but java class with getter and setter instead of property

There was a bug which allowed to write something to such synthetic property, which may lead to runtime exceptions:

public class Box<T> {
    public Box(T x) {this.x = x;}

    private T x;

    public T getX() {return x;}

    public void setX(T x) {this.x = x;}
}

fun test(b: Box<*>) {
    b.x = 10
}

// compiles with K1
fun main() {
    val box = Box("hello")
    test(box)
    box.x.length // ClassCastException
}

[dmitriy.novozhilov]

You can call this method with unchecked cast, but only if you are really sure that it is safe and you won't get an exception at runtime (compiler warned you)

fun test(b: Box<*>) {
    @Suppress("UNCHECKED_CAST")
    (b as Box<Int>).setX(10)
}

[Alex Schiff]

still not working for me 😞 is there a warning that I can suppress for the fact that kotlin expects the value to be Nothing?

[dmitriy.novozhilov]

You need to cast to some specific type which allows you to set the value you want
Check my last example again: I'm casting Box<*> to Box<Int> to get an ability to write Int to x

[Alex Schiff]

ok let me try that

[Alex Schiff]

(producerFactory as DefaultKafkaProducerFactory<String ,Any>) worked!

[Alex Schiff]

thanks!!!

[dmitriy.novozhilov]

You are welcome

[Alex Schiff]

This is a little over my head so i appreciate your help