<@U2E974ELT> This is a MCVE for the problem mentio...

# announcements

[melatonina]

@elizarov This is a MCVE for the problem mentioned in my previous message.
Calling next() on A from this code:

interface X<T> {
    operator fun plus(n: Int) : T
    fun next(): T = this + 1
}

inline class A(val value: Int) : X<A> {
    override operator fun plus(n: Int) = A(value + n)
}

causes

Exception in thread "main" java.lang.ClassCastException: class com.example.A cannot be cast to class java.lang.Number (com.example.A is in unnamed module of loader 'app'; java.lang.Number is in module java.base of loader 'bootstrap')
	at com.example.A.next-m5RZA0A(Example.kt:32)
	at com.example.ExampleKt.test(Example.kt:5)

This version causes the same exception:

interface X<T> {
    fun add(n: Int) : T
    fun next(): T = add(1)
}

inline class A(val value: Int) : X<A> {
    override fun add(n: Int) = A(value + n)
}

This version is OK:

interface X<T> {
    fun next(): T
}

inline class A(val value: Int) : X<A> {
    override fun next() =  A(value + 1)
}

This version is OK, too:

interface X<T> {
    fun next(): T
}

inline class A(val value: Int) : X<A> {
    operator fun plus(n: Int) = A(value + n)
    override fun next() =  this + 1
}

Does it have anything to do with boxing? Is this because the plus operator is called on a reference of an interface?
Is this something limited to mathematical operators?
What should I do? Should I prefer code generation instead of interface implementation when using inline classes?

[Ruckus]

I don't have much to add, but I tried this for fun using this code:

interface X<T> {
    operator fun plus(n: Int) : T
    fun next(): T = this + 1
}

inline class A(val value: Int) : X<A> {
    override operator fun plus(n: Int) = A(value + n)
}

fun main() {
    println(A(7).next())
}

In the playground (https://pl.kotl.in/m-9bCv_Rs) I get an error:

Exception in thread "main" java.lang.ClassCastException: A cannot be cast to java.lang.Number
 at A.next-GKOAj6k (File.kt:6) 
 at FileKt.main (File.kt:11) 
 at FileKt.main (File.kt:-1)

On my local machine it works no problem (I see A(value=8))

[elizarov]

Thanks a lot. I’ve filed a bug: https://youtrack.jetbrains.com/issue/KT-45539