ClassFormatError when using Context Receivers

Support
1

I was playing around with context receivers (added the opt-in compiler argument). The following code compiles, but throws a ClassFormatException:

package test

interface Semigroup<A> {
    fun op(x: A, y: A): A
}

object PlusSemigroup : Semigroup<Int> {
    override fun op(x: Int, y: Int): Int = x + y
}

data class Box<A>(val value: A)

context(Semigroup<A>)
interface BoxSemigroup<A> : Semigroup<Box<A>> {
    override fun op(x: Box<A>, y: Box<A>): Box<A> =
        Box(op(x.value, y.value))
}

fun <A> boxSemigroup(underlying: Semigroup<A>): BoxSemigroup<A> =
    object : BoxSemigroup<A> {}

fun main() {
    val boxSemigroup = boxSemigroup(PlusSemigroup)
    println(boxSemigroup.op(Box(2), Box(5)))
}

I get the following error:

Error: Unable to initialize main class test.TestKt
Caused by: java.lang.ClassFormatError: Illegal field modifiers in class test/BoxSemigroup: 0x1012

I tried different versions of this, e.g. also using with(underlying) in the boxSemigroup function, but it made no difference.

I’m using Java 17 (OpenJDK Runtime Environment (build 17+35-2724), Windows) and Kotlin 1.8.0

Before I file a bug, I wanted to ensure that I don’t do something stupid, and that others can confirm this behavior.

2

Okay, there seems to be missing support for context receivers for interfaces:

https://youtrack.jetbrains.com/issue/KT-51881/Consider-proper-support-for-context-receivers-in-interfaces

That said, this feature would be incredible useful, much more than for classes (where it works correctly).