Type unsoundness of Kotlin

console.txt

$ kotlinc Main.kt
$ kotlin MainKt
Exception in thread "main" java.lang.ClassCastException: java.lang.Object cannot be cast to B
        at B.compareTo(Main.kt:4)
        at MainKt.main(Main.kt:9)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.jetbrains.kotlin.runner.AbstractRunner.run(runners.kt:60)
        at org.jetbrains.kotlin.runner.Main.run(Main.kt:99)
        at org.jetbrains.kotlin.runner.Main.main(Main.kt:104)

Main.kt

open class A() {
  open fun compareTo(o: Any): Int = 0
}
class B() : A(), Comparable<B> {
  override fun compareTo(o: B): Int = 0
}

fun main(args: Array<String>) {
  println(B().compareTo(Object()))
}