For some reason, I can't figure out what is going on here:

class Ref<K: Comparable<K>, T>(val key: K, val value: T) : Comparable<Ref<K, T>> {
    override fun compareTo(other: Ref<K, T>) = key.compareTo(other.key)
}

fun main(args: Array<String>) {
    val list = mutableListOf<Ref<Int, *>>()
    list.sorted()  // ERROR: Ref<Int, *> is not a subtype of Comparable<Ref<Int, *>>
}

Any ideas?

Try declaring

Ref

as

class Ref<K: Comparable<K>, T>(val key: K, val value: T) : Comparable<Ref<K, *>> {
    override fun compareTo(other: Ref<K, *>) = key.compareTo(other.key)
}

Ah, that makes sense.