compare_classes.kt

import kotlin.reflect.KClass

inline operator fun <reified L : Any, reified R : Any> KClass<L>.compareTo(other: KClass<R>): Int {
    return if (this === other) {
        0
    } else if (this.java.isAssignableFrom(other.java)) {
        -1
    } else if (other.java.isAssignableFrom(this.java)) {
        1
    } else {
        throw IllegalArgumentException("Classes doesn't have common hierarchy.")
    }
}

open class A {}
open class B: A() {}
open class C: B() {}

object Main {
    @JvmStatic
    fun main(vararg args: String) {
        println(A::class < B::class)
        println(A::class <= B::class)
        println(A::class == B::class)
        println(A::class >= B::class)
        println(A::class > B::class)

        println(A::class < A::class)
        println(A::class > A::class)

        println(B::class <= B::class)
        println(B::class >= B::class)
    }
}

Here's a shorter version that doesn't use generics and reference equality:

operator fun KClass<*>.compareTo(other: KClass<*>) = when {
    this == other -> 0
    this.java.isAssignableFrom(other.java) -> -1
    other.java.isAssignableFrom(this.java) -> 1
    else -> throw IllegalArgumentException("Classes doesn't have common hierarchy.")
}