TypeEquivalence.kt

class Covariant<out T>
class Contravariant<in T>
class Invariant<T>

infix fun KType.equiv(that: KType) = isSubtypeOf(that) && isSupertypeOf(that)

@Test
fun mooEver() {
    /* Covariant<*> =:= Covariant<Any?> =:= Covariant<out Any?> */
    /* Covariant<in Nothing> is an illegal type */
    val co: Covariant<*> = Covariant<Any?>()
    val co2: Covariant<Any?> = co

    /* Contravariant<*> =:= Contravariant<Nothing> =:= Contravariant<in Nothing> */
    /* Contravariant<out Any?> is an illegal type */
    val contra: Contravariant<*> = Contravariant<Nothing>()
    val contra2: Contravariant<Nothing> = contra

    /* Invariant<*> =:= Invariant<out Any?> */
    val inv: Invariant<*> = Invariant<Any?>() as Invariant<out Any?>
    val inv2: Invariant<out Any?> = inv

    /* Now, the same with reflection */
    val top = KTypeProjection.invariant(Any::class.createType(nullable = true))
    val bottom = KTypeProjection.invariant(Nothing::class.createType())

    val star = KTypeProjection.STAR

    val outtop = top.copy(variance = KVariance.OUT)
    val inbottom = bottom.copy(variance = KVariance.IN)

    fun covariant(arg: KTypeProjection) =
            Covariant::class.createType(arguments = listOf(arg))
    fun contravariant(arg: KTypeProjection) =
            Contravariant::class.createType(arguments = listOf(arg))
    fun invariant(arg: KTypeProjection) =
            Invariant::class.createType(arguments = listOf(arg))

    /* Covariant<*> =:= Covariant<Any?> =:= Covariant<out Any?> */
    assertTrue(covariant(top) equiv covariant(star))
    assertTrue(covariant(outtop) equiv covariant(star))
    /* Covariant<in Nothing> =:= Covariant<*> WAT??? */
    assertTrue(covariant(inbottom) equiv covariant(star))

    /* Contravariant<*> !=:= Contravariant<Nothing> WAT??? */
    assertFalse(contravariant(bottom) equiv contravariant(star))
    /* Contravariant<*> !=:= Contravariant<in Nothing> WAT??? */
    assertFalse(contravariant(inbottom) equiv contravariant(star))
    /* Contravariant<*> =:= Contravariant<out Any?> WAT??? */
    assertTrue(contravariant(outtop) equiv contravariant(star))

    /* Invariant<*> =:= Invariant<out Any?> */
    assertTrue(invariant(outtop) equiv invariant(star))
}