Sealed object instances

Extensions.kt

import kotlin.reflect.KClass

fun <T : Any> KClass<T>.sealedObjectInstances() = recursiveSealedObjectInstances()

private tailrec fun <T : Any> KClass<T>.recursiveSealedObjectInstances(
    sealedSubclasses: List<KClass<out T>> = listOf(this),
    acc: Set<T> = emptySet(),
): Set<T> = when {
    sealedSubclasses.isEmpty() -> acc
    else -> recursiveSealedObjectInstances(
        acc = acc + sealedSubclasses.mapNotNull { it.objectInstance },
        sealedSubclasses = sealedSubclasses.flatMap { it.sealedSubclasses },
    )
}

Main.kt

fun main() = Sealed::class.sealedObjectInstances()
    .joinToString(separator = "\n") { it.javaClass.name }
    .let(::println)

// Sealed$SealedObject
// Sealed$SealedObject$NestedSealedObject
// ExternalSealedObject
// Sealed$SubSealed$SubSealedObject

Sealed.kt

sealed class Sealed {

    object SealedObject : Sealed() {
        object NestedSealedObject : Sealed()
    }

    sealed class SubSealed : Sealed() {
        object SubSealedObject : SubSealed()
        data class DataSubSealed(val any: Any) : SubSealed()
    }

}

object ExternalSealedObject : Sealed()