seri.kt

package sandbox.serializationtest

import kotlinx.serialization.KSerializer
import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json

@Serializable(with = XYSerializer::class)
interface XY {
  val x: Int
  val y: Int
}

object XYSerializer : KSerializer<XY> by Point.serializer() as KSerializer<XY>

@Serializable
data class Point(
  override val x: Int,
  override val y: Int
) : XY

@Serializable
data class Car(
  val position: XY
)

fun main() {
  val originalCar = Car(Point(42, 24))
  val carJson = Json.encodeToString(Car.serializer(), originalCar)
  val decodedCar = Json.decodeFromString(Car.serializer(), carJson)
  check(originalCar == decodedCar)
}