@andylamax There seem to be an example for interfaces. I would guess there is not a way to centrally define serializer for the interface, but I think this might work

interface XY {
    val x: Int
    val y: Int
}

object XYSerializer : KSerializer<XY> {
    override val descriptor: SerialDescriptor
        get() = //TODO
    override fun deserialize(decoder: Decoder): XY {
        //TODO
    }
    override fun serialize(encoder: Encoder, value: XY) {
        //TODO
    }
}

@Serializable
data class Car(
    @Serializable(with = XYSerializer::class)
    val position: XY
)

Regarding your question, you would use a serializers module:

fun main() {
  val json = Json {
    serializersModule = SerializersModule {
      polymorphic(XY::class, Point::class, Point.serializer())
    }
  }
  println(json.encodeToString(Car.serializer(), Car(Point(42, 24))))
}

If you don’t want to register a custom serializer you can use delegation: https://gist.github.com/PaulWoitaschek/c650466b8bbc0c44797b6b10951918a5

Though that’s error prone now because once XY is not a Point it will crash

So at some point you need to do a manual step to be safe against that:

object XYSerializer : KSerializer<XY> {
  override fun deserialize(decoder: Decoder): XY = Point.serializer().deserialize(decoder)
  override val descriptor: SerialDescriptor = Point.serializer().descriptor
  override fun serialize(encoder: Encoder, value: XY) {
    encoder.encodeSerializableValue(Point.serializer(), Point(value.x, value.y))
  }
}

The custom serializer you posted is a testament that

XY

can be purely serialized with

Point.serializer()

. What I am asking now is, "Is there a current way to tell the compiler to just go ahead and use `Point.serializer()`" to Serialize

XY

Instead of having to write this custom serializer down?. In better terms, the compiler has already generated a serializer that can easily serialize and deserialize XY, it feels like there should be an easier way to tell it, "for this particular case, go ahead and use this other serializer you have already generated". This might not be a currently available feature. But there sure is a use case for it