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kotlinlang

Anyone know the API for allowing a "default if type is missing for polymorphic deserialization"? Long story short, I inherited a system that has a bunch of JSON objects like `{ "a": 1, "b":2 }`  and want to start storing a new set of polydata, where it would end up serializing `{ "a": 1, "b":2, "c":2, "type": "com.example.WibbleV2" }` ... the problem is the existing records would _*NOT*_ have the type field for it to know it was V1.... trying to avoid the work of custom deserializers.

Example (pseudo):
```@Serializable
sealed class WibbeTypes {
  @Serializable
  data class WibbleV1(val a: Int, val b: Int): WibbleTypes

  @Serializable
  data class WibbleV2(val a: Int, val b: Int, val c: Int): WibbleTypes
}```
New WibbleV1 records would serialize with the type, but getting old records out and deserializing would fail because the type field is missing.

<https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/polymorphism.md#default-polymorphic-type-handler-for-deserialization> doesn't involve manual serializers, but it does require manual registration