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Is it possible to have two different values for the `SerialName` of a polymorphic type? I basically want to migrate from one type to another whilst still supporting the old version

For JSON there's `@JsonNames`

<https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-names/|https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-names/>

For non-JSON formats you can either write a custom Serializer (which can be a hassle). 

Or, define an internal constructor property for each alternative name, make each one nullable with `null` as the default value (so it's considered optional), mark each as  internal, and define a public class property to expose whatever alternative isn't null using the elvis operator.

This sounds like a good solution for properties, but not for the value of the `type` field for polymorphic serialisation which is what I'm looking for.

Basically, I'd like a way to have something like `{ "type": "foo" }` and `{ "type": "bar" }` use the same type

Ah sorry, I misunderstood. You're right, I answered a different question! 

Are you working with JSON? If so, I'd look at using JsonContentPolymorphicSerializer

<https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/json.md#content-based-polymorphic-deserialization|https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/json.md#content-based-polymorphic-deserialization>

Haven't tried it, but maybe?

```@SerialName("Foo")
data class Foo()

@SerialName("NewFoo")
typealias NewFoo = Foo```