polymorphicDefault is never invoked

[cy6erGn0m]

Describe the bug

A serializer lookup lambda registered via polymorphicDefault in a module is never used.

To Reproduce

@Serializable
data class X(@Polymorphic val any: Any)

private val testModule = SerializersModule {
  polymorphic(Any::class, String::class, String.serializer())
  polymorphic(Any::class, Int::class, Int.serializer())
  polymorphic(Any::class, Long::class, Long.serializer())
//  polymorphic(Any::class, Float::class, Float.serializer()) // uncomment to make it work

  polymorphicDefault(Any::class) { className ->
    when (className) {
      "kotlin.Float" -> Float.serializer() // never invoked
      else -> null
    }
  }
}

fun main() {
  val json = Json {
    serializersModule = testModule
    useArrayPolymorphism = true // the only way to have polymorphic working in json
  }

  println(json.encodeToString(X(1.0f)))
}

Expected behavior
I expect polymorphicDefault to be invoked in this case with serial name kotlin.Float.

So it should print the following instead of a crash:

{"any":["kotlin.Float",1.0]}

Environment

• Kotlin version: 1.5.10
• Library version: 1.2.1
• Kotlin platforms: JVM

[pdvrieze]

polymorphicDefault is only supported for decoding, not for encoding. See:

kotlinx.serialization/core/commonMain/src/kotlinx/serialization/modules/SerializersModule.kt

Line 184 in d489d96

internal typealias PolymorphicProvider<Base> = (className: String?) -> DeserializationStrategy<out Base>?

and

kotlinx.serialization/core/commonMain/src/kotlinx/serialization/modules/SerializersModule.kt

Lines 140 to 151 in d489d96

	override fun <T : Any> getPolymorphic(baseClass: KClass<in T>, value: T): SerializationStrategy<T>? {
	if (!value.isInstanceOf(baseClass)) return null
	return polyBase2Serializers[baseClass]?.get(value::class) as? SerializationStrategy<T>
	}
	override fun <T : Any> getPolymorphic(baseClass: KClass<in T>, serializedClassName: String?): DeserializationStrategy<out T>? {
	// Registered
	val registered = polyBase2NamedSerializers[baseClass]?.get(serializedClassName) as? KSerializer<out T>
	if (registered != null) return registered
	// Default
	return (polyBase2DefaultProvider[baseClass] as? PolymorphicProvider<T>)?.invoke(serializedClassName)
	}

It is mainly designed to support handling unknown inputs from a network, not to circumvent the module system.

[sandwwraith]

Yep, it's only invoked on deserialization, but it seems there are several use-cases where it may be useful on serialization too (see similar #1317)

[cy6erGn0m]

This is inconsistent and assymmetric. Should work the same in both directions

[sandwwraith]

Duplicate of #1317 — fixed by #1686