SealedClassSerializer doesn't handle value classes correctly

[valeriyo]

Describe the bug SealedClassSerializer doesn't serialize value subclasses into the expected structure, with type property. Instead it just dumps the raw value, which won't deserialize back!

To Reproduce See test below

Expected behavior Instead of raw value, the resulting JSON should be of the same structure, as regular subclasses: "{\"type\":\".......\",\"value\":......}"

Environment

• Kotlin version: 2.0.21
• Serialization: 1.7.3
• Gradle 8.8

• Android Studio Ladybug

  
  import kotlin.test.assertEquals
  import kotlinx.serialization.SerialName
  import kotlinx.serialization.Serializable
  import kotlinx.serialization.json.Json
  import org.junit.Test

@Serializable sealed interface Sealed {

@Serializable @SerialName("DataObject") data object DataObject : Sealed

@Serializable @SerialName("DataClass") data class DataClass(val code: Int) : Sealed

@Serializable @SerialName("ValueClass") @JvmInline value class ValueClass(val code: Int) : Sealed }

class SealedTest {

@Test fun test() { val serializer = Sealed.serializer()

assertEquals(
  expected = "{\"type\":\"DataObject\"}",
  actual = Json.encodeToString(serializer, Sealed.DataObject),
)

assertEquals(
  expected = "{\"type\":\"DataClass\",\"code\":111}",
  actual = Json.encodeToString(serializer, Sealed.DataClass(111)),
)

assertEquals(
  expected = "222", // WRONG! should be "{\"type\":\"DataClass\",\"value\":222}"
  actual = Json.encodeToString(serializer, Sealed.ValueClass(222)),
)

} }

[sandwwraith]

This is caused by the fact that your ValueClass is an inline class over Int. There is simply nowhere to add type, because Int is not an object. See https://github.com/Kotlin/kotlinx.serialization/issues/2049#issuecomment-1476647228