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 #2049 (comment)