Sealed value class is serialized/deserialized incorrectly with default json polymorphism style

[Stream29]

Describe the bug
Value class that contains a primitive value cannot be serialized polymorphically with 'type' parameter.
The kotlinx.serialization-json library does not mention it and produces incorrect behaviour.

To Reproduce

@Serializable
sealed interface SealedInterface

@Serializable
@JvmInline
value class SealedValue(val value: Int) : SealedInterface

@Serializable
enum class SealedEnum : SealedInterface {
    A, B
}

        val json = Json {
            prettyPrint = true
            isLenient = true
            ignoreUnknownKeys = true
        }
        val valueClassInstance = SealedValue(1)
        val valueJson = json.encodeToString<SealedInterface>(valueClassInstance)
        println(valueJson)
        val deserializedValue = json.decodeFromString<SealedInterface>(valueJson)

This code outputs:

1

and then throw exception:

kotlinx.serialization.json.internal.JsonDecodingException: Expected JsonObject, but had JsonLiteral as the serialized body of SealedInterface at element: $
JSON input: 1
	at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:24)
	at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:32)
	at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:400)
	at kotlinx.serialization.json.Json.decodeFromString(Json.kt:149)

Expected behavior

When facing the sealed value class that contains primitive type, a exception should be thrown directly, like the exception on sealed enum:

java.lang.IllegalStateException: Enums cannot be serialized polymorphically with 'type' parameter. You can use 'JsonBuilder.useArrayPolymorphism' instead
	at kotlinx.serialization.json.internal.PolymorphicKt.checkKind(Polymorphic.kt:71)
	at kotlinx.serialization.json.internal.StreamingJsonEncoder.encodeSerializableValue(StreamingJsonEncoder.kt:253)
	at kotlinx.serialization.json.internal.JsonStreamsKt.encodeByWriter(JsonStreams.kt:99)
	at kotlinx.serialization.json.Json.encodeToString(Json.kt:125)

Environment

• Kotlin version: [2.1.0]
• Library version: [1.8.0]
• Kotlin platforms: [all]

[Stream29]

Hello, I'd like to fix this issue.

[pdvrieze]

@Stream29 To "fix" the issue, the question needs to be what the correct serialization form should be. Should it revert to the array form of polymorphism?

[Stream29]

Yes, so I add a error when serializing value class polymorphically.
PR
Now it works as expected, throwing exception whose message is Value class that contains primitive kind cannot be serialized polymorphically with 'type' parameter. You can use 'JsonBuilder.useArrayPolymorphism' instead

[sandwwraith]

The decision to allow serialization of value class over primitive to (somewhat) incorrect form was deliberate: see #2049 (comment). Yes, we currently cannot deserialize such data back. We plan to address it systematically (as a part of #2835, most likely) rather than prohibiting writing such data completely. Besides, introducing such an exception would be a breaking change in the library.

[Stream29]

Thank you for replying!