Polymorphic (de)serialization crashes with inline class

[bubenheimer]

Describe the bug

A sealed interface hierarchy containing an inline class fails on polymorphic (de)serialization. Depending on specific (de)serialization method used, different errors occur:

With decodeFromString():

kotlinx.serialization.json.internal.JsonDecodingException: Expected class kotlinx.serialization.json.JsonObject (Kotlin reflection is not available) as the serialized body of MyPoly, but had class kotlinx.serialization.json.JsonLiteral (Kotlin reflection is not available)

With encodeToJsonElement():

kotlinx.serialization.SerializationException: No tag in stack for requested element

To Reproduce

@Serializable
sealed interface MyPoly {
    @Serializable
    object Invalid : MyPoly

    @Serializable
    @JvmInline
    value class Valid(val value: String) : MyPoly
}

Environment

• Kotlin version: 1.7.20
• Library version: 1.4.0
• Kotlin platforms: JVM

As for a workaround: is there anything simple besides turning the inline class into a data class?

[sandwwraith]

The problem is actually on the side of initial serialization — it should be prohibited.
We can't serialize primitives polymorphically, because there's no place to insert "type":"MyPoly.Valid" class discriminator.
When we serialize a value class polymorphically, we simply get "value" string — and the deserializer expects an object to find a type key within it.

As a workaround, you can switch to Json { useArrayPolymorphism = true } or replace the value class with a data class.

[bubenheimer]

Is it possible to do this with a custom serializer for the value class? I had tried a simple one (with PrimitiveKind.String), but that only lead me toward useArrayPolymorphism = true as you mentioned. Would a different, non-PrimitiveKind custom serializer work, or is this just not an option?

I am hesitant about useArrayPolymorphism because

This is an option for legacy JSON format and should not be generally used.

[bubenheimer]

To answer my own question: polymorphic serialization of value class works with non-PrimitiveKind custom serializer.

Ultimately, the best approach for my particular use case may be a custom serializer at the sealed interface level.

[sandwwraith]

After giving it a second thought, I can say that problem with the writing of a value class is a duplicate of #1774. Once it is fixed, it would be possible to write a value class (without a discriminator, though). It is still impossible to read it back automatically, but you can fix it with a custom content-based serializer like here: #2159