Optional properties are not working on Kotlin 1.5.31

I’m dealing with demanding API, that has 3 states: value, null value, no value. It’s a case described in blog post here: https://medium.com/livefront/kotlinx-serialization-de-serializing-jsons-nullable-optional-properties-442c7f0c2614

To Reproduce I have serialiser as in blog post:

open class OptionalPropertySerializer<T>(
    private val valueSerializer: KSerializer<T>
) : KSerializer<OptionalProperty<T>> {
    final override val descriptor: SerialDescriptor = valueSerializer.descriptor

    final override fun deserialize(decoder: Decoder): OptionalProperty<T> =
        OptionalProperty.Present(valueSerializer.deserialize(decoder))

    final override fun serialize(encoder: Encoder, value: OptionalProperty<T>) {
        when (value) {
            OptionalProperty.NotPresent -> throw SerializationException(
                "Tried to serialize an optional property that had no value present." +
                        " Is encodeDefaults false?"
            )
            is OptionalProperty.Present ->
                valueSerializer.serialize(encoder, value.value)
        }
    }
}

on Kotlin 1.5.30 this valueSerializer was injected correctly. However on 1.5.31 I’m always getting there instance of: kotlinx.serialization.internal.UnitSerializer

Expected behavior It should work as in Kotlin 1.5.30. I’m not sure what created that regression (https://github.com/JetBrains/kotlin/releases/tag/v1.5.31) or is it serialisation issue or Kotlin’s.

Environment

• Kotlin version: 1.5.31
• Library version: 1.3.0, also tested with the same result on 1.2.2
• Kotlin platforms: JVM, native
• Gradle version: 7.2

[shanshin]

As a workaround, you need to remove the @Serializable(OptionalPropertySerializer::class) annotation from the Optional class and add it to each property of your data class (like in blog post).

[C2H6O]

Related YouTrack ticket: https://youtrack.jetbrains.com/issue/KT-50764