qh.i: Class 'j' is not registered for polymorphic serialization in the scope of 'h0'.

Describe the bug

After building the app in the release variant polymorphic serialization doesn’t work.

To Reproduce

SocketEvent.kt

@Serializable
sealed interface SocketEvent

AnalyticsEvent.kt

@Serializable
sealed interface AnalyticsEvent : SocketEvent {
    @Serializable
    data class PlayerState(
        @SerialName("player_state") val playerState: State,
     ) : AnalyticsEvent

    @Serializable
    data class ButtonClick(
        @SerialName("screen_name") val screenName: String,
     ) : AnalyticsEvent
}

SocketConnectionEvent.kt

@Serializable
sealed class SocketConnectionEvent(
    @SerialName("event") open val event: String,
) : SocketEvent {

    @Serializable
    class PingEvent : SocketConnectionEvent(
        event = SocketEventName.PING.event
    )
}

WebSocketClient.kt

override suspend fun send(event: SocketEvent) {
        val trimmed = event.trimType()
        session?.run {
            sendSerialized(trimmed)
        } ?: Timber.e("No session")
    }

    /**
     * As kotlinx-serialization adds type discriminator to serialized object we need to trim it.
     * See: https://github.com/Kotlin/kotlinx.serialization/issues/464
     */
    private fun SocketEvent.trimType(): JsonElement {
        val json = Json.encodeToJsonElement(this) // <- here we get the Exception
        val valuesWithoutType = json.jsonObject.filterNot { it.key == "type" }
        return Json.encodeToJsonElement(valuesWithoutType)
    }

Proguard configuration

# Keep `Companion` object fields of serializable classes.
# This avoids serializer lookup through `getDeclaredClasses` as done for named companion objects.
-if @kotlinx.serialization.Serializable class **
-keepclassmembers class <1> {
    static <1>$Companion Companion;
}

# Keep `serializer()` on companion objects (both default and named) of serializable classes.
-if @kotlinx.serialization.Serializable class ** {
    static **$* *;
}
-keepclassmembers class <2>$<3> {
    kotlinx.serialization.KSerializer serializer(...);
}

# Keep `INSTANCE.serializer()` of serializable objects.
-if @kotlinx.serialization.Serializable class ** {
    public static ** INSTANCE;
}
-keepclassmembers class <1> {
    public static <1> INSTANCE;
    kotlinx.serialization.KSerializer serializer(...);
}

# @Serializable and @Polymorphic are used at runtime for polymorphic serialization.
-keepattributes RuntimeVisibleAnnotations,AnnotationDefault

Expected behavior

Object serialization works for release build as it works for debug

Environment

• Kotlin version: 1.7.10
• Library version: 1.4.0
• Kotlin platforms: JVM
• Gradle version: 7.5.1
• Android Gradle Plugin: 7.4.0-beta02

[bubenheimer]

So to wrap this up as far as R8 fullMode goes, I suggest adding the following to the sample Proguard in the README. It should not hurt or change anything when not using R8 fullMode, yet without these rules, running fullMode is borked.

-keep, allowoptimization, allowobfuscation class kotlinx.serialization.Serializable { *; }

-if @kotlinx.serialization.Serializable class **
-keep, allowshrinking, allowoptimization, allowobfuscation class <1>

There is more context in the linked Android issuetracker reference.

Edit: based on the update to the R8 FAQ, the least restrictive rule would actually be as follows, but this one only works for R8 users, it sounds like Proguard would show an error:

-if @kotlinx.serialization.Serializable class **
-keep, allowshrinking, allowoptimization, allowobfuscation, allowaccessmodification class <1>

[bubenheimer]

I just ran into this same problem, except I even have a custom serializer on the base class to eclipse the polymorphism. So it seems that the custom serializer is lost entirely as well, despite using the standard recommended Proguard config from the kotlinx.serialization repo. Or at least this is what I think the error implies. (I use R8.)

The custom serializer class is still there, but it seems like it is not being used, as if the annotation parameter had gone missing or did not get obfuscated the same way.