Cannot pass JsonObject containing JsonArray to Swift in a Lambda/Interface

[bitsycore]

Describe the bug

When I try to pass a JsonObject to Swift i get the following error
Could not cast value of type "KListAsNSArray" to "Kotlinx_serialization_jsonJsonElement"
It seem that the interface List on JsonArray is taking over JsonElement in that situation.

To Reproduce

// Crash (also if it's an interface like MyListener { fun onAnswer(inData: JsonObject) }
fun test(listener: (JsonObject) -> Unit) {
    listener(
        buildJsonObject {
            put("test", "test")
            putJsonArray("test2") {
                add("test")
                add("test2")
            }
        }
    )
}

If it's directly a JsonArray it work

// Work
fun test2(listener: (JsonArray) -> Unit) {
    listener(
        buildJsonArray {
            add("test")
            add("test2")
        }
    )
}

If it's in a returned value it also work

// Work
fun test3() : JsonObject {
      return buildJsonObject { 
            put("test", "test")
            putJsonArray("test2") {
                  add("test")
                  add("test2")
            }
      }
}

Expected behavior

It should detect it as a jsonElement not a list and so work in this case

Environment

• Kotlin version: 2.1.20
• Library version: 1.7 and 1.8.1
• Kotlin platforms: Native (Objc/Swift)
• Gradle version: 8.11
• Xcode 16.2 / iOS simulator 18.2

[sandwwraith]

See #2919 (comment) — I think it is the same problem. You can also vote for the issue in the tracker here: https://youtrack.jetbrains.com/issue/KT-48079

[bitsycore]

Any workaround ?
It make kotlin.serialization.json not usable for a multiplatform library

[sandwwraith]

As a workaround, you can write all JsonElement-related functions in Kotlin and use Swift only to call these functions. It is far from ideal, but this is how Kotlin/Swift interop is currently designed.

So if you have function like test3() in your example that only returns JsonObject or even a Map/String you should be fine

[bitsycore]

It's for asynchrone call and it also crash with suspend/async :/

I wrapped JsonObject in a expect/actual that is JsonObject in Android but a wrapping class for iOS, it seem to work but it's annoying behavior

[sandwwraith]

Closing in favor of https://youtrack.jetbrains.com/issue/KT-48079