Unable to create @Body converter for java.util.Map<java.lang.String, java.lang.Object> when using kotlinx.serialization Converter

[Goooler]

interface NetworkApi {
  suspend fun examplePost(@Body params: Map<String, @JvmSuppressWildcards Any>): Unit

  companion object {
    private val retrofit = Retrofit.Builder()
      .baseUrl("https://example.com/")
      .addConverterFactory(
          Json.asConverterFactory(
              "application/json; charset=UTF8".toMediaType(),
          ),
      )
      .build()

    private val api = retrofit.create(NetworkApi::class.java)
    
    suspend fun examplePost() {
      val params = mapOf(
        "name" to "Tom",
        "age" to 20,
      )
      api.examplePost(params)
    }
  }
}

[Goooler]

This is the issue from Kotlin/kotlinx.serialization#746, workaround:

interface NetworkApi {
  ...

  companion object {
    /**
     * Workaround from https://github.com/Kotlin/kotlinx.serialization/issues/746#issuecomment-779549456.
     */
    private object AnySerializer : KSerializer<Any> {
      override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Any")

      override fun serialize(encoder: Encoder, value: Any) {
        val jsonEncoder = encoder as JsonEncoder
        val jsonElement = serializeAny(value)
        jsonEncoder.encodeJsonElement(jsonElement)
      }

      private fun serializeAny(value: Any?): JsonElement = when (value) {
        is Map<*, *> -> {
          val mapContents = value.entries.associate { mapEntry ->
            mapEntry.key.toString() to serializeAny(mapEntry.value)
          }
          JsonObject(mapContents)
        }
        is List<*> -> {
          val arrayContents = value.map { listEntry -> serializeAny(listEntry) }
          JsonArray(arrayContents)
        }
        is Number -> JsonPrimitive(value)
        is Boolean -> JsonPrimitive(value)
        else -> JsonPrimitive(value.toString())
      }

      override fun deserialize(decoder: Decoder): Any {
        val jsonDecoder = decoder as JsonDecoder
        val element = jsonDecoder.decodeJsonElement()

        return deserializeJsonElement(element)
      }

      private fun deserializeJsonElement(element: JsonElement): Any = when (element) {
        is JsonObject -> {
          element.mapValues { deserializeJsonElement(it.value) }
        }
        is JsonArray -> {
          element.map { deserializeJsonElement(it) }
        }
        is JsonPrimitive -> element.toString()
      }
    }

    private val json = Json {
      serializersModule = SerializersModule {
        contextual(AnySerializer)
      }
    }

    private val retrofit = Retrofit.Builder()
      .baseUrl("https://example.com/")
      .addConverterFactory(
        json.asConverterFactory(
          "application/json; charset=UTF8".toMediaType(),
        ),
      )
      .build()

      ...
  }
}