Polymorphic serialization: properties named "type" are not serialized with a unique key

Describe the bug When using the polymorphic serializer, if a class has a property called type, it will be serialized with two keys called type – one with the class name, and one with the value of the class property.

To Reproduce

class SampleTests {
    @Serializable
    abstract class Base

    @Serializable
    data class Derived(val type: String) : Base()

    @Test
    fun test() {
        val j = Json {
            serialModule = SerializersModule {
                polymorphic(Base::class) {
                    addSubclass(Derived.serializer())
                }
            }
        }

        val derived = Derived("string")
        val stringified = j.stringify(Base.serializer(), derived)
        val parsed = j.parse(Base.serializer(), stringified) // throws

        assertEquals(
            derived,
            parsed
        )
    }
}

Expected behavior An error should be emitted that the key type is reserved, with instructions to use SerialName. It could also be useful to allow the discriminator property name to be configurable

Environment

• Kotlin version: [1.3.30]
• Library version: [0.11.0]
• Kotlin platforms: [e.g. JVM, JS, and Native]
• Gradle version: [e.g. 5.4]

[sandwwraith]

Discriminator name is configurable, see in JsonConfiguration class

[SerVB]

Now sealed classes are supported automatically and there is the same problem:

package com

import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
import kotlinx.serialization.json.JsonConfiguration

@Serializable
sealed class Base

@Serializable
data class Child(val type: String) : Base()

object Tmp {

  @JvmStatic
  fun main(args: Array<String>) {
    val json = Json(JsonConfiguration.Stable)

    val child = Child("1")

    val string = json.stringify(Base.serializer(), child)
    println(string)

    val parsed = json.parse(Base.serializer(), string)

    println(parsed)
  }
}

The output:

{"type":"com.Child","type":"1"}
Exception in thread "main" kotlinx.serialization.SerializationException: 1 is not registered for polymorphic serialization in the scope of class com.Base
	at kotlinx.serialization.internal.AbstractPolymorphicSerializerKt.throwSubtypeNotRegistered(AbstractPolymorphicSerializer.kt:99)
	at kotlinx.serialization.internal.AbstractPolymorphicSerializerKt.access$throwSubtypeNotRegistered(AbstractPolymorphicSerializer.kt:1)
	at kotlinx.serialization.internal.AbstractPolymorphicSerializer.findPolymorphicSerializer(AbstractPolymorphicSerializer.kt:83)
	at kotlinx.serialization.SealedClassSerializer.findPolymorphicSerializer(SealedSerializer.kt:92)
	at kotlinx.serialization.json.internal.PolymorphicKt.decodeSerializableValuePolymorphic(Polymorphic.kt:41)
	at kotlinx.serialization.json.internal.StreamingJsonInput.decodeSerializableValue(StreamingJsonInput.kt:33)
	at kotlinx.serialization.CoreKt.decode(Core.kt:80)
	at kotlinx.serialization.json.Json.parse(Json.kt:126)
	at com.Tmp.main(Tmp.kt:24)