Discriminator field is omitted when child type has its own codec

[sergeykolbasov]

Simple reproduction:

https://scastie.scala-lang.org/ApPHtGwrRE2GCGUpECBGww

import com.github.plokhotnyuk.jsoniter_scala.core.*
import com.github.plokhotnyuk.jsoniter_scala.macros.*

given CodecMakerConfig.PrintCodec with {}

sealed trait Parent

object Parent {
  given JsonValueCodec[Parent] = JsonCodecMaker.make(CodecMakerConfig.withDiscriminatorFieldName(Some("type")))
}

case class Child(a: Int) extends Parent

object Child {
  given JsonValueCodec[Child] = JsonCodecMaker.make(CodecMakerConfig.withDiscriminatorFieldName(Some("type")))
}

val x: Parent = Child(42)

writeToString[Parent](x) // {"a": 42}

Expected behavior is to include a type discriminator field whenever value is encoded using a parent codec.

From what I see in the generated macro Parent codec, the encoding is fully delegated to the child codec whenever it exists, and the child codec on its own disregards the discriminator.

I'm not sure if it's a consistent behavior from the user perspective, given there is an explicit discriminator configuration set on the parent codec.

[plokhotnyuk]

Just remove all codec derivation for non top-level case classes.

Deriving codecs in companion objects is anti-pattern for jsoniter-scala:

1. It floods data-model definition with a lot of boilerplate.
2. It is inefficient from the point of performance view due to introducing a lot of megamorphic virtual calls for JVM interfaces.

Macros will never try to cooperate and guess your intention ;)