jsoniter: parameterless singletons (case object / no-arg enum cases) encode but fail to decode

[MateuszKubuszok]

Summary

kindlings-jsoniter-derivation's KindlingsJsonValueCodec encodes a parameterless singleton member of a sealed hierarchy (a case object, or a no-arg Scala 3 enum case) as {"Name":{}}, but the generated decoder cannot read that back — it throws:

com.github.plokhotnyuk.jsoniter_scala.core.JsonReaderException: expected '}' after wrapped enum value, offset: 0x00000007

So encode → decode is not a round-trip for parameterless singletons. Members that carry fields (case class Circle(r: Int)) round-trip fine, so this is specific to the empty-{} wrapped payload the encoder emits for parameterless cases.

Reproduction

//> using scala 3.8.4
//> using dep com.kubuszok::kindlings-jsoniter-derivation:0.3.1

import com.github.plokhotnyuk.jsoniter_scala.core.*
import hearth.kindlings.jsoniterderivation.{ JsoniterConfig, KindlingsJsonValueCodec }

given JsoniterConfig = JsoniterConfig()

sealed trait Color derives KindlingsJsonValueCodec
object Color {
  case object Red extends Color
  case object Green extends Color
}

@main def run(): Unit =
  val value: Color = Color.Red
  val json = writeToString(value)
  println(s"encoded: $json")               // {"Red":{}}
  val decoded = readFromString[Color](json) // <-- throws
  println(s"decoded: $decoded")
  assert(decoded == value)
  println("round-trip OK")

Actual behaviour

encoded: {"Red":{}}
Exception in thread "main" com.github.plokhotnyuk.jsoniter_scala.core.JsonReaderException: expected '}' after wrapped enum value, offset: 0x00000007, buf:
+----------+-------------------------------------------------+------------------+
|          |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f | 0123456789abcdef |
+----------+-------------------------------------------------+------------------+
| 00000000 | 7b 22 52 65 64 22 3a 7b 7d 7d                   | {"Red":{}}       |
+----------+-------------------------------------------------+------------------+

Offset 0x07 is the opening { of the inner {} — after reading the "Red" discriminator key the reader does not consume the empty-object value the encoder wrote.

Expected behaviour

readFromString[Color](writeToString(v)) == v for every case, i.e. {"Red":{}} decodes to Color.Red (or encoder and decoder agree on some other symmetric shape for parameterless singletons).

Scope

• Reproduces on kindlings 0.3.1 and 0.3.0 (Scala 3.8.4, JVM).

• Affects both sealed trait + case object and parameterless Scala 3 enum cases:

  enum Direction derives KindlingsJsonValueCodec:
    case North, South
  // writeToString(Direction.North) == {"North":{}}  -> same decode failure

• Does not affect subtypes that carry fields — final case class Circle(r: Int) extends Shape encodes as {"Circle":{"r":3}} and round-trips correctly.

• Default JsoniterConfig(). Setting discriminatorFieldName changes the encoded shape to {"type":"Red"} but the decoder still fails (expected end of input), so it is not a config workaround.

Environment

• Scala 3.8.4, JDK (temurin) 17 / JVM
• com.kubuszok::kindlings-jsoniter-derivation:0.3.1 (and 0.3.0)
• jsoniter-scala-core (transitive)

Found while migrating branchtalk-backend; a sealed SessionType { case object UserSession; case object OAuth } in an HTTP response body failed to deserialize in integration tests.