DeriveJsonEncoder and DeriveJsonDecoder don't pick up implicit Codec instances

[leszekgruchala]

The DeriveJsonEncoder and DeriveJsonDecoder don't pick up implicit Codec instances so it is not possible to override their behavior.

I had my custom implicit JsonCodec[Instant] because I don't want my Instants to be decoded as Strings, but numbers:

implicit val InstantJsonCodec: JsonCodec[Instant] = JsonCodec(
    JsonEncoder.long.contramap[Instant](_.toEpochMilli),
    JsonDecoder.long.map(Instant.ofEpochMilli)
  )

unfortunately, even though this implicit is in the scope, the derived JsonCodec ignores it and keeps returning a string

case class MyDTO(
  id: Int,
  startAt: Instant
)

object MyDTO {

  implicit val InstantJsonCodec: JsonCodec[Instant] = JsonCodec(
    JsonEncoder.long.contramap[Instant](_.toEpochMilli),
    JsonDecoder.long.map(Instant.ofEpochMilli)
  )

  implicit val codec: JsonCodec[MyDTO] = JsonCodec(
    DeriveJsonEncoder.gen,
    DeriveJsonDecoder.gen
  )
}

Example: https://scastie.scala-lang.org/V2A1rnwwQAGVtjSILEe71g

Fortunately, there is a workaround. We need to define the encoder and decoder directly.
DeriveJsonEncoder doesn’t pick up the implicit Codec, only implicit Encoders.

Workound: https://scastie.scala-lang.org/V2A1rnwwQAGVtjSILEe71g