ADT derivation succeeds on decoder but fails on encoder

[matthughes]

The following example ADT fails to derive the Encoder for Foo but can generate the Decoder. Changing this to use generic.semiauto works. This uses the latest 0.13.0-M4 release.

import io.circe._
import io.circe.derivation._

sealed trait Foo {
  def id: String
}
object Foo {
  // could not find implicit value for parameter instance: io.circe.Encoder.AsObject[Bar]
  implicit val encoder: Encoder[Foo] = deriveEncoder[Foo] 
  implicit val decoder: Decoder[Foo] = deriveDecoder[Foo]
}

final case class Bar (
    id: String
) extends Foo 
object Bar {
  implicit val encoder: Encoder[Bar] =
    deriveEncoder[Bar]
  implicit val decoder: Decoder[Bar] =
    deriveDecoder[Bar]
}

final case class Baz(
    id: String
) extends Foo 

object Baz {
  implicit val encoder: Encoder[Baz] = deriveEncoder[Baz]
  implicit val decoder: Decoder[Baz] = deriveDecoder[Baz]
}

[radusw]

@matthughes You are missing an import for the implicit conversion:
import io.circe.generic.auto.exportEncoder

[matthughes]

@radusw Why would I need to pull in something from circe.generic to make circe.derivation work? I thought they were two different approaches to building the macros.