Semi-automatic derivation

Or.scala

import io.circe._
import io.circe.generic.semiauto._
import org.scalacheck._, derive._

sealed trait Or[+A, +B] extends Product with Serializable
final case class Left[A](a: A) extends Or[A, Nothing]
final case class Right[B](b: B) extends Or[Nothing, B]

object Or {

  // Compiles
  implicit def decoder[A: Decoder, B: Decoder]: Decoder[A Or B] = deriveDecoder

  // Does not compile
  implicit def cogen[A: Cogen, B: Cogen]: Cogen[A Or B] = MkCogen[A Or B].cogen

}

What I can't really work out yet is how circe manages this behaviour:

implicitly[Decoder[Int Or Boolean]] // Fine

case class Foobar(foo: Int, bar: Boolean)

implicitly[Decoder[Int Or Foobar]] // Missing implicit

How does it know to derive instances for Left[Int] and Right[Boolean], but not Foobar ?!