src/test/test2.scala:43: could not find implicit value for parameter c: test2.Combined[[x]test2.Coproduct[test2.T1,test2.T2,x],test2.T2]
   val t2 = t1.test(_ => Test2(1))
                   ^

package test2

import scala.language.higherKinds
import scala.language.implicitConversions

case class Coproduct[F[_], G[_], x](value: Either[F[x], G[x]])

trait Combined[F[_], G[_]] {
  type T[x]
}

trait LowPriority {

  implicit def pair[F[_], G[_]] =
    new Combined[F, G] {
      type T[x] = Coproduct[F, G, x]
    }
}

object Combined extends LowPriority {

  type Aux[F[_], G[_], O[_]] = Combined[F, G] { type T[x] = O[x] }

  implicit def single[F[_]] =
    new Combined[F, F] {
      type T[x] = F[x]
    }
}

trait T1[A]
case class Test1(a: String) extends T1[String]
trait T2[A]
case class Test2(a: Int) extends T2[Int]

object test {

  class Test[F[_], A](f: F[A]) {
    def test[G[_], B](fg: A => G[B])(implicit c: Combined[F, G]): Test[c.T, B] = ???
  }

  val t = new Test(Test1("a"))
  val t1 = t.test(_ => Test2(1))
  val t2 = t1.test(_ => Test2(1))
}