byname implicit parameter🎉 https://github.com/scala/scala/pull/6050

build.sbt

resolvers += "scala-pr" at "https://scala-ci.typesafe.com/artifactory/scala-integration/"

scalaVersion := s"2.13.0-pre-874bf03"

scalacOptions += "-language:_"

Cofree.scala

package example

trait Eq[A] {
  def equal(x: A, y: A): Boolean
}

final case class Cofree[F[_], A](head: A, tail: F[Cofree[F, A]])

object Cofree {
  implicit def cofreeEq[F[_], A](implicit a: Eq[A], f: => Eq[F[Cofree[F, A]]]): Eq[Cofree[F, A]] =
    (x, y) => {
      a.equal(x.head, y.head) && f.equal(x.tail, y.tail)
    }
}

object Main {
  implicit def intEq: Eq[Int] = _ == _

  implicit def optionEq[A](implicit a: Eq[A]): Eq[Option[A]] = {
    case (Some(x), Some(y)) =>
      a.equal(x, y)
    case (None, None) =>
      true
    case (_, _) =>
      false
  }

  def main(args: Array[String]): Unit = {
    val c: Cofree[Option, Int] = Cofree(1, Option(Cofree(2, Option(Cofree[Option, Int](3, None)))))
    println(implicitly[Eq[Cofree[Option, Int]]].equal(c, c))
  }
}