krinart

trait Flattanable[F[_]] {
  def flatten[A](ff: F[F[A]]): F[A]
}

object Flattanable {
  implicit def optionFlattanable: Flattanable[Option] = new Flattanable[Option] {
    override def flatten[A](ff: Option[Option[A]]): Option[A] = ff match {
      case Some(Some(v)) => Some(v)
      case _             => None
    }
  }

  def apply[A[_]: Flattanable]: Flattanable[A] = implicitly[Flattanable[A]]
}

// This one works
val fltn: Flattanable[Option] = Flattanable.optionFlattanable

// This one one works too
val fltn2: Flattanable[Option] = Flattanable[Option]

// This expression is invalid
val fltn3: Flattanable[Option] = Flattanable[Option[_]]

// This expression is invalid too
def fltn4[A]: Flattanable[Option[A]] = Flattanable[Option[A]]