WTF?

Transformer

trait Functor[F[_]]

object Functor {
  implicit val OptionFunctor: Functor[Option] =
    error("")
 
}

case class OptionT[F[+_], +A]

object OptionT {
  implicit def OptionTFunctor[F[+_]](implicit F0: Functor[F]): Functor[({type λ[α] = OptionT[F, α]})#λ] =
    error("")
}

trait TTT {
  // fine
  val a = implicitly[Functor[Option]]

  // fine
  val b = implicitly[Functor[({type lam[+a]=OptionT[Option, a]})#lam]]

  // not fine
  val c = implicitly[Functor[({type lam[+a]=OptionT[({type l[+b]=OptionT[Option, b]})#l, a]})#lam]]
}

Yeah, that's exactly the point. I will hopefully take a look at it closely soon.