IsNotFuture

trait IsNotFuture[+F]

object IsNotFuture{
  def apply[F](implicit isf: IsNotFuture[F]) = isf

  implicit def mkFuture[A] = new IsNotFuture[Future[A]] {}
  implicit object isf extends IsNotFuture[Any] {}
}

//No need for a dependent type. All you need is the implicit ambiguity.
def inSession[T](fn: (Session) => T)(implicit isNotFuture: IsNotFuture[T]): T = {
  db withSession fn
}

// Compiler fails as desired 
inSession { session => Future(1) }