WithFallback.scala

sealed trait WithFallback[+A, +B] {
  def fold[C](f: A => C, g: B => C): C
}
case class Preferred[+A](a: A) extends WithFallback[A, Nothing] {
  def fold[C](f: A => C, g: B => C): C = f(a)
}
case class Fallback[+B](b: B) extends WithFallback[Nothing, B] {
  def fold[C](f: A => C, g: B => C): C = g(b)
}

trait WithFallbackLow {
  implicit def fallback[A, B](implicit fb: B): WithFallback[A, B] = Fallback(fb)
}

object WithFallback extends WithFallbackLow {
  implicit def preferred[A, B](implicit pa: A): WithFallback[A, B] = Preferred(pa)
}


...


def reduce[A](implicit st: CMonoid[A] WithFallback Monoid[A]) = st.fold({ implicit m => ... }, { implicit m => ... })