1-works.scala

trait TesterTrait {
  type Out

  def unapply(s: String): Out
}

object Tester extends TesterTrait {
  override type Out = Boolean
  def unapply(s: String) = false
}

object Matcher1 extends TesterTrait {
  override type Out = Option[String]
  def unapply(s: String) = Some("1")
}

object Matcher2 extends TesterTrait {
  override type Out = Option[(String, String)]
  def unapply(s: String) = Some("1" -> "2")
}

object R {
  "" match {
    case Tester()       => true
    case Matcher1(m)    => true
    case Matcher2(a, b) => true
  }
}

2-doesnt.scala

trait Unapply[Reg] {
  type Out
  def apply(in: String): Out
}

trait TesterTrait[T] {
  def unapply(s: String)(implicit stu: Unapply[T]): stu.Out = stu(s)
}

object Tester extends TesterTrait[Boolean]

object Matcher1 extends TesterTrait[String]

object R {
  implicit def stringToBool: Unapply[Boolean] = new Unapply[Boolean] {
    type Out = Boolean
    def apply(in: String): Out = false
  }
  implicit def stringToOpt: Unapply[String] =
    new Unapply[String] {
      type Out = Option[String]
      override def apply(in: String): Out = Some("X")
    }

  "" match {
    case Tester()    => true
    case Matcher1(m) => true
  }
}

3-nor-with-aux.scala

trait Unapply[Reg] {
  type Out
  def apply(in: String): Out
}

object Unapply {
  type Aux[I, O] = Unapply[I] { type Out = O }
}

trait TesterTrait[T] {
  def unapply[O](s: String)(implicit stu: Unapply.Aux[T, O]): stu.Out = stu(s)
}

object Tester extends TesterTrait[Boolean]

object Matcher1 extends TesterTrait[String]

object R {
  implicit def stringToBool: Unapply.Aux[Boolean, Boolean] =
    new Unapply[Boolean] {
      type Out = Boolean
      def apply(in: String): Out = false
    }
  implicit def stringToOpt: Unapply.Aux[String, Option[String]] =
    new Unapply[String] {
      type Out = Option[String]
      override def apply(in: String): Out = Some("X")
    }

  "" match {
    case Tester()    => true
    case Matcher1(m) => true
  }
}