Abstract Type vs Type parameter

abvsTypeDep.scala

trait AnimalMouth
class Beak extends AnimalMouth

object Test1 {

  //================================Generic type works as in the following =======================

  trait Animal[Mouth <: AnimalMouth] {
    def mouth : Mouth
    def feedAnother(m: Mouth): Unit = ()
  }

  class Bird extends Animal[Beak] {
    def mouth: Beak = new Beak
  }

  // compiles, but when called with two instances of Bird, gives `type arguments [Mouth] do not conform to trait Animal's type parameter bounds [Mouth <: AnimalMouth]`
  //def feed[Mouth <: AnimalMouth, A <: Animal[Mouth]](mother: A, child: A) : Unit = mother.feedAnother(child.mouth)

  def feed[Mouth <: AnimalMouth](mother: Animal[Mouth], child: Animal[Mouth]): Unit = mother.feedAnother(child.mouth)

}

//==============Make mouth an abstract type and it would be hard ===========================

object Test2 {
  trait Animal {
    type Mouth <: AnimalMouth
    def mouth : Mouth
    def feedAnother(m: Mouth): Unit = ()
  }

  class Bird extends Animal {
    type Mouth = Beak
    def mouth: Mouth = new Beak
  }

  def feed[A <: Animal, B](mother: A { type Mouth = B}, child: A {type Mouth = B} ) : Unit = mother.feedAnother(child.mouth)
}

//=============== Work Around using Aux ================

object Test3 {

  trait Animal {
    type Mouth <: AnimalMouth
    def mouth : Mouth
    def feedAnother(m: Mouth): Unit = ()
  }

  class Bird extends Animal {
    type Mouth = Beak
    def mouth: Mouth = new Beak
  }


  object Animal {
    type Aux[Mouth0] = Animal { type Mouth = Mouth0 }
  }

  def feed[Mouth](mother: Animal.Aux[Mouth], child: Animal.Aux[Mouth]) : Unit = mother feedAnother child.mouth

}

object Test {
  def main(args: Array[String]): Unit = {
    {
      import Test1._
      val mother = new Bird()
      val child = new Bird()
      feed(mother, child)
    }

    {
      import Test2._
      val mother = new Bird()
      val child = new Bird()
      feed(mother, child)
    }

    {
      import Test3._
      val mother = new Bird()
      val child = new Bird()
      feed(mother, child)
    }
  }

}