FlatMap Covariant override in scala

CovariantOverride.scala

class AbsBuilder[+A] {

  type This[+X] <: AbsBuilder[X]

  def add: AbsBuilder[A] = this

  def flatMap[B](f: A => This[B]): AbsBuilder[B] =
    new AbsBuilder[B]

  def map[B](f: A => B): AbsBuilder[B] =
    new AbsBuilder[B]
}

class CBuilder[+A] extends AbsBuilder[A] {
  override type This[+B] = CBuilder[B]

  override def add: CBuilder[A] = this

  override def flatMap[B](f: A => This[B]): CBuilder[B] =
    new CBuilder[B]

  override def map[B](f: A => B): CBuilder[B] =
    new CBuilder[B]

  def cspecific: CBuilder[A] = this
}

new CBuilder[Unit]
  .cspecific
  .add
  .add
  .add
    .map(identity)
  .cspecific
    .flatMap(_ => new CBuilder[Int])
  .cantUpcast
  .cspecific
    .cantUpcast
    .cspecific


implicit class AOps[A](a: AbsBuilder[A]) {
  def cantUpcast: AbsBuilder[A] = a
}

implicit class COps[A](c: CBuilder[A]) {
  def cantUpcast: CBuilder[A] = c
}