Test.scala

import annotation.unchecked.uncheckedVariance

trait Builder[-A, +To]
trait IterableFactory[+CC[_]] {
  def from[A](source: It[A]): CC[A]
  def newBuilder[A]: Builder[A, CC[A]]
}

trait ItOps[+A, +CC[_], +C] {
  protected type IterableCC[X] = CC[X] @uncheckedVariance
  protected type IterableC = C @uncheckedVariance

  protected def newSpecificBuilder: Builder[A @uncheckedVariance, IterableC]
  protected def fromSpecific(coll: It[A @uncheckedVariance]): IterableC
  def iterableFactory: IterableFactory[IterableCC]

  def foo: C = fromSpecific(null)
}

trait It[+A] extends ItOps[A, It, It[A]] {
  protected def fromSpecific(coll: It[A @uncheckedVariance]): IterableC = iterableFactory.from(coll)
  protected def newSpecificBuilder: Builder[A @uncheckedVariance, IterableC] = iterableFactory.newBuilder[A]
  def iterableFactory: IterableFactory[IterableCC] = It
}

object It extends IterableFactory[It] {
  def from[A](source: It[A]): It[A] = new It[A]{}
  def newBuilder[A]: Builder[A,It[A]] = new Builder[A, It[A]]{}
}

trait SqOps[A, +CC[_], +C] extends ItOps[A, CC, C]

trait Sq[A] extends It[A] with SqOps[A, Sq, Sq[A]] {
  def flup = 0
}

object Test {
  def main(args: Array[String]): Unit = {
    val sq = new Sq[String] { }
    sq.foo.flup 
  }
}