test/Test.scala:

package test;

abstract class Base {
  type A <: Ax;
  
  abstract class Ax {
    def a = null;
    def string = "A";
  }
}

trait ExtA extends Base {
  type A <: Ax;
  trait Ax extends super.Ax {
    def b = null;
    override def string = super.string + "B";
  }
}
trait ExtB extends Base {
  type A <: Ax;
  trait Ax extends super.Ax {
    def c = null;
    override def string = super.string + "C";
  }
}

trait ExtC extends ExtA with ExtB {
  type A <: Ax;
  trait Ax extends super[ExtA].Ax with super[ExtB].Ax {
    a
    b
    c
    def d = null;
    override def string = super.string + "D";
  }
}

test/Main.scala

package test;

object Main {
  def main(args : Array[String]) : Unit = {
    new ExtC {
      type A = Ax;
      class Ax extends super.Ax;
    }
  }
}