Class B will get a method Mumu which corresponds to the case class defined inside trait A. This method forwards \
to A$class.Mumu, but such a method does not exist.

trees after mixin:
  class B extends java.lang.Object with A with scala.ScalaObject {
    case def Mumu(): A$Mumu = A$class.Mumu(B.this);
..
  abstract trait A$class extends  {
    def /*A$class*/$init$($this: A): scala.Unit = {
      ()
    }
  };