bug1279a.scala:34: error: type mismatch;
 found   : first.selfType
 required: M{type T <: this.T}
   def all4Impl[U](first: M {type T <: U}): Stream[M {type T <: U}] = Stream.cons(first, all4Impl(first.next))
                                                                                                        ^
one error found