module M_outer = struct
  module M_inner = struct
    type t
    let f () = () (* this has a zero_alloc variable (and a modality variable) *)
  end
end

(* In [M_outer_strong], the type of [M_inner] will be
   [Mty_alias M_outer.M_inner] *)
module type M_outer_strong =  sig
  module M_inner = M_outer.M_inner
end

(* As a result of the destructive substition, we'll eliminate the alias.  When
   this happens, we shouldn't keep the zero_alloc variables on [f] *)
module type M_outer_subst = M_outer_strong
  with type M_inner.t := M_outer.M_inner.t

(* If we had kept the zero_alloc or modality variables, this would give an
   error. *)
module M : M_outer_subst = struct
  module M_inner = struct
    let f () = ()
  end
end