Fix MPR#7414

[garrigue]

Fix MPR#7414.
The problem was that non-generalizable variable lowering, introduced in 4.04 for modules, didn't work for functors.
The first attempt uses Subst.modtype for its side-effect.

[gasche]

@vprevosto linked this fix to a typing regression for his (safe) codebase on the caml-list:

module Foo : sig
  module M(E: sig type t end) : sig
  type t
  val u: t -> t -> t
  end
end = struct
  module M(E: sig type t end) = struct
  type t = (E.t, unit) Hashtbl.t
  let u = Hashtbl.fold (fun x () h -> Hashtbl.add h x (); h)
  end
end

If I understand correctly, this regression is expected and users have to change their code -- this is what we concluded for the non-functor case in MPR#7313, MP#7401.

[gasche]

For the record, other OCaml software that have to be fixed after this change are camomile (opam-builder report) and fury-puyo (opam-builder report). I am planning on sending patches to the affected packages.

[garrigue]

Indeed, this is a typical case of signature annotation outside of module with nesting, which requires a more local annotation since 4.04.

[thierry-martinez]

Coccinelle has also to be fixed. Here is another bit of code that was correctly typed by 4.04 and is now rejected with 4.05 beta.

module type S = sig
  type 'a t = N

  val r: (unit -> unit t) ref
end

module F (X: S) = struct
end

module M = struct
  type 'a t = N

  let r = ref (fun () -> raise Not_found)
end

module N = F (M)