The change of behavior was part of PR#6090:
  http://caml.inria.fr/mantis/view.php?id=6090
It was a case of the type-checker refusing more programs than necessary.

My understanding of the compatibility story of the type-system is that
future versions will not rule out semantically valid programs accepted
by previous ones (note that buggy programs previously accepted could
be ruled out), but may accept strictly more semantically valid
programs. If you need to be sure that your code compile under older
versions, you should test them explicitely.

On Tue, Jan 28, 2014 at 10:19 PM, Ashish Agarwal <agarwal1975@gmail.com> wrote:
> Is the following an expected change from OCaml 4.00.1 to 4.01.0?
>
> Module Foo defines a private type and this module is included in Common. The
> signature within Common is defined using `module type of` with a constraint.
> The different behavior shown below occurs when the private type is a record,
> but not if it is a base type like string or int. In that case, the code
> compiles with both 4.00.1 and 4.01.0.
>
> $ cat a.ml
> module Foo : sig
>   type t = private {a:int}
> end = struct
>   type t = {a:int}
> end
>
> module Common : sig
>   module F : module type of Foo with type t = Foo.t
> end = struct
>   module F = Foo
> end
>
> $ ocaml -version
> The OCaml toplevel, version 4.01.0
>
> $ ocaml a.ml
> (* no errors *)
>
> Now, change version of OCaml being used.
>
> $ ocaml -version
> The OCaml toplevel, version 4.00.1
>
> $ ocaml a.ml
> File "a.ml", line 9, characters 6-33:
> Error: Signature mismatch:
>        Modules do not match:
>          sig module F : sig type t = Foo.t = private { a : int; } end end
>        is not included in
>          sig module F : sig type t = Foo.t = { a : int; } end end
>        In module F:
>        Modules do not match:
>          sig type t = Foo.t = private { a : int; } end
>        is not included in
>          sig type t = Foo.t = { a : int; } end
>        In module F:
>        Type declarations do not match:
>          type t = Foo.t = private { a : int; }
>        is not included in
>          type t = Foo.t = { a : int; }
>        File "a.ml", line 8, characters 37-51: Expected declaration
>        File "a.ml", line 2, characters 7-26: Actual declaration
>        A private type would be revealed.
>