[Caml-list] first class module types with "with type"

["Milan Stanojević" <milanst@gmail.com>]

My search foo is failing me, I'm pretty sure this was discussed before
but I can't find it.

Why is the following disallowed:
module type A = sig type 'a t end
type foo = { a : (module A with type 'a t = 'a list)}

File "z.ml", line 2, characters 29-30:
Error: Syntax error

But this is ok
module type A = sig type 'a t end
module type S = A with type 'a t = 'a list
type a = (module S)

If [t] is not polymorphic then I don't need intermediate module type
module type A = sig type t end
type a = (module A with type t = int)

What is going on here?