Can't import module types containing GADTs

[CPCTC]

Code:

(* foo.ml *)
module type T = sig
    type _ t = None: unit t
end

(* bar.ml *)
module type T = [%import: (module Foo.T)]

Result:

File "bar.ml", line 1, characters 34-39:
1 | module type T = [%import: (module Foo.T)]
                                      ^^^^^
Error: The type variable name '_ is not allowed in programs