gistfile1.txt

! [Type Error] at "bug.saty", line 11, character 0 to line 15, character 3:
    The implementation of value 'make' has type
      ('a phantom) M.t
    which is inconsistent with the type required by the signature
      ('#a phantom) M.t

phantom-type-vs-module.saty

type 'a phantom = Phantom

module M : sig
  type 'a t
  val make : 'a -> 'a t
end = struct
  type 'a t = 'a option
  let make _ = None
end

module N : sig
  val make : ('a phantom) M.t
end = struct
  let make = M.make (Phantom)
end