Bug: cannot handle some functions with optional module parameters

[cbarcenas]

ocamlformat seems to have an issue processing certain let-bindings that contain first-class modules as optional parameters.

Specifically, it fails when encountering a let-binding that takes an optional module argument with a default value, such as in scenario 2 below.

Let-bindings containing optional module parameters without a default value, such a scenario 1, are unaffected. Let-bindings containing non-optional module parameters are also fine.

Error output

ocamlformat version 0.12 with default configuration

ocamlformat: Cannot process "example.ml".
  Please report this bug at https://github.com/ocaml-ppx/ocamlformat/issues.
  BUG: generating invalid ocaml syntax.

Reproduction steps

module type X_typ = sig
  val f : int -> int
end

module X_add_one : X_typ = struct
  let f = (+) 1
end

(* Scenario 1: No issue with no default value *)
let helper ?x = match x with
  | Some (module X:X_typ) -> X.f
  | None                  -> X_add_one.f

(* Scenario 2: Fails with "BUG: generating invalid ocaml syntax." *)
let helper ?x:((module X)=(module X_add_one:X_typ)) = X.f

let () = Printf.printf "%d\n" (helper 41)

[cbarcenas]

I patched ocamlformat to print the malformed formatted code and found the offending line:

-let helper ?x:((module X)=(module X_add_one:X_typ)) = X.f
+let helper ?x:(module X = (module X_add_one : X_typ)) = X.f

ocamlformat should not be removing the parentheses (...) surrounding module X, as these are required.