Formatter removes `()` from functor type which results invalid code

[hackwaly]

Version: 12.0.0-beta.2

Before formatted:

module type S = {}
module type Config = {}
module rec Make: (Config, ()) => S = (Config: Config, ()): S => {}

After formatted:

module type S = {}
module type Config = {}
module rec Make: (Config, ) => S = (Config: Config, ()): S => {}

[nojaf]

Thank you for reporting this.
This problem occurs as well in v11.