Parsing error with parenthesized explicit arg with attributes

[mtzguido]

let _ = x:int -> bool
let _ = #x:int -> bool
let _ = #[@@@1]x:int -> bool
let _ = [@@@1]x:int -> bool

let _ = (x:int) -> bool
let _ = (#x:int) -> bool
let _ = (#[@@@1]x:int) -> bool
let _ = ([@@@1]x:int) -> bool

Only the last line fails to parse.

* Error 168 at B.fst(11,21-11,21):
  - Syntax error