failure to parse strict field in GADT style type declaration

[ganesh]

This file parses fine in ghc:

{-# LANGUAGE GADTs #-}
module Foo where

data X where
  X :: !Int -> X

HSE (e.g. via hlint) fails to parse it with:

Foo.hs:5:8: Parse error
Error message:
  Parse error: !
Code:
    data X where
  >   X :: !Int -> X

This is with version 1.9.4 but that's not an available version.

[ganesh]

btw by "not an available version" I mean in the "Version" field of this trac.

I've had a quick look at fixing this, but it seems non-trivial as there's no existing syntactic element for a function type where the individual arguments can have bangs.