Make ($!) representation-polymorphic

Now that `($)` is representation-polymorphic, `($!)` should
surely follow suit.

Reviewers: hvr, bgamari, simonpj

Reviewed By: bgamari, simonpj

Subscribers: simonpj, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4382

docs/users_guide/8.6.1-notes.rst

@@ -49,6 +49,8 @@ Template Haskell
 ``base`` library
 ~~~~~~~~~~~~~~~~
 
+``($!)`` is now representation-polymorphic like ``($)``.
+
 
 Build system
 ~~~~~~~~~~~~

libraries/base/GHC/Base.hs

@@ -1327,8 +1327,8 @@ f $ x =  f x
 -- argument, evaluates the argument to weak head normal form (WHNF), then calls
 -- the function with that value.
 
-($!)                    :: (a -> b) -> a -> b
-f $! x                  = let !vx = x in f vx  -- see #2273
+($!) :: forall r a (b :: TYPE r). (a -> b) -> a -> b
+f $! x = let !vx = x in f vx  -- see #2273
 
 -- | @'until' p f@ yields the result of applying @f@ until @p@ holds.
 until                   :: (a -> Bool) -> (a -> a) -> a -> a

libraries/base/changelog.md

 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
+## 4.12.0.0 *TBA*
+
+  * `($!)` is now representation-polymorphic like `($)`.
+
 ## 4.11.1.0 *TBA*
   * `System.IO.openTempFile` is now thread-safe on Windows.