[GHC] #9127: Don't warn about pattern-bindings of the form `let !_ = rhs`

[GHC]

#9127: Don't warn about pattern-bindings of the form `let !_ = rhs`
------------------------------------+-------------------------------------
       Reporter:  refold            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 With GHC 7.8.2, code like

 {{{
         let !_           = rqPostParams rq
         let !_           = rqParams rq
 }}}

 triggers the following warnings:

 {{{
 src/Snap/Internal/Test/RequestBuilder.hs:150:13: Warning:
     This pattern-binding binds no variables: !_ = rqPostParams rq

 src/Snap/Internal/Test/RequestBuilder.hs:151:13: Warning:
     This pattern-binding binds no variables: !_ = rqParams rq
 }}}

 I think that `let !_ = rhs` shouldn't trigger a warning, just like `let _
 = rhs` doesn't.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9127>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler