[GHC] #15957: Provide warning for when RecordWildCards LHS {..} doesn't bind anything.

[GHC]

#15957: Provide warning for when RecordWildCards LHS {..} doesn't bind anything.
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        Reporter:  Fuuzetsu          |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by carter):

 If you compile will wall you don’t get an unused variable warning?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15957#comment:1>
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