[GHC] #9756: Warn about unnecessary unsafeCoerce

[GHC]

#9756: Warn about unnecessary unsafeCoerce
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by rwbarton):

 I would tend to think it should be opt-in anyways, at least for a couple
 compiler versions, since it would just annoy people who want to support
 those older compiler versions without `coerce`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9756#comment:2>
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