[GHC] #9757: Warn about derivable instances

[GHC]

#9757: Warn about derivable instances
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       Reporter:  dfeuer             |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 GHC can derive a lot of instances by itself, and can sometimes (with
 `GeneralizedNewtypeDeriving`) do so better than anyone else. I'd like to
 be able to get a warning about any instance that's "obviously equivalent"
 to the one that would be derived, for some suitable values of "obviously"
 and "equivalent".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9757>
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