[GHC] #9757: Warn about derivable instances

[GHC]

#9757: Warn about derivable instances
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 To my knowledge, GHC can ''not'' derive instances better than anyone else.
 Since the advent of `coerce`, `GeneralizedNewtypeDeriving` has no magic --
 any GND instance can be written by hand, sometimes with lots of type
 annotations and `ScopedTypeVariables`, though.

 Personally, I would want to see nice definitions of "obviously" and
 "equivalent" before thinking too much harder about this. This seems hard
 to do and not terribly useful, to me.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9757#comment:1>
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