[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 rwbarton):

 Thinking more about about this ticket and #9756, I think the purpose of
 compiler warnings should be to warn about code that is probably bad, not
 code that is probably fine that could be written in a (potentially) better
 way. (After all, we don't warn about excess parentheses in code either.)
 That is the domain of hlint, as Richard notes.

 Now #9756 is possibly an exception, if GHC is in a unique position to
 decide whether the replacement of `coerce` will type check, but I think
 the principle is a good one.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9757#comment:4>
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