[GHC] #13065: Prohibit user-defined Generic and Generic1 instances

[GHC]

#13065: Prohibit user-defined Generic and Generic1 instances
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        Reporter:  dfeuer            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  Generics
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 Can you comment on why this is different from our stance on `Data`, which
 addresses largely similar problems but for which custom instances are
 apparently approved (e.g., `Data.Map.Map` has one)?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13065#comment:4>
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