[GHC] #8263: allow duplicate deriving / standalone deriving

[GHC]

#8263: allow duplicate deriving / standalone deriving
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        Reporter:  aavogt            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I'm also inclined to reject this idea. Given the current richness of
 `deriving` language, it's possible to derive two instances which are
 //almost// the same but with subtle differences, e.g.,

 {{{#!hs
 deriving stock instance Show Bool
 deriving newtype instance Show Bool
 }}}

 I imagine trying to detect duplicate instances would be a large headache.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8263#comment:3>
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