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

[GHC]

#8263: allow duplicate deriving / standalone deriving
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       Reporter:  aavogt            |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 I would like to avoid conditional compilation. Currently I need to write:

 {{{
 #if MIN_VERSION_Cabal(1,18,0)
 -- cabal 1.18 has these instances
 #else
 deriving instance Typeable Flag
 #endif
 }}}

 Would it be sane to have the deriving instance do nothing if the imported
 instance that clashes was also derived?

 reifyInstances together with a fixed #8100 could allow faking this
 feature.

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