[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
      Resolution:                    |         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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Comment (by aavogt):

 Flag comes from cabal: that deriving (Typeable) was added in version 1.18.
 No code is needed to support that version. Things get messy to support
 older versions of Cabal which do not export a Typeable instance.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8263#comment:2>
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