[GHC] #12163: Surprising error message when deriving Functor for a GADT

[GHC]

#12163: Surprising error message when deriving Functor for a GADT
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        Reporter:  benjamin.hodgson  |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  MacOS X           |         Architecture:  x86_64
 Type of failure:  Incorrect         |  (amd64)
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"cc92a446d6932cf06364529e71c866289088c59a/ghc" cc92a44/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="cc92a446d6932cf06364529e71c866289088c59a"
 Improve error message in deriving( Functor )

 Fixes Trac #12163.  Pretty simple.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12163#comment:4>
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