[GHC] #9305: GHC panic when deriving Functor on a Fixed type

[GHC]

#9305: GHC panic when deriving Functor on a Fixed type
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        Reporter:  andreyLevushkin     |            Owner:
            Type:  bug                 |           Status:  new
        Priority:  normal              |        Milestone:
       Component:  Compiler            |          Version:  7.8.2
      Resolution:                      |         Keywords:
Operating System:  Linux               |     Architecture:  x86_64 (amd64)
 Type of failure:  Compile-time crash  |       Difficulty:  Unknown
       Test Case:  T9305               |       Blocked By:
        Blocking:                      |  Related Tickets:
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Changes (by nomeata):

 * testcase:   => T9305


Comment:

 I get
 {{{
 T9305.hs:8:48:
     No instance for (Functor Event)
       arising from the first field of ‘EventF’ (type ‘F (Event a)’)
     Possible fix:
       use a standalone 'deriving instance' declaration,
         so you can specify the instance context yourself
     When deriving the instance for (Functor EventF)
 }}}
 when trying with GHC head. I’m not quite sure if its the best error
 message, but it looks better than what you report, hence adding it as a
 (non-broken) test case.

 If convenient, can you check if it still happens with 7.8.3? Might be an
 already fixed and merged bug. Otherwise we should identify the fix and
 merge it for 7.8.4.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9305#comment:2>
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