[GHC] #13117: Derived functor instance for void types handles errors badly

[GHC]

#13117: Derived functor instance for void types handles errors badly
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           Reporter:  dfeuer         |             Owner:  dfeuer
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If we have

 {{{#!hs
 data V a deriving (Functor)
 }}}

 and we evaluate

 {{{#!hs
 f <$> (error "Boo!" :: Void)
 }}}

 we should really get a "Boo" error, rather than a useless "Void fmap" one.
 The offending code is in `compiler/typecheck/TcGenFunctor.hs`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13117>
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