[GHC] #13717: Pattern synonym exhaustiveness checks don't play well with EmptyCase

[GHC]

#13717: Pattern synonym exhaustiveness checks don't play well with EmptyCase
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1-rc1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms,
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 For the record, I can implement `finZAbsurd` in a messier way that the
 pattern checker accepts:

 {{{#!hs
 finZAbsurd :: Fin 'Z -> a
 finZAbsurd = finZAbsurd' Refl
   where
     finZAbsurd' :: n :~: 'Z -> Fin n -> a
     finZAbsurd' pf FZ = case pf of
     finZAbsurd' pf (FS _) = case pf of
 }}}

 but that's a lot uglier.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13717#comment:5>
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