[GHC] #10426: matchGroupArity panic with PatternSynonyms

[GHC]

#10426: matchGroupArity panic with PatternSynonyms
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              Reporter:  Iceland_jack  |             Owner:
                  Type:  bug           |            Status:  new
              Priority:  normal        |         Milestone:
             Component:  Compiler      |           Version:  7.10.1
              Keywords:                |  Operating System:  Linux
          Architecture:  x86           |   Type of failure:  None/Unknown
             Test Case:                |        Blocked By:
              Blocking:                |   Related Tickets:
Differential Revisions:                |
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 {{{#!hs
 {-# LANGUAGE PatternSynonyms, ViewPatterns, EmptyCase #-}

 pattern Id <- (id -> _) where
 }}}

 which gives:

 {{{#!hs
 % ghc -ignore-dot-ghci tmp.FbbzjZ48OO.hs
 [1 of 1] Compiling Main             ( tmp.FbbzjZ48OO.hs, tmp.FbbzjZ48OO.o
 )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.0.20150316 for i386-unknown-linux):
         matchGroupArity

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10426>
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