[GHC] #12007: Pattern families regression

[GHC]

#12007: Pattern families regression
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           Reporter:  MikeIzbicki    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           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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 The following code compiles fine for me in GHC 7.10.1, but fails in GHC
 8.1.20160502

 {{{
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ViewPatterns #-}

 data Foo a = Foo a a

 pattern A a1 a2 = Foo a1 a2
 pattern B a1 a2 = A a1 a2
 }}}

 The problem is the nested pattern.  Compiling gives:

 {{{
 [1 of 1] Compiling Main             ( Bug1.hs, interpreted )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.1.20160502 for x86_64-unknown-linux):
         kindPrimRep.go rep_a85f

 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/12007>
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