[GHC] #11283: PatternSynonms and DisambiguateRecordFields causes panic

[GHC]

#11283: PatternSynonms and DisambiguateRecordFields causes panic
-------------------------------------+-------------------------------------
           Reporter:  adamgundry     |             Owner:  adamgundry
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #9975
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following module causes a panic in HEAD:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, DisambiguateRecordFields #-}
 data P a = MkP a
 pattern S{x} = MkP x
 e = S{x = 3}
 }}}

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.11.20151223 for x86_64-unknown-linux):
         find_tycon
   S
   [S pattern synonym defined at PatSynBug.hs:3:1]
 }}}

 This is essentially the same bug as #9975; for some reason it became
 slightly harder to tickle. Fix incoming.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11283>
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