[GHC] #9867: PatternSynonyms + ScopedTypeVariables triggers an internal error

[GHC]

#9867: PatternSynonyms + ScopedTypeVariables triggers an internal error
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              Reporter:  antalsz     |            Owner:  cactus
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by cactus):

 I think we need to zonk the typechecked pattern before passing it to
 `tc_patsyn_finish`, so that the signature in it can be replaced with
 `[a{tv awx}]`. `TcHsSyn` doesn't currently export `zonkPat` but hopefully
 it will be enough to pass it an empty `ZonkEnv`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9867#comment:3>
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