[GHC] #12698: GHC panic on pattern synonym

[GHC]

#12698: GHC panic on pattern synonym
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"a693d1cb0ee9499af3145d73b1aebe5b6df0da98/ghc"
 a693d1cb/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a693d1cb0ee9499af3145d73b1aebe5b6df0da98"
 Correct order of existentials in pattern synonyms

 Trac #12698 exposed a nasty bug in the typechecking for
 pattern synonmys: the existential type variables weren't
 being put in properly-scoped order.

 For some reason TcPatSyn.tcCollectEx was colleting them as a
 set, not as a list!  Easily fixed.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12698#comment:2>
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