[GHC] #10426: matchGroupArity panic with PatternSynonyms

[GHC]

#10426: matchGroupArity panic with PatternSynonyms
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  T10426
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1665
       Wiki Page:                    |
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Comment (by Matthew Pickering <matthewtpickering@…>):

 In [changeset:"8d954125604e4585167306c4f1d4807275be0a61/ghc"
 8d954125/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8d954125604e4585167306c4f1d4807275be0a61"
 Disallow empty where bindings in pattern synonym declarations.

 Reviewers: austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1665

 GHC Trac Issues: #10426
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10426#comment:5>
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