[GHC] #8961: Require PatternSynonyms language extension to *use* pattern synonyms

[GHC]

#8961: Require PatternSynonyms language extension to *use* pattern synonyms
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       Reporter:  cactus                   |             Owner:  cactus
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:  7.8.1
      Component:  Compiler (Type checker)  |           Version:  7.8.1-rc2
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Easy (less than 1 hour)  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 To keep in the spirit of #2905, especially in light of the immaturity of
 the pattern synonyms extensions, GHC should require the `PatternSynonyms`
 language extension to be turned on when encountering a pattern synonym
 occurance.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8961>
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