[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:  closed
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler (Type    |          Version:  7.8.1-rc2
  checker)                           |         Keywords:
      Resolution:  fixed             |     Architecture:  Unknown/Multiple
Operating System:  Unknown/Multiple  |       Difficulty:  Easy (less than 1
 Type of failure:  None/Unknown      |  hour)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by cactus):

 I did this because I thought requiring the language pragma at use sites is
 the more conservative choice, given that `PatternSynonyms` is more of a
 tech preview than a full feature yet (see #8581, #8582, #8583, #8779 for
 some of the missing features from the original proposal; #8584 and #8761
 for more pressing issues, #8968 for a potentially deeper rabbit hole).

 If that is the consensus, I have no problems with reverting this, but I
 think for 7.8.1 we should leave this in so that it requires effort (and
 thus it cannot go unnoticed) if you use this feature. A future release
 with the above-mentioned problems addressed could then remove this hoop to
 jump through. So my vote is on keeping this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8961#comment:12>
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