[GHC] #11233: Improve optimisation of pattern synonym matching

[GHC]

#11233: Improve optimisation of pattern synonym matching
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           Reporter:  mpickering     |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:  newcomer,      |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11224
Differential Rev(s):                 |         Wiki Page:
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 Currently GHC does a very poor job of optimising pattern matches which
 include pattern synonyms.

 It would be good to modify `sameGroup` in `compiler/deSugar/Match.hs` to
 be a lot smarter about when it is safe to group two pattern matches
 together.

 Grouping was originally disabled as it is a bit trickier than originally
 thought. See #11224 for details.

 The rule was originally to group two pattern matches together `PgSyn p1`
 and `PgSyn p2` when `p1 == p2`. This rule wasn't safe when `p1` had a
 polymorphic return type and thus could have a different type on each
 branch even though syntactically identical. A good solution to this ticket
 would come up with a smarter rule about when it is and isn't safe to group
 two together in the same spirit as the more complicated rules for view
 patterns.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11233>
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