[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  mpickering
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2669
       Wiki Page:                    |
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Comment (by rwbarton):

 Here's another example continuing the `Either` theme. Suppose I have a
 synonym
 {{{#!hs
 pattern LL <- Left (Left _)
 }}}
 and a function
 {{{#!hs
 f LL = 0
 f (Left (Right x)) = x
 f (Right y)) = -y
 }}}
 How can I get the exhaustiveness checker to accept this program with
 `COMPLETE`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:51>
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