[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
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        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 mpickering):

 I was specifically referring to the second bullet point at the end of
 comment:31 .

 > GHC understands that if `p1, ..., pn` are exhaustive patterns for the
 result type of `f`, then `f -> p1, ..., f -> pn` are exhaustive patterns
 for the input type of f. This to some extent solves the problem of mixing
 patterns defined in different modules, as long as those patterns are
 defined in terms of the same "view functions".

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:48>
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