[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 goldfire):

 Replying to [comment:46 mpickering]:

 Thanks for this detailed reply! I'm happy with all your responses save
 one:

 > >  - The spec says "We verify that the result types of each constructor
 in a complete match agrees with each other." What does this sentence mean?
 Consider the possibility of pattern synonyms that match against functions,
 polymorphic patterns, and possibly even higher-rank patterns.
 > >
 >
 > It means that we look at the result type of each conlike and then verify
 that the type constructor for each type is the same. In the case of a set
 containing polymorphic patterns, at least one pattern in the set must have
 a definite type or you must specify a type signature to fix the type for
 the whole set.

 I'm not sure what you mean here. Where do you specify the type signature?
 In the `COMPLETE` pragma?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:53>
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