[GHC] #10339: PatternSynonyms confuse exhaustiveness check

[GHC]

#10339: PatternSynonyms confuse exhaustiveness check
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              Reporter:  hvr         |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 I just noticed some odd behaviour when (ab)using pattern synonyms in the
 following code (whose style I don't really endorse):

 {{{#!hs
 {-# LANGUAGE PatternSynonyms, LambdaCase #-}

 module PatSyn1 where

 import System.Directory (doesFileExist)

 pattern Found    = True
 pattern NotFound = False

 readConfigFile :: FilePath -> IO ()
 readConfigFile filePath = doesFileExist filePath >>= \case
     Found    -> putStrLn =<< readFile filePath
     NotFound -> putStrLn "File does not exist."


 pattern Void = ()

 foo :: () -> ()
 foo Void = ()
 }}}

 Results in

 {{{
 /home/hvr/Haskell/PatSyn1.hs:(11,54)-(13,47): Warning:
     Pattern match(es) are non-exhaustive
     In a case alternative: Patterns not matched: _

 /home/hvr/Haskell/PatSyn1.hs:19:1-13: Warning:
     Pattern match(es) are non-exhaustive
     In an equation for ‘foo’: Patterns not matched: _
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10339>
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