[GHC] #14547: Wrong warning by -Wincomplete-patterns

[GHC]

#14547: Wrong warning by -Wincomplete-patterns
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        Reporter:  YoshikuniJujo     |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  incomplete-
                                     |  patterns OverloadedLists,
                                     |  PatternMatchWarnings TypeFamilies
Operating System:  Linux             |         Architecture:  x86
 Type of failure:  Incorrect         |            Test Case:
  error/warning at compile-time      |  deSugar/should_compile/T14547
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4624
       Wiki Page:                    |
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Comment (by simonpj):

 I'm a bit lost with this example.  I think that, because of
 `OverloadedLists` it desugars as
 {{{
         foo (toList -> []) = Empty
         foo (toList -> (x : xs)) = x :! foo xs
 }}}
 But then we actually need the `[]` (list) instance of `toList` which is
 the identity function?   Then what?   How does the bogus error message
 arise?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14547#comment:3>
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