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

1 Dec 2017

      #14547: Wrong warning by -Wincomplete-patterns
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           Reporter:  YoshikuniJujo  |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:  incomplete-    |  Operating System:  Linux
  patterns OverloadedLists           |
  TypeFamilies                       |   Type of failure:  Incorrect
       Architecture:  x86            |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Wrong warning occur where I use -Wincomplete-patterns with OverloadedLists
 and TypeFamilies.

 {{{#!hs
 {-# LANGUAGE OverloadedLists #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# OPTIONS_GHC -Wall -fno-warn-tabs #-}

 class Foo f where
         type It f
         foo :: [It f] -> f

 data List a = Empty | a :! List a deriving Show

 instance Foo (List a) where
         type It (List a) = a
         foo [] = Empty
         foo (x : xs) = x :! foo xs
 }}}

 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/14547
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