[GHC] #16289: GHC thinks pattern match is exhaustive

5 Feb 2019


      #16289: GHC thinks pattern match is exhaustive
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           Reporter:  dnspies        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 {-# OPTIONS_GHC -Werror #-}

 module Lib where

 data Value = Finite Integer | Infinity
   deriving (Eq)

 instance Num Value where
   (+)         = undefined
   (*)         = undefined
   abs         = undefined
   signum      = undefined
   negate      = undefined
   fromInteger = Finite

 isSpecial :: Value -> Bool
 isSpecial Infinity = True
 isSpecial 0        = True
 isSpecial 1        = True
 isSpecial _        = False

 }}}


 {{{
...
ghc Lib.hs
 [1 of 1] Compiling Lib              ( Lib.hs, Lib.o )
Lib.hs:19:1: error: [-Woverlapping-patterns, -Werror=overlapping-patterns]
     Pattern match is redundant
     In an equation for ‘isSpecial’: isSpecial 1 = ...
    |
 19 | isSpecial 1        = True
    | ^^^^^^^^^^^^^^^^^^^^^^^^^

 Lib.hs:20:1: error: [-Woverlapping-patterns, -Werror=overlapping-patterns]
     Pattern match is redundant
     In an equation for ‘isSpecial’: isSpecial _ = ...
    |
 20 | isSpecial _        = False
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16289
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