[GHC] #11245: Non-exhaustive pattern, "Patterns not matched" list is empty

[GHC]

#11245: Non-exhaustive pattern, "Patterns not matched" list is empty
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
           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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 Example:

 {{{#!haskell
 module Main where

 maybeOdd :: Int -> Maybe Int
 maybeOdd i = if odd i then Just i else Nothing

 main :: IO ()
 main = do
   let x = maybeOdd 10

   let a | Just i <- x
         , odd i
         = True

         | Nothing <- x
         = False

   print x
   print a
 }}}

 Warning printed by GHC HEAD:

 {{{
 Exhaustive.hs:10:7: warning:
     Pattern match(es) are non-exhaustive
     In an equation for ‘a’: Patterns not matched:
 Linking Exhaustive ...
 }}}

 The problem with this message is; if it couldn't come up with an example
 unmatched pattern, then how can it know that the pattern is non-
 exhaustive? If it came up with an example, why is that example not
 printed?

 UPDATE: I just realized it's actually worse that I first thought. If I
 change {{{a}}} in this example:

 {{{#!haskell
   let a | Just i <- x
         = True
 }}}

 This message is printed:

 {{{
 [1 of 1] Compiling Main             ( Exhaustive.hs, Exhaustive.o )

 Exhaustive.hs:10:7: warning:
     Pattern match(es) are non-exhaustive
     In an equation for ‘a’: Patterns not matched:

 Exhaustive.hs:10:16: warning: Defined but not used: ‘i’
 Linking Exhaustive ...
 }}}

 NOTE: Tried with GHC 7.10 too. It seems like in the case where the checks
 are not exhaustive, both 7.10 and HEAD are giving the same warning(with
 empty list of non-checked patterns). HEAD is better in detecting
 exhaustive patterns.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11245>
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