[GHC] #12949: Pattern coverage mistake

[GHC]

#12949: Pattern coverage mistake
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        Reporter:  dfeuer            |                Owner:  gkaracha
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 In case it's helpful, I've come up with a much simpler case:

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 module Exh where

 class Foo a where
   foo :: Maybe a

 data Result a b = Neither | This a | That b | Both a b

 q :: forall a b . (Foo a, Foo b) => Result a b
 q = case foo :: Maybe a of
       Nothing -> case foo :: Maybe b of
                    Nothing -> Neither
                    Just c -> That c
       Just i -> case foo :: Maybe b of
                    Nothing -> This i
                    Just c -> Both i c
 }}}

 This produces the following warnings, both of them incorrect:

 {{{
 Exh.hs:13:20: warning: [-Woverlapping-patterns]
     Pattern match is redundant
     In a case alternative: Just c -> ...
    |
 13 |                    Just c -> That c
    |                    ^^^^^^^^^^^^^^^^

 Exh.hs:15:20: warning: [-Woverlapping-patterns]
     Pattern match is redundant
     In a case alternative: Nothing -> ...
    |
 15 |                    Nothing -> This i
    |                    ^^^^^^^^^^^^^^^^^
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12949#comment:11>
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