[GHC] #11390: GHC does not warn about redundant patterns

[GHC]

#11390: GHC does not warn about redundant patterns
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  warnings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 I thought that was what the documentation for `EmptyCase` is alluding to:

 > With dependently-typed features it is more useful (see Trac #2431). For
 example, consider these two candidate definitions of absurd:
 >
 > {{{#!hs
 > data a :==: b where
 >   Refl :: a :==: a
 >
 > absurd :: True :~: False -> a
 > absurd x = error "absurd"    -- (A)
 > absurd x = case x of {}      -- (B)
 > }}}
 >
 > We much prefer (B). Why? Because GHC can figure out that `(True :~:
 False)` is an empty type. So (B) has no partiality and GHC should be able
 to compile with [http://downloads.haskell.org/~ghc/master/users-guide
 /using-warnings.html#ghc-flag--Wincomplete-patterns -Wincomplete-
 patterns]. (Though the pattern match checking is not yet clever enough to
 do that.) On the other hand (A) looks dangerous, and GHC doesn’t check to
 make sure that, in fact, the function can never get called.

 I wasn't sure whether the "Though the pattern match checking is not yet
 clever enough to do that." statement had changed in 8, I'll edit the code
 to enable `LambdaCase` as well.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11390#comment:6>
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