[GHC] #11806: GHC does not warn for mistakenly empty case

[GHC]

#11806: GHC does not warn for mistakenly empty case
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        Reporter:  dfeuer            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by gkaracha):

 Replying to [ticket:11806 dfeuer]:
 > {{{#!hs
 > {-# LANGUAGE EmptyCase #-}
 > {-# OPTIONS_GHC -Wall -fwarn-incomplete-uni-patterns #-}
 >
 > oops :: Int -> a
 > oops x = case x of {}
 > }}}
 >
 > Since `Int` is inhabited, I would expect a warning, but I get none.
 Since empty case is typically used in situations where programmers want
 GHC to see an absurdity, this seems most unfortunate.

 Indeed. Yet, I feel like there is a big misconception here, from the you
 said

 > Since `Int` is inhabited

 It is indeed inhabited, but **like any other type in Haskell**, where
 every type is inhabited. That is, for the following (see #11390) a warning
 should also be expected:
 {{{#!hs
 silly1 :: Void -> Void
 silly1 x = case x of {}
 }}}

 There have been separate discussions on this (see related tickets #10746,
 #7669, #11390) and I think that we have a design choice to make here.
 Unless you force the argument in some other way like:

 {{{#!hs
 silly2 :: Void -> Void
 silly2 x = x `seq` case x of {}
 }}}
 or
 {{{#!hs
 silly3 :: Void -> Void
 silly3 (!x) = case x of {}
 }}}

 then it is indeed non-exhaustive. This means that in almost all cases, an
 empty case expression will issue a warning (which I think is the right
 thing to do).
 Of course, in `silly2` and `silly3` the warning is too conservative but
 remember that type inhabitation is undecidable. Hence, I see two choices
 here. We can:

 1. Always issue a non-exhaustive warning for empty case expressions, or
 2. Never  issue a non-exhaustive warning for empty case expressions.

 I am pointing this out because before #7669 we would issue the warning you
 seek here and it has been intentionally changed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11806#comment:2>
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