Empty case causes warning

[goldfire]

The following code

{-# LANGUAGE EmptyCase #-}
{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
data Void

foo :: Void -> ()
foo x = case x of {}

causes this warning

/Users/rae/temp/Scratch.hs:6:9: Warning:
    Pattern match(es) are non-exhaustive
    In a case alternative: Patterns not matched:
Ok, modules loaded: Main.

[simonpj@…]

commit 9162d159a62d19d4b371b7348eb1b524001fddcd

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Wed Feb 13 17:37:17 2013 +0000

    Simplify the base case for 'check', and thereby fix Trac #7669

 compiler/deSugar/Check.lhs |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

[simonpj]

Thanks. Turns out I could cure this and make the code simpler!

Simon

[monoidal]

However, previously empty case always gave a warning; now it never does.

f :: Bool -> a
f x = case x of { }

[simonpj]

Very true.

Would anyone like to look at the overlapping-pattern warnings? It's a well-defined problem, restricted to a single module in GHC (deSugar/Check.lhs). We have a bunch of tickets about it: at least #595, #5728, #3927, #5724, #5762, #4139, #6124.

A lovely project awaits!

Meanwhile I think I'll leave the code in its simplified state.

Simon