[commit: ghc] master: Add a missing case to Lint's understanding of empty cases (1b282fc)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/1b282fcc5e31cdb6dd1ccdb5a3dcf1b46ab3532b/ghc

>---------------------------------------------------------------

commit 1b282fcc5e31cdb6dd1ccdb5a3dcf1b46ab3532b
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Nov 22 15:03:15 2013 +0000

    Add a missing case to Lint's understanding of empty cases
    
       case x:Int# of {}
    
    is OK


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1b282fcc5e31cdb6dd1ccdb5a3dcf1b46ab3532b
 compiler/coreSyn/CoreLint.lhs |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/compiler/coreSyn/CoreLint.lhs b/compiler/coreSyn/CoreLint.lhs
index 2b76a4a..f38a69a 100644
--- a/compiler/coreSyn/CoreLint.lhs
+++ b/compiler/coreSyn/CoreLint.lhs
@@ -550,8 +550,17 @@ checkCaseAlts :: CoreExpr -> OutType -> [CoreAlt] -> LintM ()
 checkCaseAlts e ty alts = 
   do { checkL (all non_deflt con_alts) (mkNonDefltMsg e)
      ; checkL (increasing_tag con_alts) (mkNonIncreasingAltsMsg e)
-     ; checkL (isJust maybe_deflt || not is_infinite_ty)
-	   (nonExhaustiveAltsMsg e) }
+
+          -- For types Int#, Word# with an infinite (well, large!) number of
+          -- possible values, there should usually be a DEFAULT case
+          -- But (see Note [Empty case alternatives] in CoreSyn) it's ok to
+          -- have *no* case alternatives.
+          -- In effect, this is a kind of partial test. I suppose it's possible
+          -- that we might *know* that 'x' was 1 or 2, in which case
+          --   case x of { 1 -> e1; 2 -> e2 }
+          -- would be fine.   
+     ; checkL (isJust maybe_deflt || not is_infinite_ty || null alts)
+              (nonExhaustiveAltsMsg e) }
   where
     (con_alts, maybe_deflt) = findDefault alts