Allow cases with empty alterantives

This patch allows, for the first time, case expressions with an empty
list of alternatives. Max suggested the idea, and Trac #6067 showed
that it is really quite important.

So I've implemented the idea, fixing #6067. Main changes

 * See Note [Empty case alternatives] in CoreSyn

 * Various foldr1's become foldrs

 * IfaceCase does not record the type of the alternatives.
   I added IfaceECase for empty-alternative cases.

 * Core Lint does not complain about empty cases

 * MkCore.castBottomExpr constructs an empty-alternative case
   expression   (case e of ty {})

 * CoreToStg converts '(case e of {})' to just 'e'

1 files changed, 12 insertions, 0 deletions

diff --git a/compiler/stgSyn/CoreToStg.lhs b/compiler/stgSyn/CoreToStg.lhs
index 71bdfe97c9..c4f289c68e 100644
--- a/compiler/stgSyn/CoreToStg.lhs
+++ b/compiler/stgSyn/CoreToStg.lhs
@@ -363,6 +363,18 @@ coreToStgExpr (Cast expr _)
 
 -- Cases require a little more real work.
 
+coreToStgExpr (Case scrut _ _ []) 
+  = coreToStgExpr scrut   
+    -- See Note [Empty case alternatives] in CoreSyn If the case
+    -- alternatives are empty, the scrutinee must diverge or raise an
+    -- exception, so we can just dive into it.
+    --
+    -- Of course this may seg-fault if the scrutinee *does* return.  A
+    -- belt-and-braces approach would be to move this case into the
+    -- code generator, and put a return point anyway that calls a
+    -- runtime system error function.
+        
+
 coreToStgExpr (Case scrut bndr _ alts) = do
     (alts2, alts_fvs, alts_escs)
        <- extendVarEnvLne [(bndr, LambdaBound)] $ do