Fix the pruning of dead case alternatives

[ghc-hetmet.git] / compiler / simplCore / SimplUtils.lhs

diff --git a/compiler/simplCore/SimplUtils.lhs b/compiler/simplCore/SimplUtils.lhs
index 5223fe0..2e9c83d 100644 (file)
--- a/compiler/simplCore/SimplUtils.lhs
+++ b/compiler/simplCore/SimplUtils.lhs
@@ -40,7 +40,7 @@ import SimplMonad
 import Type
 import TyCon
 import DataCon
-import TcGadt  ( dataConCanMatch )
+import Unify   ( dataConCannotMatch )
 import VarSet
 import BasicTypes
 import Util
@@ -1199,7 +1199,7 @@ prepareAlts scrut case_bndr' alts
        ; default_alts <- prepareDefault dflags scrut case_bndr' mb_tc_app 
                                         imposs_deflt_cons maybe_deflt
 
-       ; let trimmed_alts = filter possible_alt alts_wo_default
+       ; let trimmed_alts = filterOut impossible_alt alts_wo_default
              merged_alts = mergeAlts trimmed_alts default_alts
                -- We need the mergeAlts in case the new default_alt 
                -- has turned into a constructor alternative.
@@ -1215,10 +1215,10 @@ prepareAlts scrut case_bndr' alts
                    Var v -> otherCons (idUnfolding v)
                    other -> []
 
-    possible_alt :: CoreAlt -> Bool
-    possible_alt (con, _, _) | con `elem` imposs_cons = False
-    possible_alt (DataAlt con, _, _) = dataConCanMatch inst_tys con
-    possible_alt alt               = True
+    impossible_alt :: CoreAlt -> Bool
+    impossible_alt (con, _, _) | con `elem` imposs_cons = True
+    impossible_alt (DataAlt con, _, _) = dataConCannotMatch inst_tys con
+    impossible_alt alt                = False
 
 
 --------------------------------------------------
@@ -1306,9 +1306,8 @@ prepareDefault dflags scrut case_bndr (Just (tycon, inst_tys)) imposs_cons (Just
                                -- which would be quite legitmate.  But it's a really obscure corner, and
                                -- not worth wasting code on.
   , let imposs_data_cons = [con | DataAlt con <- imposs_cons]  -- We now know it's a data type 
-       is_possible con  = not (con `elem` imposs_data_cons)
-                          && dataConCanMatch inst_tys con
-  = case filter is_possible all_cons of
+       impossible con  = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con
+  = case filterOut impossible all_cons of
        []    -> return []      -- Eliminate the default alternative
                                -- altogether if it can't match
 
@@ -1361,7 +1360,7 @@ mkCase :: OutExpr -> OutId -> OutType
 -- put an error case here insteadd
 mkCase scrut case_bndr ty []
   = pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $
-    return (mkApps (Var eRROR_ID)
+    return (mkApps (Var rUNTIME_ERROR_ID)
                   [Type ty, Lit (mkStringLit "Impossible alternative")])