Fix Trace #1494

[ghc-hetmet.git] / compiler / deSugar / MatchLit.lhs

diff --git a/compiler/deSugar/MatchLit.lhs b/compiler/deSugar/MatchLit.lhs
index 2cdab30..50fec2f 100644 (file)
--- a/compiler/deSugar/MatchLit.lhs
+++ b/compiler/deSugar/MatchLit.lhs
@@ -27,12 +27,10 @@ import TcType
 import Type
 import PrelNames
 import TysWiredIn
-import PrelNames
 import Unique
 import Literal
 import SrcLoc
 import Ratio
-import SrcLoc
 import Outputable
 import Util
 import FastString
@@ -183,7 +181,8 @@ matchLiterals :: [Id]
              -> DsM MatchResult
 
 matchLiterals (var:vars) ty sub_groups
-  = do {       -- Deal with each group
+  = ASSERT( all notNull sub_groups )
+    do {       -- Deal with each group
        ; alts <- mapM match_group sub_groups
 
                -- Combine results.  For everything except String