Improve error message when there is instance overlap

diff --git a/compiler/types/InstEnv.lhs b/compiler/types/InstEnv.lhs
index 560c4fc..7aaea5c 100644 (file)
--- a/compiler/types/InstEnv.lhs
+++ b/compiler/types/InstEnv.lhs
@@ -430,10 +430,11 @@ lookupInstEnv (pkg_ie, home_ie) cls tys
     (pkg_matches,  pkg_unifs)  = lookup pkg_ie  
     all_matches = home_matches ++ pkg_matches
     all_unifs   = home_unifs   ++ pkg_unifs
-    pruned_matches 
-       | null all_unifs = foldr insert_overlapping [] all_matches
-       | otherwise      = all_matches  -- Non-empty unifs is always an error situation,
-                                       -- so don't attempt to pune the matches
+    pruned_matches = foldr insert_overlapping [] all_matches
+       -- Even if the unifs is non-empty (an error situation)
+       -- we still prune the matches, so that the error message isn't
+       -- misleading (complaining of multiple matches when some should be
+       -- overlapped away)
 
     --------------
     lookup env = case lookupUFM env cls of