1 files changed, 15 insertions, 0 deletions
diff --git a/compiler/rename/RnUtils.hs b/compiler/rename/RnUtils.hs
index 3a743b56fb..8d103142ed 100644
--- a/compiler/rename/RnUtils.hs
+++ b/compiler/rename/RnUtils.hs
@@ -14,6 +14,7 @@ module RnUtils (
         addFvRn, mapFvRn, mapMaybeFvRn,
         warnUnusedMatches, warnUnusedTypePatterns,
         warnUnusedTopBinds, warnUnusedLocalBinds,
+        warnUnusedRecordWildcard,
         mkFieldEnv,
         unknownSubordinateErr, badQualBndrErr, typeAppErr,
         HsDocContext(..), pprHsDocContext,
@@ -222,6 +223,16 @@ warnUnusedTopBinds gres
                                else                 gres
          warnUnusedGREs gres'
 
+
+-- | Get to see whether at least one name from each RecordWildcard is used.
+warnUnusedRecordWildcard :: [Name] -> FreeVars -> RnM ()
+warnUnusedRecordWildcard ns used_names = do
+  let used = filter (`elemNameSet` used_names) ns
+  traceRn "warnUnused" (ppr ns $$ ppr used_names $$ ppr used)
+  warnIfFlag Opt_WarnUnusedRecordWildcards (null used)
+    unusedRecordWildcardWarning
+
+
 warnUnusedLocalBinds, warnUnusedMatches, warnUnusedTypePatterns
   :: [Name] -> FreeVars -> RnM ()
 warnUnusedLocalBinds   = check_unused Opt_WarnUnusedLocalBinds
@@ -296,6 +307,10 @@ addUnusedWarning flag occ span msg
          nest 2 $ pprNonVarNameSpace (occNameSpace occ)
                         <+> quotes (ppr occ)]
 
+unusedRecordWildcardWarning :: SDoc
+unusedRecordWildcardWarning
+  = text "No variables bound in the record wildcard match are used"
+
 addNameClashErrRn :: RdrName -> [GlobalRdrElt] -> RnM ()
 addNameClashErrRn rdr_name gres
   | all isLocalGRE gres && not (all isRecFldGRE gres)