X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcTyClsDecls.lhs;h=bc20d3d8d9bc90b6245ec89ac8498d2880e0bf9f;hb=9a4c93a59e008ddc376fde5f9eb468b762f0d0a7;hp=0e59f0167bdcc7f05a5ef792c5cc2a0aa81b7a66;hpb=a6f29db07ac47b8a924a65c7e07ce73bc491d0e5;p=ghc-hetmet.git

diff --git a/compiler/typecheck/TcTyClsDecls.lhs b/compiler/typecheck/TcTyClsDecls.lhs
index 0e59f01..bc20d3d 100644
--- a/compiler/typecheck/TcTyClsDecls.lhs
+++ b/compiler/typecheck/TcTyClsDecls.lhs
@@ -37,7 +37,6 @@ import VarSet
 import Name
 import Outputable
 import Maybes
-import Monad
 import Unify
 import Util
 import SrcLoc
@@ -49,6 +48,7 @@ import Unique		( mkBuiltinUnique )
 import BasicTypes
 
 import Bag
+import Control.Monad
 import Data.List
 \end{code}
 
@@ -1257,12 +1257,19 @@ mkRecSelBind (tycon, sel_name)
     -- Add catch-all default case unless the case is exhaustive
     -- We do this explicitly so that we get a nice error message that
     -- mentions this particular record selector
-    deflt | length cons_w_field == length all_cons = []
+    deflt | not (any is_unused all_cons) = []
 	  | otherwise = [mkSimpleMatch [nlWildPat] 
 	    	      	    (nlHsApp (nlHsVar (getName rEC_SEL_ERROR_ID))
     	      		    	     (nlHsLit msg_lit))]
 
-    unit_rhs = L loc $ ExplicitTuple [] Boxed
+	-- Do not add a default case unless there are unmatched
+	-- constructors.  We must take account of GADTs, else we
+	-- get overlap warning messages from the pattern-match checker
+    is_unused con = not (con `elem` cons_w_field 
+			 || dataConCannotMatch inst_tys con)
+    inst_tys = tyConAppArgs data_ty
+
+    unit_rhs = mkLHsTupleExpr []
     msg_lit = HsStringPrim $ mkFastString $ 
               occNameString (getOccName sel_name)