Take account of GADTs when reporting patterm-match overlap

[ghc-hetmet.git] / compiler / typecheck / TcTyClsDecls.lhs

diff --git a/compiler/typecheck/TcTyClsDecls.lhs b/compiler/typecheck/TcTyClsDecls.lhs
index 0e59f01..049276d 100644 (file)
--- a/compiler/typecheck/TcTyClsDecls.lhs
+++ b/compiler/typecheck/TcTyClsDecls.lhs
@@ -1257,11 +1257,18 @@ 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))]
 
+       -- 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 = L loc $ ExplicitTuple [] Boxed
     msg_lit = HsStringPrim $ mkFastString $ 
               occNameString (getOccName sel_name)