Fix desugaring of record update (fixes Trac #2735)

diff --git a/compiler/deSugar/DsExpr.lhs b/compiler/deSugar/DsExpr.lhs
index 37129d8..b91380d 100644 (file)
--- a/compiler/deSugar/DsExpr.lhs
+++ b/compiler/deSugar/DsExpr.lhs
@@ -451,24 +451,32 @@ dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
 
     do { record_expr' <- dsLExpr record_expr
        ; field_binds' <- mapM ds_field fields
+       ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
+             upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']
 
        -- It's important to generate the match with matchWrapper,
        -- and the right hand sides with applications of the wrapper Id
        -- so that everything works when we are doing fancy unboxing on the
        -- constructor aguments.
-       ; alts <- mapM mk_alt cons_to_upd
+       ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
        ; ([discrim_var], matching_code) 
                <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
 
        ; return (add_field_binds field_binds' $
                  bindNonRec discrim_var record_expr' matching_code) }
   where
-    ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Id, CoreExpr)
+    ds_field :: HsRecField Id (LHsExpr Id) -> DsM (Name, Id, CoreExpr)
+      -- Clone the Id in the HsRecField, because its Name is that
+      -- of the record selector, and we must not make that a lcoal binder
+      -- else we shadow other uses of the record selector
+      -- Hence 'lcl_id'.  Cf Trac #2735
     ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
-                           ; return (unLoc (hsRecFieldId rec_field), rhs) }
+                           ; let fld_id = unLoc (hsRecFieldId rec_field)
+                           ; lcl_id <- newSysLocalDs (idType fld_id)
+                           ; return (idName fld_id, lcl_id, rhs) }
 
     add_field_binds [] expr = expr
-    add_field_binds ((b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
+    add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)
 
        -- Awkwardly, for families, the match goes 
        -- from instance type to family type
@@ -476,7 +484,7 @@ dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
     in_ty     = mkTyConApp tycon in_inst_tys
     in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
 
-    mk_alt con
+    mk_alt upd_fld_env con
       = do { let (univ_tvs, ex_tvs, eq_spec, 
                  eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
                 subst = mkTopTvSubst (univ_tvs `zip` in_inst_tys)
@@ -487,6 +495,8 @@ dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
           ; arg_ids    <- newSysLocalsDs (substTys subst arg_tys)
           ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
                                         (dataConFieldLabels con) arg_ids
+                 mk_val_arg field_name pat_arg_id 
+                     = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
                 inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
                        -- Reconstruct with the WrapId so that unpacking happens
                 wrap = mkWpApps theta_vars `WpCompose` 
@@ -514,11 +524,6 @@ dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
                                         , pat_ty = in_ty }
           ; return (mkSimpleMatch [pat] wrapped_rhs) }
 
-    upd_field_ids :: NameEnv Id        -- Maps field name to the LocalId of the field binding
-    upd_field_ids = mkNameEnv [ (idName field_id, field_id) 
-                             | rec_fld <- fields, let field_id = unLoc (hsRecFieldId rec_fld) ]
-    mk_val_arg field_name pat_arg_id 
-      = nlHsVar (lookupNameEnv upd_field_ids field_name `orElse` pat_arg_id)
 \end{code}
 
 Here is where we desugar the Template Haskell brackets and escapes

diff --git a/compiler/typecheck/TcExpr.lhs b/compiler/typecheck/TcExpr.lhs
index 540292c..51d6f4b 100644 (file)
--- a/compiler/typecheck/TcExpr.lhs
+++ b/compiler/typecheck/TcExpr.lhs
@@ -1189,9 +1189,11 @@ tcRecordBinds data_con arg_tys (HsRecFields rbinds dd)
        do { rhs' <- tcPolyExprNC rhs field_ty
           ; let field_id = mkUserLocal (nameOccName field_lbl)
                                        (nameUnique field_lbl)
-                                       field_ty loc
-               -- The field_id has the *unique* of the selector Id
-               -- but is a LocalId with the appropriate type of the RHS
+                                       field_ty loc 
+               -- Yuk: the field_id has the *unique* of the selector Id
+               --          (so we can find it easily)
+               --      but is a LocalId with the appropriate type of the RHS
+               --          (so the desugarer knows the type of local binder to make)
           ; return (Just (fld { hsRecFieldId = L loc field_id, hsRecFieldArg = rhs' })) }
       | otherwise
       = do { addErrTc (badFieldCon data_con field_lbl)