Index: sinfo.ads
===================================================================
--- sinfo.ads	(revision 197766)
+++ sinfo.ads	(working copy)
@@ -807,7 +807,10 @@
    --    This flag is set on N_Selected_Component nodes to indicate that a
    --    discriminant check is required using the discriminant check routine
    --    associated with the selector. The actual check is generated by the
-   --    expander when processing selected components.
+   --    expander when processing selected components. In the case of
+   --    Unchecked_Union, the flag is also set, but no discriminant check
+   --    routine is associated with the selector, and the expander does not
+   --    generate a check.
 
    --  Do_Division_Check (Flag13-Sem)
    --    This flag is set on a division operator (/ mod rem) to indicate
Index: sem_res.adb
===================================================================
--- sem_res.adb	(revision 197777)
+++ sem_res.adb	(working copy)
@@ -8798,8 +8798,6 @@
         and then Ekind_In (Entity (S), E_Component, E_Discriminant)
         and then Present (Original_Record_Component (Entity (S)))
         and then Ekind (Original_Record_Component (Entity (S))) = E_Component
-        and then Present (Discriminant_Checking_Func
-                           (Original_Record_Component (Entity (S))))
         and then not Discriminant_Checks_Suppressed (T)
         and then not Init_Component
       then
Index: exp_ch4.adb
===================================================================
--- exp_ch4.adb	(revision 197777)
+++ exp_ch4.adb	(working copy)
@@ -9198,6 +9198,7 @@
       Loc   : constant Source_Ptr := Sloc (N);
       Par   : constant Node_Id    := Parent (N);
       P     : constant Node_Id    := Prefix (N);
+      S     : constant Node_Id    := Selector_Name (N);
       Ptyp  : Entity_Id           := Underlying_Type (Etype (P));
       Disc  : Entity_Id;
       New_N : Node_Id;
@@ -9273,18 +9274,27 @@
       --  Deal with discriminant check required
 
       if Do_Discriminant_Check (N) then
+         if Present (Discriminant_Checking_Func
+                      (Original_Record_Component (Entity (S))))
+         then
+            --  Present the discriminant checking function to the backend, so
+            --  that it can inline the call to the function.
 
-         --  Present the discriminant checking function to the backend, so that
-         --  it can inline the call to the function.
+            Add_Inlined_Body
+              (Discriminant_Checking_Func
+                (Original_Record_Component (Entity (S))));
 
-         Add_Inlined_Body
-           (Discriminant_Checking_Func
-             (Original_Record_Component (Entity (Selector_Name (N)))));
+            --  Now reset the flag and generate the call
 
-         --  Now reset the flag and generate the call
+            Set_Do_Discriminant_Check (N, False);
+            Generate_Discriminant_Check (N);
 
-         Set_Do_Discriminant_Check (N, False);
-         Generate_Discriminant_Check (N);
+         --  In the case of Unchecked_Union, no discriminant checking is
+         --  actually performed.
+
+         else
+            Set_Do_Discriminant_Check (N, False);
+         end if;
       end if;
 
       --  Ada 2005 (AI-318-02): If the prefix is a call to a build-in-place