[Ada] Improve error message for dispatching subprogram formals

2019-12-12  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch8.adb: Improve error message for dispatching subprogram
	formals.

From-SVN: r279301

gcc/ada/ChangeLog

+2019-12-12  Ed Schonberg  <schonberg@adacore.com>
+
+	* sem_ch8.adb: Improve error message for dispatching subprogram
+	formals.
+
 2019-12-12  Gary Dismukes  <dismukes@adacore.com>
 
 	* exp_ch9.adb (Build_Protected_Entry): Set the Scope of the new

gcc/ada/sem_ch8.adb

@@ -3453,9 +3453,14 @@ package body Sem_Ch8 is
                   if Old_S_Ctrl_Type /= New_S_Ctrl_Type
                     or else No (New_S_Ctrl_Type)
                   then
-                     Error_Msg_NE
-                       ("actual must be dispatching subprogram for type&",
-                        Nam, New_S_Ctrl_Type);
+                     if No (New_S_Ctrl_Type) then
+                        Error_Msg_N
+                          ("actual must be dispatching subprogram", Nam);
+                     else
+                        Error_Msg_NE
+                          ("actual must be dispatching subprogram for type&",
+                           Nam, New_S_Ctrl_Type);
+                     end if;
 
                   else
                      Set_Is_Dispatching_Operation (New_S);