[Ada] Improved handling of circular compilation dependencies

2019-12-12  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_ch10.adb (Install_With_Clause): Check for the case of a
	circular dependency involving a predefined (or GNAT-defined)
	unit and handle that case by generating an appropropriate error
	message.

From-SVN: r279299

gcc/ada/ChangeLog

+2019-12-12  Steve Baird  <baird@adacore.com>
+
+	* sem_ch10.adb (Install_With_Clause): Check for the case of a
+	circular dependency involving a predefined (or GNAT-defined)
+	unit and handle that case by generating an appropropriate error
+	message.
+
 2019-12-12  Gary Dismukes  <dismukes@adacore.com>
 
 	* sem_ch4.adb, sem_res.adb: Minor reformatting.

gcc/ada/sem_ch10.adb

@@ -5336,6 +5336,20 @@ package body Sem_Ch10 is
             Error_Msg_N
               ("instantiation depends on itself", Name (With_Clause));
 
+         elsif not Analyzed (Uname)
+           and then Is_Internal_Unit (Current_Sem_Unit)
+           and then not Is_Visible_Lib_Unit (Uname)
+           and then No (Scope (Uname))
+         then
+            if Is_Predefined_Unit (Current_Sem_Unit) then
+               Error_Msg_N
+                 ("predefined unit depends on itself", Name (With_Clause));
+            else
+               Error_Msg_N
+                 ("GNAT-defined unit depends on itself", Name (With_Clause));
+            end if;
+            return;
+
          elsif not Is_Visible_Lib_Unit (Uname) then
 
             --  Abandon processing in case of previous errors