[Ada] Object_Size clause specifying 0 bits is illegal

The patch gives an error message on "for T'Object_Size use 0;".

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* sem_ch13.adb (Object_Size): Give an error for zero. It really
	rubs me the wrong way that we don't honor "for T'Object_Size use
	0;", but it's not important enough to fix. In any case, if we're
	not going to obey the clause, we should give an error.

gcc/testsuite/

	* gnat.dg/object_size1.adb: New testcase.

From-SVN: r274722

gcc/ada/ChangeLog

 2019-08-20  Bob Duff  <duff@adacore.com>
 
+	* sem_ch13.adb (Object_Size): Give an error for zero. It really
+	rubs me the wrong way that we don't honor "for T'Object_Size use
+	0;", but it's not important enough to fix. In any case, if we're
+	not going to obey the clause, we should give an error.
+
+2019-08-20  Bob Duff  <duff@adacore.com>
+
 	* errout.adb (Error_Msg_Internal): Set Warn_Err in case of
 	Is_Style_Msg.
 	* erroutc.adb (Output_Msg_Text): Do Warnings_Treated_As_Errors

gcc/ada/sem_ch13.adb

@@ -5812,6 +5812,9 @@ package body Sem_Ch13 is
                if ASIS_Mode then
                   null;
 
+               elsif Size <= 0 then
+                  Error_Msg_N ("Object_Size must be positive", Expr);
+
                elsif Is_Scalar_Type (U_Ent) then
                   if Size /= 8 and then Size /= 16 and then Size /= 32
                     and then UI_Mod (Size, 64) /= 0

gcc/testsuite/ChangeLog

+2019-08-20  Bob Duff  <duff@adacore.com>
+
+	* gnat.dg/object_size1.adb: New testcase.
+
 2019-08-20  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gcc.c-torture/execute/20190820-1.c: New test.

gcc/testsuite/gnat.dg/object_size1.adb

+--  { dg-do compile }
+
+with Text_IO; use Text_IO;
+
+procedure Object_Size1 is
+  type Zero_Size_Type is (Solo);
+
+  for Zero_Size_Type'Size use 0;
+  for Zero_Size_Type'Object_Size use 0;  --  { dg-error "Object_Size must be positive" }
+begin
+  Put_Line (Zero_Size_Type'Size'Image);
+  Put_Line (Zero_Size_Type'Object_Size'Image);
+end Object_Size1;