[Ada] Semantics of Delete for fixed strings

This patch corrects a bug in the implementation of Delete in an unusual
boundary case: the RM describes the semantics of Delete as equivalent to
that of Replace_String with a null argument. As a result, deleting a
null string that starts past the end of its argument is a noop and must
not raise Index_Error.

2019-07-08  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* libgnat/a-strfix.adb (Delete): The RM describes the semantics
	of Delete as equivalent to that of Replace_String with a null
	argument. As a result, deleting a null string that starts past
	the end of its argument is a noop and must not raise
	Index_Error.

gcc/testsuite/

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

From-SVN: r273205

gcc/ada/ChangeLog

+2019-07-08  Ed Schonberg  <schonberg@adacore.com>
+
+	* libgnat/a-strfix.adb (Delete): The RM describes the semantics
+	of Delete as equivalent to that of Replace_String with a null
+	argument. As a result, deleting a null string that starts past
+	the end of its argument is a noop and must not raise
+	Index_Error.
+
 2019-07-08  Javier Miranda  <miranda@adacore.com>
 
 	* exp_disp.adb (Register_Primitive): When registering a

gcc/ada/libgnat/a-strfix.adb

@@ -192,7 +192,15 @@ package body Ada.Strings.Fixed is
       elsif From not in Source'Range
         or else Through > Source'Last
       then
+         --  In most cases this raises an exception, but the case of deleting
+         --  a null string at the end of the current one is a special-case, and
+         --  reflects the equivalence with Replace_String (RM A.4.3 (86/3)).
+
+         if From = Source'Last + 1 and then From = Through then
+            return Source;
+         else
             raise Index_Error;
+         end if;
 
       else
          declare

gcc/testsuite/ChangeLog

+2019-07-08  Ed Schonberg  <schonberg@adacore.com>
+
+	* gnat.dg/fixed_delete.adb: New testcase.
+
 2019-07-08  Javier Miranda  <miranda@adacore.com>
 
 	* gnat.dg/interface9.adb, gnat.dg/interface9_root-child.ads,

gcc/testsuite/gnat.dg/fixed_delete.adb

+--  { dg-do run }
+
+with Ada.Text_IO; use Ada.Text_IO;
+with Ada.Strings.Fixed; use Ada.Strings.Fixed;
+
+procedure Fixed_Delete is
+   Str  : String := "a";
+   Str1 : String := Replace_Slice (Str, 2, 2, "");
+   Str2 : String := Delete (Str, 2, 2);
+begin
+   if Str1 /= "a" then
+      raise Program_Error;
+   end if;
+   if Str2 /= "a" then
+      raise Program_Error;
+   end if;
+end Fixed_Delete;