[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

1 files changed, 9 insertions, 1 deletions

diff --git a/gcc/ada/libgnat/a-strfix.adb b/gcc/ada/libgnat/a-strfix.adb
index 6bf825b..b8b5f42 100644
--- a/gcc/ada/libgnat/a-strfix.adb
+++ b/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
-         raise Index_Error;
+         --  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