[Ada] Semantics of Delete for fixed strings

[Pierre-Marie de Rodat <derodat@adacore.com>]

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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.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.

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--- gcc/ada/libgnat/a-strfix.adb
+++ 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

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/fixed_delete.adb
@@ -0,0 +1,17 @@
+--  { 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;