Syntax of extended return statement

[Bill Findlay <news@findlayw.plus.com>]

I have the following compilation warning from GNAT:

   function pop
   return KDF9.word is
   begin
      the_nest_depth := the_nest_depth - 1;
      return result : KDF9.word := the_nest(the_nest_depth) do
             |
>>> warning: "result" is not modified, could be declared constant
         the_nest(the_nest_depth) := 0;
      end return;
   end pop;

If I change the return statement to:

      return result : constant KDF9.word := the_nest(the_nest_depth) do
         the_nest(the_nest_depth) := 0;
      end return;

it compiles without warning.

My problem is that I see no justification for that in the Ada 2005 RM, which
gives:

extended_return_statement ::=

return defining_identifier:[aliased] return_subtype_indication[:=expression]
[do
   handled_sequence_of_statements
end return]; 

return_subtype_indication ::= subtype_indication | access_definition

Can anyone enlighten me?

-- 
Bill Findlay
with blueyonder.co.uk;
use  surname & forename;