access_to_constant as subprogram paramater

[nelson@blaze-net.com]

Why does the language disallow an access_to_constant as a parameter
to a subprogram?

For example:

--
-- Example illustrating desire to pass access_to_constant to a procedure.
-- Refer to RM 3.10(12) and 6.1.
--
procedure Access_Parameter is

   PathName : aliased constant String := "/a/b/c.adb";
      -- Declared as constant in order to produce compilation error


   procedure Parse (Input : access String) is
--
-- Would like to declare this:
--    procedure Parse (Input : access constant String) is
-- as Parse never modifies the input string
--
   begin
      null;  -- Parse a pathname string into components
   end Parse;

begin
   Parse(PathName'Access);  -- line 22
end Access_Parameter;



for which gnat produces:


cd /usr/nelson/example/
gnatmake -c access_parameter.adb
gcc -c access_parameter.adb
access_parameter.adb:22:10: access-to-variable designates constant
gnatmake: "access_parameter.adb" compilation error

Compilation exited abnormally with code 2 at Sat Sep 19 13:23:25

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