Range checking not working as expected?

[Maciej Sobczak <no.spam@no.spam.com>]

Hello,

Consider this:

with Ada.Text_IO, Ada.Integer_Text_IO;
procedure Overflow is
    A : Integer := 0;
    D : constant Integer := 500000000;
begin
    for I in 1..20 loop
       Ada.Integer_Text_IO.Put(A);
       Ada.Text_IO.New_Line;
       A := A + D;
    end loop;
end Overflow;

On my machine (PC+Linux+GNAT) it produces the following output:

           0
   500000000
  1000000000
  1500000000
  2000000000
-1794967296
-1294967296
  -794967296
  -294967296
   205032704
   705032704
  1205032704
  1705032704
-2089934592
-1589934592
-1089934592
  -589934592
   -89934592
   410065408
   910065408

Not good - I expected constraint error instead of going round the counter.
What has really confused me is that declaring A as Natural gives the 
same effect.
Further investigation with the following declarations:

    type MyType is range 0..Integer'Last;
    A : MyType := 0;

(and type casts later, when necessary)

showed exactly the same effect. Following this path, I tried this:

    type MyType is range 0..Integer'Last - 1;
    A : MyType := 0;

And *now* I got what I wanted - the constraint error.

What's going on? Well, I understand the low-level mechanics of this 
difference (and also why this might be a border-case for the compiler), 
but the question is rather high-level: what does *that* mean? :)


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/