Re: Weird behavior with modular type

[Adam Beneschan <adam@irvine.com>]

On Feb 7, 9:24 am, Yannick Duchêne (Hibou57)
<yannick_duch...@yahoo.fr> wrote:
>
> Now, the following, with added "Natural'Pos (...)", did not even compile:
>
>     with Ada.Text_IO;
>     procedure Bug is
>
>        type Modular_Type is mod 256;
>        subtype Index_Type is Natural range 1 .. 3;
>
>        E : Modular_Type := 255;
>        I : Index_Type := 3;
>        B : Boolean;
>
>     begin
>        B := E < Natural'Pos (10 ** I);
>        Ada.Text_IO.Put_Line (Boolean'Image (B));
>     end Bug;
>
> That's counter intuitive. I would expect the first to behave as the
> second, with either a failure to compile or range error at run-time.

By the way, I don't see any reason this shouldn't compile, although
offhand I think it should raise a Constraint_Error at runtime (since
1024 would be the right argument of a function "<" (Left, Right : E)
return Boolean and 1024 can't be converted to E without a
Constraint_Error).  When I compile on GNAT (4.5.2), it says "static
expression fails Constraint_Check".  This looks like a GNAT bug,
though, because there's no static expression.  Apparently it thinks
it's static because it knows the value of I must still be 3 at that
point, but that doesn't make it a static expression according to the
language rules, so the program shouldn't be rejected.  (Inserting a
call to a null nested procedure before the first line of Bug makes the
error message go away.)

                       -- Adam