Re: Constraints?

["David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>]

Simon Wright wrote in message ...
>(I think I forgot to post this one, it hasn't appeared here yet; if
>duplicated, sorry)
>
>GNAT thinks the code below is OK, Object Ada (special edition)
>doesn't.  If OA is right, as I fear it may be, can anyone offer a way
>of getting the effect I'm after?
>
>(t is some abstract base container, i is an abstract iterator over
>such a container, n is a concrete container, r is a concrete iterator
>over an n)
>
>package foo is
>    type t is abstract tagged null record;
>    type i (f : access t'class) is abstract tagged limited null record;
>    type n is new t with null record;
>    type r (f : access n) is new i (f) with null record;
>    -- OA says foo.ads: Error: line 5 col 37 LRM:3.7(15), Subtype of
>    -- discriminant must be statically compatible with subtype of the
>    -- corresponding parent discriminant
>end foo;
The real problem is a violation of LRM:3.7(13).
Both compilers will accept the code below.
package foo is
    type t is abstract tagged null record;
    type i (f : access t'class) is abstract tagged limited null record;
    type n is new t with null record;
    type a_n is access all n'class;
    type r (f : a_n) is new i (f) with null record;
end foo;


David C. Hoos, Sr.