Constraints?

[Simon Wright <simon@pogner.demon.co.uk>]

(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;