generic's and 'size... gnat bug?

[Al Johnston <sofeise@mindspring.com>]

I have some code that I am porting that looks like this
assume
   subtype foo_id is natural;

generic
    type foo_gentype is (<>);
package some_package  is
    procedure some_routine;
end some_package;

package body some_package is
    procedure some_routine is
        function to_footype is new
unchecked_conversion(foo_gentype,foo_id);
        my_foo_gentype : foo_gentype;
        my_foo_id : foo_id_type;
    begin
        my_foo_id := to_footype(my_foo_gentype)
    end some_routine;
end some_package;

which is inst'd as
    package my_foo_pkg is new some_package(foo_id_type);

My problem: compile warning pointing to the line above; but addressing
the unchecked_conversion between foo_gentype and foo_id.  it says
foo_gentype'size is 32 and foo_id_type'size is 31.  I have run across
the "fact" that a natural uses 31 bits in gnat(linux/x86), but why does
the compiler think/use the value of 32 for the generic type?

is a bug?  or is this a language "feature"

thanks.