Dmitry A. Kazakov wrote:
> 
> I think that Ludovic's view is more consistent: checks are just a part of
> the conversion which otherwise is an identity function, because *presently*
> the representation is required to be same. But representation is an
> implementation detail and thus cannot count. Moreover it is very desirable
> to allow subtypes having different representations.

Try this with your favorite compiler:

Ada.Text_IO.Put_Line
    (Item => "Integer'Size =" & Integer'Image (Integer'Size) ):
Ada.Text_IO.Put_Line
    (Item => "Natural'Size =" & Integer'Image (Natural'Size) ):

You're likely to get something like

Integer'Size = 32
Natural'Size = 31

because representations of subtypes are not required to be the same.

We can also consider

type Rec (Length : Positive) is record
    V : String (1 .. Length);
end record;

subtype Small is Rec (Length =>     1);
subtype Big   is Rec (length => 1_000);

where Small and Big have significantly different representations. Then 
there are subtypes of variant record types with discriminants that 
select different variants.