why "unconstrained subtype in component declaration" while said component given default value ?

[Mehdi Saada <00120260a@gmail.com>]

Hello.
Why is this case not allowed ? Is there really an implementation issue here ? I doubt so. Why should it be any different from the non-issue of initializing variables of unconstrained subtypes vs declaring uninitialized variables of constrained subtypes ?
 type A (N: Positive) is record
      II : Integer;
      Chaine : String := Give_String(N);
   end record;

Also, there's something that would be really cool, and I'm curious why it could not be allowed: Why can't we provide a private constrained subtype, of an inconstrained private type ?

package III is
   type A (<>) is private;
   subtype B is A;  -- should be a constrained subtype
private
   function Donne_String (N : Positive) return String;
   type A (N: Positive) is record
      II : Integer;
      Chaine : String(1..N) := Give_String(N);
   end record;
end III;