Rationale for this not to compile?

[Victor Porton <porton@narod.ru>]

Is there any rationale why this does not compile? If it compiled, it would 
be probably useful in practice with a real example similar to the below toy 
example (as to make the type T not private, I must create an *.Internal.* 
package to serve as a "private repository" for types like T, as real private 
types cannot be used). It is called a hack.

gnatmake -q -c -gnatc -u -P/home/porton/t/default.gpr test.ads
test.ads:10:32: premature use of private type
gnatmake: "/home/porton/t/test.ads" compilation error

package Test is

   generic
      type T is limited private;
   package Gen is
   end Gen;
   
   type X is private;

   package Instance is new Gen(X);
   
private
   
   type X is null record;
   
end Test;

-- 
Victor Porton - http://portonvictor.org