Cannot call primitive op for generic formal derived type: why not?

["Matthew Heaney" <matthew_heaney@acm.org>]

I have a type hierarchy, rooted at P.T, with a private, primitive operation,
here called Private_Op.

The root package P has a generic child, P.C, with a generic formal type that
derives from T, here called NT.  P.C has a public operation, Op, that takes
NT as an access parameter.

The body of P.C.Op tries to call the primitive operation Private_Op.  This
seems like it should be legal, because NT derives from T, and T has
Private_Op as a primitive operation (so NT should have inherited it).

Yet my compiler is telling me that the call to Private_Op is illegal:

p-c.adb:5:19: expected type access to "T" defined at p.ads:9
p-c.adb:5:19: found type access to "Nt" defined at p-c.ads:5

It seems as if the compiler doesn't acknowledge that Private_Op is primitive
for NT.

What's up with that?  Is this a compiler bug, or is this program not legal
Ada?

I thought the whole point of importing a derived type is precisely to be
able to call the primitive operations for types in that class.  Is this
expectation incorrect?

Thanks in advance,
Matt


--STX
package P is

   type T is abstract tagged limited private;

private

   type T is abstract tagged limited null record;

   procedure Private_Op (O : access T);

end P;


package body P is

   procedure Private_Op (O : access T) is
   begin
      null;
   end;

end P;


generic
   type NT is new T with private;
package P.C is

   procedure Op (O : access NT);

end P.C;

package body P.C is

   procedure Op (O : access NT) is
   begin
      Private_Op (O);
   end Op;

end P.C;