Deriving from non-null abstract type

[benari@zeus.datasrv.co.il (Moti Ben-Ari)]

I would like to derive from a _non-null_ abstract tagged type:

  type Abstract_Type is abstract tagged
    record
      General_Info: Some_Type;
    end record;

  type Derived_Type is new Abstract_Type with
    record
      Special_Info: Some_Other_Type;
    end record;

Of course, you can't create a value of the abstract type,
but it turns out that it is impossible to create a value
of the derived type:

    D: Derived_Type := (G with S);

because G would have to be an aggregate of an abstract type 
which is illegal. The only thing you can do is:

  D: Derived_Type := (Abstract_Type with S);

which means that the component General_Info has to
be initialized in a separate statement.
This seems to be against the philosophy of Ada which
provides aggregates so that you won't forget to initialize
a component.

Is there some way of creating a value that I don't see?
If not, why aren't abstract types restricted to null records?

Moti

Moti Ben-Ari
benari@datasrv.co.il