Upcasting

["Matthew Baulch" <matt@greenroom.com.au>]

For the records:

...
type Base is tagged with
  record
    Base1, Base2 : Integer;
  end record;

type Derived is new Base with
  record
    Derived1, Derived2 : Float;
  end record;
...
Foo1 : Base;
Foo2 : Derived;
...

Obviously to assign Foo2 to Foo1 you would:
	Foo1 := Base(Foo2);

But if you wanted to assign Foo1 to Foo2:
	Foo2 := (Foo1 with 5.0, 10.0);
would work.

My question is: How would you do this 'upcasting' if varying
types existed? Obviously the compiler could tell where each
value must go based on their type if only one of their type
was needed in the transition. How does this type of thing work?
(or does the A95 compiler not allow it)