Conversion of Access Types Question

["Paul S. Heidmann" <psh@neta.com>]

Greetings fellow Ada lovers!

I'm an experienced Ada83 programmer that is learning to use Ada95.
Things are going very well, but I don't understand the problems
that I'm having converting access types that point to variables of
class wide types.  Consider the following piece of code:

=======================================

procedure Types is

   type T1 is tagged
      record
         I : Integer;
      end record;
   type A_T1 is access all T1'Class;

   type T2 is new T1 with
      record
         I2 : Integer;
      end record;
   type A_T2_Classwide is access all T2'Class;
   type A_T2 is access T2;

   Thing1 : A_T1;
   Thing2 : A_T2_Classwide;
   Thing3 : A_T2;

begin
   Thing1 := new T2;
   Thing2 := A_T2_Classwide (Thing1);
   Thing3 := A_T2 (Thing1);  -- illegal, types are not convertable.
end Types;

=======================================

My question is, why is the last line an illegal conversion, but the
second to last line not?

Thanks!

Paul Heidmann