Dynamically tagged expression required

[Maciej Sobczak <no.spam@no.spam.com>]

Hi,

procedure Hello is

    type Shape is tagged null record;
    type Triangle is new Shape with
      record
         SideLen : Positive;
      end record;

    A : Shape;
    B : Shape'Class := A;             -- (1)
    C : Triangle := (SideLen => 7);

begin

    A := C;                           -- (2)
    B := C;                           -- (3)

end Hello;


(2) does not compile, and this is what was expected.
(3) does not compile, neither, beucase "dynamically tagged expression 
required".

Interestingly, (1) is fine.

Why is (1) allowed?
Why is (3) not allowed?

Regards,

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/