prefix of dereference must be a name?

[Stephen Leake <stephen_leake@stephe-leake.org>]

I'd like to be able to do the following:

type T is record ... end record;
type T_Access is access all T;

function "+" (Left, Right in : ...) return T_Access;

A : T_Access := new T;
B : T_Access := new T;
C : T_Access := A + B;

A.all := (C + B).all;

but GNAT complains that "prefix for selection is not a name", pointing
to (C + B).all.

So then I tried:

function "-" (Token : in T_Access) return T_Access
is begin
    return Token;
end;

A.all := -(C + B).all;

and got the same error. Finally I tried:

function Copy (Token : in T_Access) return T_Access
is begin
    return Token;
end;

A.all := Copy (C + B).all;

And the compiler was happy.

What's the rationale for this? 

I assume one issue here is that "-" might be an intrinsic, but since
the type involved is not a numeric type, I don't see how that could be
a problem.

Similarly for ().all; when could that be a problem?

-- 
-- Stephe