Passing the same actual as both in and out formal parameters?

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Consider:

   type T is tagged private;
   procedure P (A : in T; B : out T) is separate;
   Object : T;
begin
   P (A => Object, B => Object);

This seems legal but I suspect the execution might lead to bugs if P
reads and writes components of A and B in arbitrary order, e.g.

type T is tagged record
   L, M : Integer;
end record;

procedure P (A : in T; B : out T) is
begin
   B.L := A.M; -- does this change A.L too?
   B.M := A.L; -- bug: A.L has been clobbered, now B.M = B.L?
end P;

My concern stems from the fact that T is tagged (I cannot change
that), so Object is passed by reference as both A and B.

Am I right to be concerned?

--
Ludovic Brenta.