Access to Unconstrained Arrays

[John Harbaugh <john.s.harbaugh2@boeing.com>]

When trying to pass pointers to objects of unconstrained array type, I
am getting confusing compiler errors.  For example:

procedure Main is
   type Lists is array(Natural range <>) of Integer;
   type Lists_Ptrs is access all Lists;

   procedure Do_Something(To : in out Lists_Ptrs) is...

   List1 : Lists(1..4) := (1,2,3,4);
   List2 : Lists       := (1,2,3,4);

begin
   Do_Something(To => List1);  -- Compilation Error.  Why?
   Do_Something(To => List2);  -- No problemo!!!
end Main;

List1 should be an anonymous constrained subtype of type Lists.  I was
under the impression that subtypes are simply a subset of their base
type.  Is this not the case for constrained subtypes of unconstrained
types?

Thanks in advance for any who may respond.

   - John