Operator visibility/inheritance for derived private type? (Repost)

[Rod Chapman <rod@praxis-cs.co.uk>]

Confused of Bath writes:

  Imagine I have a package exporting a private type with an
explicitly redeclared "=" operator - Ada.Task_Identification.Task_ID
for instance.  I want to derive another private type from
this, exporting only a subset of the operations of the original,
so I write

with Ada.Task_Identification; 
package Rod.TI is
   type Task_ID is private;
   function Current_Task return Boolean;
private
   type Task_ID is new Ada.Task_Identification.Task_ID;
end Rod.TI;

If a user of this package does

   A, B : Rod.TI.Task_ID;
 begin
   if A = B then ...

which "=" is called?  Do I get some simple comparison of
A and B or should I get a call of
Ada.Task_Identification."="?

GNAT (3.12a3 on NT) seems to be generating the former,
but I'm not sure if the language requires the latter,
and GNAT is simply doing an optimisation.  If
I _definintely_ want the call to Ada.Task_Identification."=",
so I need to explicitly redeclare my own "=" for
Rod.TI.Task_ID?  Which language rule(s)
am I missing to explain what going on?

Cheers,
 Rod Chapman
 Praxis Critical Systems