!ASIS Issue #094
!topic  Default "=" /= Is_Equal?
!reference  ASIS 95-8,9,10,13,20,21,22
!from  Bill Stanley  98-04-03
!keywords  "=" operator Is_Equal Is_Identical abstract
!discussion

It appears that there is a small problem with the 
Asis specification.  The Is_Equal function is required
because the default "=" operator might not properly
evaluate the test for equality (or inequality).  This
is fine, but it opens the door for possible programming 
errors.  It is very easy during a moment of inattention 
to use the "=" operator instead of the Is_Equal function.
If the default "=" operator does not always give the 
correct results, we then have an incorrect program.

To make matters worse, there are no compiler errors
generated by the improper use of the "=" operator.
I ran into this issue when using a generic linked 
list package and instantiated a linked list of
Asis.Elements from this package.  The generic package
uses the  "=" operator to test for equality between
the generic items.  Since it is a relativly easy matter 
to implement the "=" and "/=" operators, The logical 
thing to do is to override the default "=" and "/="
functions.  This will eliminate a possible source
of errors.   

!resolution  Accept, with Modification.
!date 98-04-30

Add the following function to clause 3.5, type Context:

   function "=" (Left  : in Context;
                 Right : in Context)
                 return Boolean is abstract;

Add the following function to clause 3.6, type Element:

   function "=" (Left  : in Element;
                 Right : in Element)
                 return Boolean is abstract;

Add the following function to clause 3.10, type Compilation_Unit:

   function "=" (Left  : in Compilation_Unit;
                 Right : in Compilation_Unit)
                 return Boolean is abstract;

Add the following function to clause 9.1, type Container:

   function "=" (Left  : in Container;
                 Right : in Container)
                 return Boolean is abstract;

Add the following function to clause 20.1, type Line:

   function "=" (Left  : in Line;
                 Right : in Line)
                 return Boolean is abstract;

Add the following function to clause 21.1, type Id:

   function "=" (Left  : in Id;
                 Right : in Id)
                 return Boolean is abstract;

Add the following function to clause 22.1, type Record_Component:

   function "=" (Left  : in Record_Component;
                 Right : in Record_Component)
                 return Boolean is abstract;

Add the following function to clause 22.3, type Array_Component:

   function "=" (Left  : in Array_Component;
                 Right : in Array_Component)
                 return Boolean is abstract;

Add appropriate rationale to the new section D.4.4, Abstract "=" for
private types, in Annex D:

D.4.4 Abstract "=" for private types

An Is_Equal function has been defined for the private types Context
(Clause 3.5), Element (Clause 3.6), Compilation_Unit (Clause 3.10),
Container (Clause 9.1), Line (Clause 20.1), Id (Clause 21.1),
Record_Component (Clause 22.1), and Array_Component (Clause 22.3). An
abstract function "=" has been defined for each private type because
the semantics for the Is_Equal function for these types may differ
from the semantics of the default "=" operator.  This prevents
improper use of the "=" operator.