Re: Renaming an abstract function

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

Mark Biggar <mark.a.biggar@home.com> writes:

> <snip example of renaming an abstract function returning an abstract
> type>

> > >    package Test1 is
> > >       type T is abstract tagged limited private;
> > >       function P (X, Y: in T) return T is abstract;
> > >       function "*" (A, B: in T) return T renames P; -- error line
> > >    private
> > >       type T is abstract tagged limited null record;
> > >    end;

> There is an AI that addresses this problem:
> 
> >!standard 8.5.4     (7)                             00-07-12  >AI95-00211/06
> >!class ramification 98-11-18
> >!status Response 2000 00-01-24
> >!status WG9 approved 99-06-12
> >!status ARG Approved (with changes) 8-0-1  99-03-24
> >!status work item 98-11-18
> >!priority Medium
> >!difficulty Easy
> >!qualifier Clarification
> >!subject Can an abstract subprogram be renamed?
> >
> >!summary
> >
> >An abstract subprogram can be renamed, and the renamed view is also
> >abstract. Such a renaming must appear in a place where the declaration
> >of an abstract subprogram would be legal. Similarly, the "shall be >overridden"
> >property of 3.9.3(6) applies to a renamed view. Thus, any renaming of an
> >inherited subprogram that must be overridden is illegal.
> 
> This explains the compilation error as the renaming "*" is a must be
> overridden function as it has an abstract return type and thus is
> illegal.
> 

Well, the full text of the AI talks about renaming an _inherited_
subprogram, not the abstract subprogram itself. It does not discuss
exactly what should happen for this case.

Simply following the rules for inheriting abstract subprograms, you
are required to give an overriding subprogram for any derived type.
Thus, if the renaming is valid, you'd have to override both P and "*",
which is surely not the intent. So it seems reasonable to conclude
that the renaming itself is invalid.

We could propose a "renaming inheritance" rule that says an equivalent
renaming is applied to the overriding subprogram; that is the intent
of this renaming.

-- 
-- Stephe