Re: Renaming an abstract function

[Mark Biggar <mark.a.biggar@home.com>]

Adam Beneschan wrote:
> 
> Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov> wrote in message news:<uvggau5ro.fsf@gsfc.nasa.gov>...
> > "Nick Roberts" <nickroberts@adaos.worldonline.co.uk> writes:
> >
> > > When given the program:
> > >
> > >    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;
> > >
> > > GNAT 3.12p on Windows 95 returns the error message:
> > >
> > >    function that returns abstract type must be abstract
> > >
> > > Is this a bug in: (a) GNAT; (b) the RM95; or (c) my brain?
> >
> > Same error in gnat 3.14 (you should at least upgrade to 3.13p).
> >
> > RM 8.5.4 (2) says:
> >
> >   2.      subprogram_renaming_declaration ::=
> >              subprogram_specification renames callable_entity_name;
> >
> > The term "callable_entity_name" is not mentioned elsewhere in the
> > reference manual, which I find odd.
> 
> Note that "callable_entity" is in italics, while "name" is not.  In RM
> notation, that means that the language expects the syntax of a "name"
> (defined in 4.1), while "callable_entity" is a description that tells
> you what the name has to represent.  I guess if you're searching a
> text file version of the RM, there's probably no indication that part
> of it is in italics.  "callable entity" is easily found in the index.
> 
> A "callable entity" is defined (in 6(2)) to be a subprogram or entry.
> Thus an abstract subprogram is a callable entity even though you can't
> really call it.
> 
> I don't see any prohibition on renaming an abstract subprogram.
> However, renaming subprograms don't quite take on all the
> characteristics of the subprograms they rename; see 8.5.4(12) for
> instance.  I suspect that has something to do with why renaming an
> abstract subprogram could lead to an error, but I don't have time
> right this minute to look into it further.

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.

--
Mark Biggar
mark.a.biggar@home.com