bug#11410: 24.1.50; package.el `emacs' package information is broken

[Toby Cubitt]

On Sat, May 05, 2012 at 08:37:14AM -0400, Stefan Monnier wrote:
> > The Elisp manual "Conventional Headers for Emacs Libraries" node says:
> >     The package code automatically defines a package named `emacs'
> >     with the version number of the currently running Emacs.  This can
> >     be used to require a minimal version of Emacs for a package.
> 
> BTW I highly doubt the usefulness of such a dependency.  Package.el can
> use dependencies to automatically download&update needed packages, but
> it currently can't do that for `emacs', and in general it's can't do
> that at all (the same set of installed packages might even be used by
> different versions of Emacs).
> 
> So such a dependency makes more sense as a load-time test (written in
> Elisp directly in the package's code) rather than as
> a package.el dependency.

Surely the idea is for packages to refuse to even *install* under Emacs
versions where they won't work (package.el displays an "emacs x.x not
available" in this case.)

As I understand it, with a load-time test package.el will still downland
and install the package successfully, even though the package won't
work. With the `emacs' version dependency, install will instead fail with
an informative message.

That seems like a useful feature to me.

(Even more useful when the package in question is actually being
installed as part of a chain of dependencies.)

Toby
-- 
Dr T. S. Cubitt
Mathematics and Quantum Information group
Department of Mathematics
Complutense University
Madrid, Spain

email: tsc25@cantab.net
web:   www.dr-qubit.org