bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Ergus]

Yes, internally it won't produce any issue (I am using it since I
implemented it the first time), but some external packages will need to
update some of their faces...; and others could remove some of the hacks
they implemented to emulate the functionality this change provides.

Do we have something in defface that we can "recommend" to conditionally
specify this attribute when version >= 27 only (maybe a syntax sugar)?

In any case maybe we need to add a recommendation in NEWS about how to
update.

On Wed, Oct 16, 2019 at 02:42:09PM +0300, Eli Zaretskii wrote:
> > Date: Wed, 16 Oct 2019 13:10:04 +0200
> > From: Ergus <spacibba@aol.com>
> > Cc: Andrey Orst <andreyorst@gmail.com>, 37774@debbugs.gnu.org
> >
> > I have seen these reports and also the ones in reddit. Do you think that
> > we should/must/can do anything about?
>
> Maybe, I'm not yet sure I understand the magnitude of the problem.
> Let's see where this discussion leads us.