bug#54156: 28.0.91; set-face-attribute with a nil FRAME doesn't change the default

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: damien@cassou.me,  54156@debbugs.gnu.org
> Date: Sat, 26 Feb 2022 16:04:28 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > So we need a special trick to override defface with 'unspecified', and
> > that trick is this call:
> >
> >   (set-face-attribute 'region t :background 'unspecified)
> >
> > This is handled specially in internal-set-lisp-face-attribute to do
> > what Damien wants.
> 
> So perhaps set-face-attribute should do that automatically when handed a
> nil value?

The fact that we handle nil as 'unspecified' in the case of the color
is for compatibility with Emacs 20.  In Emacs 21 and later, a color
cannot be nil.  This is well-documented: a color must be a string or
'unspecified'.  I don't think we want to resurrect that old convention
of nil; it's just that Damien and others will have to get used to not
using nil for a color.