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

[Eli Zaretskii <eliz@gnu.org>]

> Resent-From: Lars Ingebrigtsen <larsi@gnus.org>
> Original-Sender: "Debbugs-submit" <debbugs-submit-bounces@debbugs.gnu.org>
> Resent-CC: bug-gnu-emacs@gnu.org
> Resent-Sender: help-debbugs@gnu.org
> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 25 Feb 2022 13:15:32 +0100
> Cc: 54156@debbugs.gnu.org
> 
> Damien Cassou <damien@cassou.me> writes:
> 
> > I have this in my init.el file:
> >
> > (set-face-attribute 'finsit-javascript-html-tag-face nil :background nil)
> 
> It's perhaps easier to reproduce with a standard face:
> 
> (set-face-attribute 'region nil :background nil)
> 
> Eval this and mark some area and see that it marking a region no longer
> has a background.  `C-x 5 2' and everything is back to normal in the new
> frame.
> 
> However, this works:
> 
> (set-face-attribute 'region nil :background "red")
> 
> So it looks like setting something to nil is forgotten?  I.e., there's
> no difference between nil and "use the default", I'm guessing, but I
> haven't tried to debug further.  Anybody familiar with the code in this
> area?

Does this issue happen only with the nil values of :background?  If
so, may I ask what do you and Damien think should be the effect of
setting the background color of a face to nil?