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

[Eli Zaretskii]

> From: Kévin Le Gouguec <kevin.legouguec@gmail.com>
> Cc: Ergus <spacibba@aol.com>,  Eli Zaretskii <eliz@gnu.org>,
>   jonas@bernoul.li,  37774@debbugs.gnu.org
> Date: Thu, 05 Dec 2019 17:48:22 +0100
> 
> I think there might have been a regression since this commit: in-between
> 1c29ba0340 (2019-11-17) and 21790e5473 (2019-12-05), something caused
> the backgrounds to no longer extend for faces with unspecified :extend
> inheriting faces with :extend t.

Right.

> (custom-theme-set-faces
>  'extend-inherit
>  '(default ((t (:background "black" :foreground "gainsboro"))))
>  '(highlight ((t (:background "gray10"))))
>  '(diff-added ((t (:background "#12222f" :extend t))))
>  '(diff-removed ((t (:background "#2f1e00" :extend t))))
>  '(diff-refine-added ((t (:background "#1b3347"))))
>  '(diff-refine-removed ((t (:background "#472e00"))))
>  '(ediff-current-diff-A ((t (:inherit (diff-removed)))))

Why are you using values for :inherit that are lists of one element?
Why not just ":inherit FACE"?

But yes, this is a bug: the value of :inherit can legitimately be a
list.  Should be fixed now, thanks.