> If you replace all instances of :fontset above with :font, fac-font
will not return nil.

Actually, if you pass the fontset through the `:font' attribute, only the ASCII font from the fontset is used. This applies to both `set-face-attribute' and `face-remap-add-relative'. AFAIU, I think when it comes to the face attributes, the fontset feature is more like a way to set different font for diferent character sets, making it able to "display the whole range of characters that Emacs supports" for a given face, since setting the `:family' attribute or assigning a single font to `:font' would suffice for setting font only for ASCII characters. So the case is, either the `:font' attribute needs to consider the full fontset, or the whole fontset feature is, as you said, "obsolete".

________________________________
From: Eli Zaretskii <eliz@gnu.org>
Sent: Wednesday, December 27, 2023 1:49
To: Hanwen Guo <g.hanwen@outlook.com>
Cc: 68042@debbugs.gnu.org <68042@debbugs.gnu.org>
Subject: Re: bug#68042: 30.0.50; `face-font' returns nil after adding remap relative for fontset

tags 68042 notabug
thanks

> From: Hanwen Guo <g.hanwen@outlook.com>
> Date: Tue, 26 Dec 2023 11:47:22 +0000
> msip_labels:
>
> Providing the `:fontset' or `:font' face attribute in the `SPEC'
> argument of `face-remap-add-relative' with `FACE' argument being
> `'default' will make the `face-font' function return nil.
>
> The following Emacs Lisp code can reproduce this behavior under 'emacs
> -Q'.
>
> (create-fontset-from-fontset-spec
>  (font-xlfd-name
>   (font-spec :family "Source Serif"
>              :registry "fontset-variable pitch regular")))
>
> (set-face-attribute 'variable-pitch nil
>                     :family "Source Serif"
>                     :fontset "fontset-variable pitch regular")
>
> (let ((var-pitch (face-attribute 'variable-pitch :family))
>       (var-fontset (face-attribute 'variable-pitch :fontset)))
>   (setq mixed-pitch-variable-cookie
>         (face-remap-add-relative 'default :family var-pitch :fontset var-fontset)))
>
> (face-font 'default) ;; nil
>
> (face-remap-remove-relative mixed-pitch-variable-cookie)
>
> (face-font 'default) ;; non-nil

Don't use :fontset as a face attribute; use :font instead.  The value
of :font can be a fontset, so you don't need to use the (obsolete and
not really supported) :fontset attribute.  The doc string of
set-face-attribute documents :font, not :fontset, and that is not an
accident.

If you replace all instances of :fontset above with :font, fac-font
will not return nil.

This is not a bug.