bug#68042: 30.0.50; `face-font' returns nil after adding remap relative for fontset

[Hanwen Guo]

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


In GNU Emacs 30.0.50 (build 1, x86_64-w64-mingw32) of 2023-12-11 built
 on MASELLUM-G14
Repository revision: 9434ad25ce2747864e0bcf5665f65eb65a079178
Repository branch: master
Windowing system distributor 'Microsoft Corp.', version 10.0.22631
System Description: Microsoft Windows 10 Pro (v10.0.2009.22631.2861)

Configured using:
 'configure --with-mailutils --without-pop --with-json
 --with-tree-sitter --with-sqlite3 --with-xpm --with-jpeg --with-tiff
 --with-gif --with-png --with-rsvg --with-webp --with-modules
 --without-dbus --without-compress-install --with-native-compilation=aot
 'CFLAGS=-O2 -fno-optimize-sibling-calls''

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY W32NOTIFY PDUMPER PNG RSVG SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XPM ZLIB

Important settings:
  value of $LANG: CHS
  locale-coding-system: cp65001

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  minibuffer-regexp-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils face-remap comp-run
bytecomp byte-compile comp-common rx time-date subr-x cl-loaddefs cl-lib
china-util rmc iso-transl tooltip cconv eldoc paren electric uniquify
ediff-hook vc-hooks lisp-float-type elisp-mode mwheel dos-w32 ls-lisp
disp-table term/w32-win w32-win w32-vars term/common-win tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow
isearch easymenu timer select scroll-bar mouse jit-lock font-lock syntax
font-core term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads w32notify w32 lcms2 multi-tty
move-toolbar make-network-process native-compile emacs)

Memory information:
((conses 16 64497 9228) (symbols 48 6044 0) (strings 32 17178 1453)
 (string-bytes 1 484111) (vectors 16 13625)
 (vector-slots 8 351812 11898) (floats 8 45 37) (intervals 56 347 19)
 (buffers 992 10))