emacs-27 071483b: Fix reverting customizations

[Eli Zaretskii]

branch: emacs-27
commit 071483b6f516b737182440264ec11c94ada53dc3
Author: Eli Zaretskii <address@hidden>
Commit: Eli Zaretskii <address@hidden>

    Fix reverting customizations
    
    * lisp/custom.el (custom-push-theme): Don't use setcar to modify
    the recorded value of PROP; instead, cons a new property list by
    deleting the old value and adding the new one.  (Bug#38812)
---
 lisp/custom.el | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lisp/custom.el b/lisp/custom.el
index ba7f999..037f6c5 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -886,7 +886,10 @@ See `custom-known-themes' for a list of known themes."
        (put theme 'theme-settings
             (cons (list prop symbol theme value)
                   (delq res theme-settings)))
-       (setcar (cdr setting) value)))
+        ;; It's tempting to use setcar here, but that could
+        ;; inadvertently modify other properties in SYMBOL's proplist,
+        ;; if those just happen to share elements with the value of PROP.
+        (put symbol prop (cons (list theme value) (delq setting old)))))
      ;; Add a new setting:
      (t
       (when (custom--should-apply-setting theme)