Subject: bug#71277: 30.0.50; last-prefix-arg lost when universal-argument used

From: Sean Whitton <[email-masked]>
Date: Thu, 30 May 2024 12:47:29 +0100

Hello,

Consider

    (defun counter ()
      (interactive)
      (when (eq last-command this-command)
	(setq current-prefix-arg
	      (+ (prefix-numeric-value current-prefix-arg)
		 (prefix-numeric-value last-prefix-arg))))
      (message "%s!" (prefix-numeric-value current-prefix-arg)))
    (global-set-key "\M-." #'counter)

If you type M-. M-. M-. then you see "3!" as expected.

Similarly if you type "C-u M-. M-. M-." then you see "6!".

However, if you type "M-. M-. C-u M-." then you get "5!".

Unlses something is wrong with my arithmetic, this is not correct.
You should get "6!" after the third example too.
The use of C-u clobbers last-prefix-arg, basically.

I believe this will fix it:

-- >8 --
Subject: [PATCH] universal-argument--preserve: Preserve last-prefix-arg

* lisp/simple.el (universal-argument--preserve): Set
current-prefix-arg to last-prefix-arg in order to preserve
last-prefix-arg, too.
---
 lisp/simple.el | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lisp/simple.el b/lisp/simple.el
index 44197c3189a..76fb81c9df5 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -5414,7 +5414,8 @@ universal-argument--description
 (add-hook 'prefix-command-preserve-state-hook
           #'universal-argument--preserve)
 (defun universal-argument--preserve ()
-  (setq prefix-arg current-prefix-arg))
+  (setq prefix-arg current-prefix-arg)
+  (setq current-prefix-arg last-prefix-arg))
 
 (defvar universal-argument-map
   (let ((map (make-sparse-keymap))
-- 
Sean Whitton