From d7fda971f1c09c6e3809447361fc83e60cefb20f Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Thu, 5 Mar 2026 16:14:43 +0000
Subject: [PATCH] unfill-paragraph: Reimplement in terms of fill-region

* lisp/textmodes/fill.el (unfill-paragraph): Reimplement in
terms of fill-region.
---
 lisp/textmodes/fill.el | 58 +++++++++++-------------------------------
 1 file changed, 15 insertions(+), 43 deletions(-)

diff --git a/lisp/textmodes/fill.el b/lisp/textmodes/fill.el
index a64bf7d36f4..0e5fdaad864 100644
--- a/lisp/textmodes/fill.el
+++ b/lisp/textmodes/fill.el
@@ -945,14 +945,9 @@ region, instead of just filling the current paragraph."
 
 (defun unfill-paragraph (arg &optional beg end)
   "Join lines of this paragraph and fix up whitespace at joins.
-If there is a fill prefix, delete it from the beginning of lines before
-joining them.
-Interactively, if the region is active, join lines of each paragraph wholly
-contained within the region.  This means that if the region begins or ends
-within a paragraph, treat the partial paragraph that's within the region as
-the whole paragraph whose lines are to be joined.
-A numeric prefix argument means join the lines of the following ARG
-paragraphs.  In this case an active region is ignored.
+Interactively, if the region is active, join lines of each paragraph in
+the region.  A numeric prefix argument means join the lines of the
+following ARG paragraphs.  In this case an active region is ignored.
 
 With an active region and no prefix argument this is roughly the same as
 `delete-indentation' with that active region, except that this command
@@ -964,41 +959,18 @@ lines within, or if ARG is nil, optional arguments BEG and END non-nil
 means to join the lines of each paragraph in the region delimited by BEG
 and END."
   (interactive "P\nR")
-  (let ((endm (make-marker)))
-    (unwind-protect
-	(cl-flet ((do-unfill ()
-		    (set-marker endm (point))
-		    (fill-forward-paragraph -1)
-		    ;; Adjust BEG and ENDM by 1 because we want to
-		    ;; preserve the newlines before and after the
-		    ;; paragraph.
-		    (delete-indentation nil (1+ (point)) (1- endm))
-		    (goto-char endm)))
-	  (save-excursion
-	    (if (or arg (not beg) (not end))
-		;; Do it ARG times or until we run out of paragraphs.
-		(let ((n (prefix-numeric-value arg))
-		      (i 0))
-		  (while (and (< i n)
-			      (zerop (fill-forward-paragraph 1)))
-		    (do-unfill)
-		    (incf i)))
-	      ;; Ensure there are newlines at beginning and end of
-	      ;; region.  This is how we handle a region starting or
-	      ;; ending within a paragraph.
-	      (goto-char beg)
-	      (unless (or (bolp) (eolp))
-		(open-line 1)
-		(setq beg (point)))
-	      (goto-char end)
-	      (unless (or (bolp) (eolp))
-		(newline 1)
-		(setq end (point)))
-	      (with-restriction beg end
-		(goto-char beg)
-		(while (zerop (fill-forward-paragraph 1))
-		  (do-unfill))))))
-      (set-marker endm nil))))
+  (when (or arg (not beg))
+    (let ((arg (prefix-numeric-value arg)))
+      (when (zerop arg)
+        (user-error "Invalid numeric argument to `unfill-paragraph'"))
+      (save-excursion
+        (fill-forward-paragraph 1)
+        (fill-forward-paragraph -1)
+        (setq beg (point))
+        (fill-forward-paragraph arg)
+        (setq end (point)))))
+  (let ((fill-column (point-max)))
+    (fill-region beg end)))
 
 (declare-function comment-search-forward "newcomment" (limit &optional noerror))
 (declare-function comment-string-strip "newcomment" (str beforep afterp))