bug#39376: [PATCH] Throw error when calling reverse-region on a single line

[Philip K <philip@warpmail.net>]

The current implementation of reverse region will delete everything
following the current line, if the region marks only a part of a
line. This is because the algorithm implemented below accidentally
deletes all following lines, and fails before being able to reinsert
them.

This patch sends an error (could be changed to an user-error) in that
case, as it makes no sense for a single line to be reversed.
---
 lisp/sort.el | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lisp/sort.el b/lisp/sort.el
index 6ea1c44060..8848c5bf24 100644
--- a/lisp/sort.el
+++ b/lisp/sort.el
@@ -543,6 +543,8 @@ reverse-region
   (if (> beg end)
       (let (mid) (setq mid end end beg beg mid)))
   (save-excursion
+    (when (< (line-beginning-position) beg end (line-end-position))
+      (error "Cannot reverse a single line"))
     ;; put beg at the start of a line and end and the end of one --
     ;; the largest possible region which fits this criteria
     (goto-char beg)
-- 
2.20.1