Fix recently extended delete-indentation behavior

* doc/lispref/text.texi (User-Level Deletion): Document new optional
arguments of delete-indentation.

* lisp/simple.el (delete-indentation): Do not barf if called
interactively when region is inactive. (bug#35021)
Do not skip blank lines. (bug#35036)
Consistently deactivate mark even when no text was changed.
Handle active region spanning a single line.

* test/lisp/simple-tests.el (simple-test--buffer-substrings):
New convenience function.
(simple-test--dummy-buffer, simple-test--transpositions): Use it.
(simple-delete-indentation-no-region)
(simple-delete-indentation-inactive-region): Update commentary.
Call delete-indentation interactively when testing for behavior with
inactive region and region is not explicitly defined.
(simple-delete-indentation-blank-line)
(simple-delete-indentation-boundaries)
(simple-delete-indentation-region)
(simple-delete-indentation-prefix): New tests.

1 files changed, 31 insertions, 23 deletions

diff --git a/lisp/simple.el b/lisp/simple.el
index f76f31ad146..306df967661 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -598,30 +598,38 @@ When called from Lisp code, ARG may be a prefix string to copy."
 If there is a fill prefix, delete it from the beginning of this
 line.
 With prefix ARG, join the current line to the following line.
-If the region is active, join all the lines in the region.  (The
-region is ignored if prefix argument is given.)"
-  (interactive "*P\nr")
-  (if arg (forward-line 1)
-    (if (use-region-p)
-        (goto-char end)))
-  (beginning-of-line)
-  (while (eq (preceding-char) ?\n)
-    (progn
-      (delete-region (point) (1- (point)))
-      ;; If the second line started with the fill prefix,
+When BEG and END are non-nil, join all lines in the region they
+define.  Interactively, BEG and END are, respectively, the start
+and end of the region if it is active, else nil.  (The region is
+ignored if prefix ARG is given.)"
+  (interactive
+   (progn (barf-if-buffer-read-only)
+          (cons current-prefix-arg
+                (and (use-region-p)
+                     (list (region-beginning) (region-end))))))
+  ;; Consistently deactivate mark even when no text is changed.
+  (setq deactivate-mark t)
+  (if (and beg (not arg))
+      ;; Region is active.  Go to END, but only if region spans
+      ;; multiple lines.
+      (and (goto-char beg)
+           (> end (line-end-position))
+           (goto-char end))
+    ;; Region is inactive.  Set a loop sentinel
+    ;; (subtracting 1 in order to compare less than BOB).
+    (setq beg (1- (line-beginning-position (and arg 2))))
+    (when arg (forward-line)))
+  (let ((prefix (and (> (length fill-prefix) 0)
+                     (regexp-quote fill-prefix))))
+    (while (and (> (line-beginning-position) beg)
+                (forward-line 0)
+                (= (preceding-char) ?\n))
+      (delete-char -1)
+      ;; If the appended line started with the fill prefix,
       ;; delete the prefix.
-      (if (and fill-prefix
-               (<= (+ (point) (length fill-prefix)) (point-max))
-               (string= fill-prefix
-                        (buffer-substring (point)
-                                          (+ (point) (length fill-prefix)))))
-          (delete-region (point) (+ (point) (length fill-prefix))))
-      (fixup-whitespace)
-      (if (and (use-region-p)
-               beg
-               (not arg)
-               (< beg (point-at-bol)))
-          (beginning-of-line)))))
+      (if (and prefix (looking-at prefix))
+          (replace-match "" t t))
+      (fixup-whitespace))))
 
 (defalias 'join-line #'delete-indentation) ; easier to find