Subject: bug#35600: [Patch]: Re: bug#35600: 27.0.50; uncomment-region fails correctly to reverse the action of comment-region.

From: Alan Mackenzie <[email-masked]>
Date: Tue, 7 May 2019 08:59:53 +0000

Hello, Emacs.

On Mon, May 06, 2019 at 10:11:00 +0000, Alan Mackenzie wrote:

> In the master branch:

> With indent-tabs-mode t, and tab-width (say) 8, start off with the
> following two line of Emacs lisp:

>     (if (null bs-state)
> 	(ignore)
>     ^
>   <tab>

> (the first of these lines being indented by four spaces, the second by a
> <tab>).

> Mark the two lines, and execute M-x comment-region.  We get this:

>     ;; (if (null bs-state)
>     ;;  (ignore)
>        ^
>      <tab>

> (where all the space is space characters except for the <tab> just
> before "(ignore)").

> Now execute M-x uncomment-region.  This should restore the region to
> what it started as.  Instead one gets:

>     (if (null bs-state)
>     	(ignore)
> ^^^^  ^
> SPCs <tab>

> , there now being four spurious spaces before the <tab> preceding
> "(ignore)".  This is a bug.

> ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

> In uncomment-region-default-1, the variable `spt' gets set to the result
> of comment-search-forward, the ";" at the beginning of the comment on
> the current line.  The function then narrows to between `spt' and BO
> next line.

> In this narrowed region, the function removes the comment starter.
> Because of the narrowing, it cannot take any account of the spaces at
> the start of the line.

> Possibly, uncomment-region needs to be amended so that that narrowing
> includes any space preceding the comment starter, and that space gets
> properly handled.

Here is a patch which fixes the bug a little differently.  After removing
a comment starter, if there are obtrusive spaces left before a <tab>,
they get removed:

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 9d919ccbbe..ab2be080a3 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -1001,7 +1001,15 @@ uncomment-region-default-1
 		       (re-search-forward sre (line-end-position) t))
 		(replace-match "" t t nil (if (match-end 2) 2 1)))))
 	  ;; Go to the end for the next comment.
-	  (goto-char (point-max))))))
+	  (goto-char (point-max)))
+        ;; Remove any obtrusive spaces left preceding a tab at `spt'.
+        (when (and (eq (char-after spt) ?\t) (eq (char-before spt) ? )
+                   (> tab-width 0))
+          (save-excursion
+            (goto-char spt)
+            (let* ((fcol (current-column))
+                   (slim (- (point) (mod fcol tab-width))))
+              (delete-char (- (skip-chars-backward " " slim)))))))))
   (set-marker end nil))
 
 (defun uncomment-region-default (beg end &optional arg)

-- 
Alan Mackenzie (Nuremberg, Germany).