bug#41793: `comment-only-p' erroneously flags blank lines as comments

[Toby Cubitt]

In GNU Emacs 26.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.16)
 of 2020-04-28

On the following text, with point at |
(comment-only-p (point) (1+ (point)))
returns t when it should probably return nil:

"
|

test"

This impacts commands like `comment-or-uncomment-region'.

This is because the current implementation of `comment-only-p' fails to check 
the return value of (comment-forward):

(defun comment-only-p (beg end)
  "Return non-nil if the text between BEG and END is all comments."
  (save-excursion
    (goto-char beg)
    (comment-forward (point-max))
    (<= end (point))))

The correct implementation should probably be:

(defun comment-only-p (beg end)
  "Return non-nil if the text between BEG and END is all comments."
  (save-excursion
    (goto-char beg)
    (and (comment-forward (point-max))
         (<= end (point)))))

Tested on 26.3, but implementation of `comment-only-p' is unchanged in latest 
git.

Cheers,
Toby
-- 
Dr T. S. Cubitt
Reader (Associate Professor) in Quantum Information
Royal Society University Research Fellow
Department of Computer Science
University College London

email: tsc25@cantab.net
web:   www.dr-qubit.org