Re: beginning-of-defun-comments bug [was: Re: 26.0.90: mark-defun problem in c-mode]

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Dec 30, 2017 at 14:53:27 +0200, Eli Zaretskii wrote:
> > Date: Sat, 30 Dec 2017 12:01:36 +0000
> > Cc: emacs-devel@gnu.org, zhang cc <ccsmile2008@outlook.com>,
> >   Noam Postavsky <npostavs@gmail.com>
> > From: Alan Mackenzie <acm@muc.de>

> > > I see the error, with point anywhere in the body of the second function.
> > > Critical seems to be there being no blank line between the functions.

> > > I think there's a bug in beginning-of-defun-comments, which I'm in the
> > > middle of edebugging.  It moves point into the first function.

> > beginning-of-defun-comments has a bug.  On doing M-x
> > beginning-of-defun-comments from the inside of a function, when there's
> > no blank lines between it and the previous function, point ends up
> > inside that previous function, not at the comments which may separate
> > them.

> > Perhaps this bug should be fixed before the next Emacs-26 pretest.

> How old is this problem?  It looks like it's new in Emacs 26?  If so,
> we should try fixing it on the release branch.

beginning-of-defun-comments came into existence on 2017-03-31, so this
problem is definitely new in Emacs 26.

I've diagnosed the bug.  At one place, it is necessary to scan a line of
text from BOL to detect any non-comment/non-space character.  The
current code tries to do this by using parse-partial-sexp with the
fourth argument STOPBEFORE non-nil.

This STOPBEFORE causes the scanning to stop at any character which
begins a sexp.  A closing brace doesn't fit into this category.  The
scan therefore reaches EOL, and the code therefore falsely assumes there
are no non-syntactic-ws characters on that line.

I'm sure I can fix this today.

-- 
Alan Mackenzie (Nuremberg, Germany).