bug#72759: 31.0.50; Emacs hangs with open-paren-in-column-0-is-defun-start set to nil

[Eli Zaretskii <eliz@gnu.org>]

> From: Eshel Yaron <me@eshelyaron.com>
> Cc: 72759@debbugs.gnu.org
> Date: Thu, 22 Aug 2024 14:37:01 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Would it make sense to have checkdoc-next-docstring bind
> > open-paren-in-column-0-is-defun-start to a non-nil value?
> 
> Not entirely: the difference in behavior of beginning-of-defun is
> unexpected, and may prove problematic in other cases as well, so it
> would be best to fix the root cause IMO.  Namely, in
> beginning-of-defun-raw, the fallback cond clause fails to move forward
> over an sexp that does starts after the beginning of the line.

How do you propose to fix the root cause, when Emacs fails to
correctly identify the beginning of a defun?  Suppose we'd want to fix
the condition of this loop:

>> (while (and (not (setq found (checkdoc--next-docstring)))
>>                 (beginning-of-defun -1)))

so it doesn't infloop -- how would you go about that, when
beginning-of-defun doesn't move and you are not at BOB?

> You should also be able to see this in effect with C-- C-M-a at the
> beginning of a line whose contents is " (foo)".  The results vary
> depending on the value of open-paren-in-column-0-is-defun-start.

Exactly!  When beginning-of-defun is confused, what are its callers
supposed to do?  They don't know better where the beginning of the
previous defun is.

Or what am I missing?