bug#61098: 29.0.60; Confusing behavior of show-paren-mode

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: rahguzar@zohomail.eu,  61098@debbugs.gnu.org,  dgutov@yandex.ru
> Date: Sun, 05 Feb 2023 10:19:24 -0500
> 
> > I guess I don't understand what show-paren-local-mode does, then.  Let
> > me explain my line of reasoning:
> >
> >   (define-minor-mode show-paren-local-mode
> >     "Toggle `show-paren-mode' only in this buffer."
> >     :variable ( show-paren-mode .
> > 		(lambda (val) (setq-local show-paren-mode val)))
> >     (cond
> >      ((eq show-paren-mode (default-value 'show-paren-mode))
> >       (unless show-paren-mode
> > 	(show-paren--delete-overlays))
> >       (kill-local-variable 'show-paren-mode))
> >      ((not (default-value 'show-paren-mode))
> >       ;; Locally enabled, but globally disabled.
> >       (show-paren-mode 1)                ; Setup the timer.
> >       (setq-default show-paren-mode nil) ; But keep it globally disabled.
> >       )
> >      (t ;; Locally disabled only.
> >       (show-paren--delete-overlays))))
> >
> > When I looked at the above function and stepped through it in the OP's
> > scenario, what I saw was that the 2nd condition of 'cond' doesn't
> > fire, because the default-value of show-paren-mode is t nowadays.  So
> > I concluded that this code relies on the mode being globally off by
> > default, and attempted to fix the condition that turns the mode on
> > locally even though it is on globally.
> 
> But if the mode is already globally enabled, the (setq-default show-paren-mode nil)
> will definitely not be right: you'd end up disabling it globally (while
> maybe keeping it enabled locally).
> 
> > The predicate used by the timer requires either a match of the buffer
> > (which doesn't happen in this case) or that the local value of
> > show-paren-mode is non-nil.  Setting the local value non-nil is the
> > job of the mode function, isn't it?
> 
> Yes, but the first branch of the `cond` incorrectly does
> `kill-local-variable` under the assumption that the local value doesn't
> matter if it's the same as the global one, but that's not true here
> because (as a wise man said):
> 
>    The predicate used by the timer requires either a match of the buffer
>    (which doesn't happen in this case) or that the local value of
>    show-paren-mode is non-nil.

This answers only half of my questions, but if you and Dmitry think
Dmitry's patch will solve this problem, then Dmitry, please install it
and close this bug.

Thanks.