bug#23949: 25.0.95; Regression in handling error caused by (string-match-p "." nil)

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: kaushal.modi@gmail.com,  schwab@suse.de,  23949@debbugs.gnu.org
> Date: Wed, 13 Jul 2016 12:00:40 -0400
> 
> >> > I also think that the "breaks a lot of Elisp code" part is at least a
> >> > tad exaggerated.
> >> Binding inhibit-changing-match-data to t will pretty much break any
> >> function that uses match-beginning or match-end.
> > But those functions aren't supposed to run when string-match is
> > called.
> 
> Yet they are in bug#23949.

No, they aren't.  They run from the debugger.

> >> > (defsubst string-match-p (regexp string &optional start)
> >> >   "\
> >> > Same as `string-match' except this function does not change the match 
> >> > data."
> >> >   (condition-case err
> >> >       (let ((inhibit-changing-match-data t))
> >> >  (string-match regexp string start))
> >> >     (error (signal (car err) (cdr err)))))
> >> That will still cause the same problems when debug-on-signal is non-nil.
> > So you don't consider this an improvement that should be installed?
> 
> No.

What about the suggestion made by Andreas?

> A simpler and more robust solution would be
> (save-match-data (string-match regexp string start))
> 
> Of course, with either solution, it means that string-match-p is even
> worse in terms of efficiency, whereas the unsuspecting coder would
> rightfully expect string-match-p to be (slightly) *more* efficient than
> string-match.

Exactly.  And we are punishing the innocent (the calls that don't
signal an error) for fear of the guilty (those that do).