GNU bug report logs - #22763
25.1.50; Feature Request -- A faster method to obtain line number at position.

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From: Eli Zaretskii <eliz <at> gnu.org>
To: Ben Levy <blevy <at> protonmail.com>
Cc: 22763 <at> debbugs.gnu.org
Subject: bug#22763: 25.1.50; Feature Request -- A faster method to obtain line number at position.
Date: Fri, 21 May 2021 08:46:45 +0300

> Date: Thu, 20 May 2021 20:40:31 +0000
> From:  Ben Levy via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
> 
> (with-temp-buffer
>   (insert "foo\nbar\nbaz")
>   (narrow-to-region 4 5)
>   (message "%s" (line-number-at-pos 1 t)))
> 
> On the latest master (ef7a6eec20), this errors with
> 
> Args out of range: 1, 4, 5
> 
> But on emacs-27.2, it prints "1".

This is the expected behavior: the POSITION argument must be in the
accessible portion of the buffer, like every position argument in
Emacs.  A Lisp program which calls this function like above has a bug
that needs to be fixed.

This is not a bug.

> >From what I can tell, this is because the lisp version of line-number-at-pos
> allowed the argument to be outside the visible range, and it widened before
> counting lines to allow this.

Right, the Lisp implementation was incorrect, and failed to do the
test before widening.

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