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

[Eli Zaretskii <eliz@gnu.org>]

> 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@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.