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

[Ben Levy]

> Please explain the connection.

Here's a minimal example (sorry I didn't try to do this earlier):

(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".

>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.