bug#23675: 30.0.50: make count-lines optionally ignore invisible lines

[J.P.]

Closed in 2020 (Emacs 28).

Perhaps I'm not understanding how the `ignore-invisible-lines' parameter
for `count-lines' is supposed to work, but it doesn't seem to make the
function consider `invisible' text properties that have lists as values.
I bring this up because ERC will likely be needing a line-counting
function that's list-aware, at least in the manner shown in the attached
tests. The change to `count-lines' accompanying these tests was merely
lifted from `forward-visible-line' to make them pass, but it's quite
possibly flawed and/or incomplete. If a proper solution ever emerges to
address this, hopefully it'll come at the hands of someone better
informed than I in the ways of Emacs invisibility. In the meantime
(2023), ERC will likely be doing its own subpar rendition unless someone
takes up the challenge for 30.1 (and Compat agrees to adopt it). Thanks.

0001-POC-Honor-invisible-text-prop-list-values-in-count-l.patch
Description: Text Data