hide-lines-matching fails for emacs 27.1

[glendeni]

In moving from Ubuntu 14.04 (emacs version 23.4.1) to Ubuntu 22.04 (emacs version 27.1), I find hide-lines fails (using the same .emacs file as before) for

(hide-lines-matching "^ *#[^ #A-Z0-9]")

with

'Wrong type argument: listp, t'

in function hide-lines-add-overlay`- apparently coming from the sequence

(member 'hl buffer-invisibility-spec)

Am over my head trying to debug this, so must simply report and hope someone more knowledgable
than I can fix this, since functionality very useful to me

[sje30]

I got the same problem with Emacs 29 pretest.

My fix was to change the following defun slightly

(defun hide-lines-add-overlay (start end)
  "Add an overlay from `START' to `END' in the current buffer.
Push the overlay onto the `hide-lines-invisible-areas' list"
  (unless (hide-lines-invisible-p start end)
    (let ((overlay (make-overlay start end)))
      (push overlay hide-lines-invisible-areas)
      (unless (and (listp buffer-invisibility-spec)
		   (member 'hl buffer-invisibility-spec))
	(add-to-invisibility-spec 'hl))
      (overlay-put overlay 'invisible 'hl))))

by adding the extra check that (list buffer-invisibility-spec) returns true. The problem from what I could see is that the variable starts off as t, rather than a list.

happy to send a PR if this is indeed the problem and right fix.

[glendeni]

That fixes my problem (had been entering backtrace when trying to use hide-show)! Now hide-show works for me. Thanks! Dunno what PR is, assume that is report to fix problem, if so please do.

[sje30]

thanks. A PR is a pull-request to fix the code, and someone else I see already fixed it before I did. See #7