Hi Tim and all,

Thanks for sticking with me here...

Tim Cross <theophilusx@gmail.com> writes:

> I just checked this when running emacs -Q and get the following
>
> comment-line-break-function is a variable defined in ‘simple.el’.
>
> Its value is ‘org-comment-line-break-function’
>
> and fill-prefix is
>
> fill-prefix is a variable defined in ‘simple.el’.
>
> Its value is nil

OK, those are the values I have too...

> and I don't get any error with M-j and cannot reproduce the issue you
> are encountering. .

Do you see an error if you explicitly call

(insert-before-markers-and-inherit nil)

?

Because that is what org-comment-line-break-function does when
fill-prefix is nil. That is the source of the error on all the versions
where I have reproduced it. If you don't see the error then, it would
indicate to me that something in your setup is suppressing it.

> Looking at the git log, I can only find these messages relating to
> default-indent-new-line
> ...
> which indicates the function was added in 2007 by RMS and made the
> default for M-j in 2019. 

Right, which is why I don't see it with M-j in Emacs 26, presumably.
Emacs 26.1, the version installed on my Debian system, was released May
28, 2018.

> my suspicion is that your org version is too old for the current Emacs
> versions... The other possibility is that you have a broken "mixed"
> installation of org.

I'm aware of the difficulties of a mixed installation and have been
careful to avoid them. I run Org from git, usually a recent pull of the
bugfix branch, which I load via use-package from my init file. But I
have also tested it with the built-in Org for various versions of Emacs
with emacs -Q. I do not have Org installed via ELPA.

Just to be extra, super sure, I built Emacs this afternoon from a
checkout of the repo, and the error is *still* there, with the same
cause. In that build, with emacs -Q, I have:

(org-version)
"9.5"

(emacs-version)
"GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo version 1.16.0)
 of 2021-11-30"

At this point I've replicated the bug on my machine in four different
builds of Emacs (version 26.1 from Debian, 27.2 and "emacs-next" from
Guix, and version 29.0.50 I built myself from source) with several
versions of Org (the built-in ones in these Emacsen and a recent build
of the bugfix branch). It is robustly reproducible for me, and the cause
is clear: default-indent-new-line calls org-comment-line-break-function,
which calls

(insert-before-markers-and-inherit nil)

which is a type error. I'm looking for help figuring out what the right
fix is. I attach a patch for the simplest fix I can think of; please let
me know if something else would be better.

-- 
Best,
Richard