bug#67683: 30.0.50; [PATCH] Fix 'M-.' on Emacs C sources

[Eli Zaretskii <eliz@gnu.org>]

> From: Manuel Giraud <manuel@ledu-giraud.fr>
> Cc: 67683@debbugs.gnu.org
> Date: Fri, 19 Jan 2024 10:18:04 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Manuel Giraud <manuel@ledu-giraud.fr>
> >> Cc: 67683@debbugs.gnu.org
> >> Date: Thu, 18 Jan 2024 22:10:48 +0100
> >> 
> >> Eli Zaretskii <eliz@gnu.org> writes:
> >> 
> >> [...]
> >> 
> >> >> I'm not using Edebug but since I currently get "(void-function nil)"
> >> >> error I did a 'toggle-debug-on-error' and this error triggers when
> >> >> calling 'tags-included-tables' from 'tags-table-extend-computed-list'
> >> >> 
> >> >> What is really weird is that if I set myself into the TAGS buffer and
> >> >> evaluate (tags-included-tables), it works and returns a list of
> >> >> filenames.
> >> >
> >> > And if you type "C-u M-." in the TAGS buffer, then you can type a
> >> > symbol, and Xref shows its definition allright?  IOW, does M-. work
> >> > from the TAGS buffer?
> >> 
> >> No "C-u M-." does not work from the TAGS buffer with the same error.
> >
> > So then the conclusion is that something in the way
> > tags-included-tables is invoked from tags-table-extend-computed-list
> > (or its callers in the M-. scenario) breaks tags-included-tables, even
> > though invoking tags-included-tables directly works as intended,
> > right?  If so, I suggest to look up the call-stack chain and try to
> > find what is it that breaks tags-included-tables.
> 
> Thanks.  I have found the problem even though I don't what causes it.
> It came from the "lisp/TAGS" buffer: in this one, evaluating
> (tags-included-tables) returns an error.

Maybe that's a bug that we should fix.  Can you try figuring out why
the function fails in that buffer?