bug#43037: Eval error in the ‘c-lang-defvar’ or ‘c-lang-setvar’

[Alan Mackenzie]

Hello, Eli and AC.

On Fri, Oct 16, 2020 at 13:45:34 +0300, Eli Zaretskii wrote:
> > Date: Fri, 16 Oct 2020 06:14:17 -0400
> > From: AC <achirvasub@gmail.com>
> > Cc: 43037@debbugs.gnu.org

> > Literally seconds ago I compiled emacs-27, started it up with 'emacs
> > -Q' and then visited the blank file '~/new.js'. That error popped
> > up.

> I cannot reproduce this with stock Emacs 27.1, but I can reproduce
> with the current emacs-27 branch.  The more detailed error message is
> this:

>   Eval error in the ‘c-lang-defvar’ or ‘c-lang-setvar’ for
>   ‘font-lock-comment-end-skip’ (fallback source eval - js-mode
>   compiled with CC Mode 5.34 but loaded with 5.34.1): (error
>   "‘c-font-lock-comment-end-skip’ got no (prior) value in js-mode
>   (might be a cyclic reference)")

> Alan, can you take a look, please?

Yes, I think the problem is precisely what the error message said.
js-mode.el was compiled (quite a long time ago) when c-version was still
5.34, but since then c-version has become 5.34.1.  So their is a clash
between what js-mode.el was compiled with and the CC Mode current in the
Emacs version.

To resolve this, I think deleting lisp/progmodes/js-mode.elc and
compiling lisp/progmodes/js-mode.el again should sort out the problem.

This problem has happened more than once.  I think the solution should
be in the Makefile to add a dependency of js-mode.elc on cc-defs.el.

-- 
Alan Mackenzie (Nuremberg, Germany).