bug#67008: 30.0.50; Multiple major mode parents

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> I tried the branch with the attached tentative addition to AUCTeX
> feature/fix-mode-names-overlap branch. I tested two modes, namely latex
> mode and japanese latex mode, with the second attachment to see whether
> the directory local variables stored in the .dir-locals.el are reflected
> or not.

Thanks for testing :-)

> For latex mode, it works as expected. Thank you, it is promising.

Great.

> On the contrary, it fails for japanese latex mode with the error:
> File mode specification error: (error Cycle in the major mode hierarchy: japanese-LaTeX-mode)
> I think the reason is that AUCTeX has
> (defalias 'japanese-latex-mode #'japanese-LaTeX-mode)
> for backward compatibility.

Oh, I see so `japanese-LaTeX-mode` declares `japanese-latex-mode` as an
extra parent but the alias makes it a cycle.
Admittedly, in the current `provided-mode-derived-p` code, we actually
don't signal errors in case of cycles, we just silently break them via
de-duplications, so I just changed my code accordingly.

> This new feature of multiple inheritance would work well for category
> [a], which retains backward compatibility by `major-mode-remap-alist'
> and doesn't use defalias. However, it doesn't work for category [b].

I believe with the code I just pushed it should work fine for [b] as well.

> It seems to me very difficult to arrange the proposed feature to cover
> category [b], looking at its way to handle mode alias.

Maybe there's something I'm missing, but the change was simple and I
believe it does address your use case.


        Stefan