bug#63564: 29.0.91; (setcdr) behaves differently between natively and byte compiled code

[Eli Zaretskii <eliz@gnu.org>]

> From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
> Date: Thu, 18 May 2023 03:40:40 +0100
> 
> 
> Users have discovered there's a markdown-mode function that behaves
> differently depending Emacs is executing byte-compiled code or natively
> compiled code.
> 
> The issue is documented
> [here](https://github.com/jrblevin/markdown-mode/issues/578).
> 
> 
> There are two examples in the issue that will produce an `Wrong type
> argument: consp, nil` error on the natively compiled version of
> `markdown-imenu-create-nested-index`, but not the byte-compiled or
> interpreted version.  A user has provided a disassembly of the natively
> compiled code for that function.
> 
> The last user has said and I can confirm the offending line seems to be `(setcdr
> sibling-alist alist)` in that function.
> 
> Much appreciate it if Andrea could take a look.

Adding Andrea.

While, of course, Andrea's help will be appreciated, there's currently
no reason to believe this is a problem in the Emacs core, and
therefore filing a bug report here could be premature.  Ideally, the
markdown-mode's developers should examine the problem first and
present convincing evidence that this is a problem with native
compilation and not with the code in markdown-mode itself.