Subject: Re: bug#61962: 30.0.50; New trouble with symbols with positions

From: Alan Mackenzie <[email-masked]>
Date: Tue, 7 Mar 2023 10:24:41 +0000

Hello, Michael.

On Tue, Mar 07, 2023 at 01:29:21 +0100, Michael Heerdegen wrote:
> Alan Mackenzie <[email-masked]> writes:

> > I think I now understand what's going on.  It's all to do with stripping
> > symbol positions in eval-and-compile forms.  Before the patch of ~two
> > weeks ago, the positions were stripped in e-and-c.  After the patch,
> > they weren't stripped.

> > I think the correct thing to do is to strip the symbol positions in the
> > `eval' part of eval-and-compile, but leave them alone in the `compile'
> > part.  This is actually quite tricky, since
> > byte-run-strip-symbol-positions works destructively.  So I need to copy
> > the code first, and there is no suitable function to do this.  copy-tree
> > is close, but can't handle circular lists.  So I will have to write a
> > safe version of copy tree.

> Sounds all plausible.  I also don't have a better idea.

I've now written safe-copy-tree, and committed it together with the fix
in bytecomp.el to master.  So I'm closing the bug with this post.

> > In the mean time, could you try out the following patch which uses
> > copy-tree as a first approximation.  I think it fixes the problem,
> > apart from the above.

> Yes, looks good.

Thanks!

> I wonder now if other cases also suffer from the problem.  What happens
> when I call `eval' in a macro expander (i.e. while generating the macro
> expansion, not in the result of an expansion)?  And how does
> `cl-eval-when' behave (this is actually a special case of the first
> question) ?

I think these are so far unsolved problems with the
symbols-with-position mechanism - sometimes the s-w-p leaks out of macro
contexts.  Are you seeing this problem in real life?

> Thanks so far,

> Michael.

-- 
Alan Mackenzie (Nuremberg, Germany).