bug#60467: 30.0.50; primitive-undo: Changes to be undone by function different from announced

[Gregory Heytings]

> > Patch attached.
>
> Hmm... scary, even if this is intended for the master branch.  Could you 
> at least keep the few comments which were in the original code, and 
> perhaps also add comments explaining all those tricky tests with consp, 
> equality to t and to old-bul, the significance of (car ptr) and (caar 
> ptr), etc.?  This would make the code more maintainer-friendly, as the 
> need to constantly consult the spec of the various undo-list elements 
> (and hope the spec is complete and accurate ;-) would be avoided.

Is the attached patch better in that respect?

> I'd prefer to fix this in Emacs 29 if feasible, but unless the changes 
> are much simpler and/or more clearly documented, so that it becomes 
> possible to assess their risks, I don't see how that could happen.

I don't see how the changes could be made simpler, sorry, but now they are 
documented.  Feel free to ask further questions if something is still 
unclear.

> I presume that all the undo tests we have still pass?

They do.

> If so, how about adding a test for this bug?

I'll see if I can do that, but it seems tricky.

Fix-combine-change-call.patch
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