bug#71819: Incorrect substitution when undo replacing regexps

[Eli Zaretskii <eliz@gnu.org>]

Ping!  Tino, can you please look into this issue?

> From: Juri Linkov <juri@linkov.net>
> Cc: Tino Calancha <tino.calancha@gmail.com>,  Sab Pyrope
>  <sab.pyrope@gmail.com>,  71819@debbugs.gnu.org
> Date: Sun, 30 Jun 2024 19:25:11 +0300
> 
> >> I encountered a bug when replacing regular expressions.
> >> If you [u]ndo a previous change during the replacement process, subsequent changes to the \2 substitution
> >> will use the old undone result.
> >>
> >> Text:
> >> ---
> >> Aaaaa. Bbbb.
> >> Ccccc. Dddd.
> >> Eeeee. Ffff.
> >> Ggggg. Hhhh.
> >> ---
> >> Input regexp: \([.]\) \([A-Z]\)
> >> Output regexp: \1  \2
> >> Bug:
> >> 1. Copy the text above to the buffer.
> >> 2. Enter the regular expressions to the interactive regexp replacement [C-M-%].
> >> 3. Click [y] twice until the Ffff is highlighted.
> >> 4. Cancel the previous replacement via [u].
> >> 5. Continue replacing [y] until the end.
> >> 6. Notice that all sentences except the first and second now begin with an incorrect letter D, which is not
> >> implied by the entered regexp, it should be F and H, as if it had been replaced, but without being undoed in
> >> the middle of the process.
> >> Output:
> >> ---
> >> Aaaaa.  Bbbb.
> >> Ccccc.  Dddd.
> >> Eeeee.  Dfff.
> >> Ggggg.  Dhhh.
> >> ---
> >
> > This is a regression between Emacs 26 and Emacs 27, perhaps as a side
> > effect of fixing bug#36328.  Juri, could you please take a look?
> 
> This is caused by the commit 30c4f35a6fc8 that introduced these lines:
> 
>   (when regexp-flag
>     (setq next-replacement (nth 4 elt)))
> 
> Maybe Tino knows how to fix this (Cc'ed).
>