bug#41445: 26.3; Query-replace triggers "match data clobbered by..."

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sat, 23 May 2020 13:36:37 +0200
> Cc: ture@turepalsson.se, 41445@debbugs.gnu.org
> 
> > My line of reasoning was that only the callers of ENCODE_FILE and
> > DECODE_FILE will not expect the match-data to be clobbered.  Code that
> > calls ucs-normalize-region directly may or may not be bothered by the
> > clobbering, so we should leave that to the caller.
> > 
> > The advantage of not doing this unconditionally is that we don't
> > unnecessarily punishing callers that don't need match-data to be
> > saved.
> 
> For callers of the ucs-normalize- functions, correctness should come first; 
> their names, semantics or descriptions do not lead the user to suspect them 
> of clobbering the match data. It is an implementation leakage which can be 
> quite unexpected, as is witnessed by this bug.

I thought we had some advice to Lisp programs not to assume that
match-data will be preserved, but maybe I'm misremembering.  Stefan,
do you remember something along these lines?

Anyway, if you feel strongly about doing this on the lowest level, I
won't fight.