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

[Eli Zaretskii <eliz@gnu.org>]

> 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.