bug#49534: 26.3; Isearch should support using filter predicates with empty search hits

[Drew Adams]

(Resending after unarchiving.)

Juri, could you please answer the questions
I asked you here?  Thx.

> > > Your code (in master) removes that boundary test
> > > altogether (no `bobp' or `eobp' test).  Why is
> > > that the right thing?  Is it because the match
> > > should be allowed to match up to `bobp' or `eobp'?
> > > If so, why was that test in isearch.el in the
> > > first place?
> >
> > If you think that you found a problem, please provide
> > a test that exposes it.
> 
> I'm asking why you removed that boundary test.
> And peripherally, why it was there to begin with?

ping.  I'd really like to know.

> > > Actually, you do still test for reaching the
> > > boundary, but only for an empty match and after
> > > filter failure.  Why is that?
> >
> > Please provide a test case if you think there is a problem.
> 
> I'm asking why you test for reaching the boundary,
> and only for an empty match and after a filter
> failure.

ping.  I'd really like to know.
 
> > > And why do you not need to back up a char after
> > > the loop, if the match was empty the last time
> > > around and the next time it fails?  It'll have
> > > advanced a char; should it stay there instead
> > > of backing up?  (Dunno, but I supposed not.)
> >
> > Please provide more tests that confirm your doubts.
> 
> I'm asking why you don't need to back up a char
> after having advanced a char in that case.

ping.  I'd really like to know.