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

[Juri Linkov]

>> > > 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.

All these changes fixed the test case that you presented in the bug report.