bug#2499: 23.0.91; syntax-ppss / beginning-of-defun not saving match data

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> >   (i) `beginning-of-defun' is allowed to not save match data. If so,
>> >       please document it.
>> 99.9% of the functions are allowed to not save match data.  
> Oh, I didn't know that! I thought the policy was the other way
> around. (Perhaps 34.6, The Match Data, should make this policy more
> explicit. If so, and you say so, I'll prepare a patch.)

Patch welcome.

>> We do not bother to document it.  We should document the functions
>> that are not allowed to do it, OTOH.
> I think it's still worth to think about making `syntax-ppss' saving the
> match data as this function may easily be implicitly invoked by
> something deep down.

Basically any function that does a non-trivial amount of work should be
expected to affect the match-data.  I.e. it's the responsability of the
match-data user to make sure she reads the match-data soon after the
regexp match took place, rather than responsability of all other
functions to preserve the match-data.


        Stefan