bug#36136: syntax-ppss fails to invalidate its cache on changes to syntax-table text properties

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

The syntax-ppss cache is not invalidated when syntax-table text
properties are set or cleared.  This is because the invalidation
function, syntax-ppss-flush-cache is invoked only as a before-change
function, but typical (?all) syntax-table property changes happen when
before-change-functions is inactive.

This is a bug.

In my debugging of a CC Mode scenario, a buffer change causes a
syntax-table text property change at an earlier part of the buffer.  This
is to do with the change making previously non-matching C++ raw string
identifiers match up.  Font lock follows the syntax-ppss state, which
spuriously records that the end part of the buffer is still in a string.
Hence the non-string part of the buffer is still fontified with
font-lock-string-face.

Suggested fix: the functions set_properties, add_properties,
remove_properties in textprop.c should check for changes to,
specifically, syntax-table properties.  When these changes are detected,
a hook called something like syntax-table-props-change-alert-hook should
be called (with some appropriate position parameters, tbd).
syntax-ppss-flush-cache will be added to this hook.

-- 
Alan Mackenzie (Nuremberg, Germany).