http://tracker.phpbb.com/browse/PHPBB3-7580
This test demonstrates that is possible to test if a
variable exists and then use it if it does, similar to
using isset(), but in template/twig syntax.
<!— IF {VAR} is defined —>{VAR}<!— ENDIF —>
PHPBB3-7580
compile_tag_if had the flawed approach of adding an isset statement for
all variables to the beginning of the if. This fails for negative
expressions, and checking those takes a considerable effort.
The easier solution is to make the variable expression itself
conditional, defaulting to null if it is not set.
Thanks to naderman for the solution.
PHPBB3-9726
