Steps to Reproduce:

Run:

a := [1];
a[b] := (b := 0);
true

Actual Results:

The filter can be evaluated fine with AbuseFilterCachingParser due to https://phabricator.wikimedia.org/diffusion/EABF/browse/master/includes/parser/AbuseFilterCachingParser.php$304. However, it has an unbound id in the original abusefilter.

Expected Results:

I think that the original abusefilter is more well-behaved. In particular, we should try to evaluate things from left to right to keep surprise at minimum.