OK, I've finally found + fixed the problem.

During the search I discovered more cases where the query generator did not produce correct results. And the search method for anydbm (Proptree in hyperdb) was originally built on the assumption that an empty list [] would not produce any results when searching for links and multilinks. This has been changed without considering the semantics and also needed fixing (originally searching for empty Link/Multilink was done with the value '-1'). Due to similar reasons the expression parser interacted with the search method of Proptree when in a subtree results need to be combined.

These two cases involve fairly rarely occurring use-cases like searching for two different attributes (say name and order) of the same transitive property, so this is probably the reason it was not found earlier.

Another area littered with bugs was the interaction with newer features like reverse multilinks: this also was never tested with the multilink expression feature.

And finally I'm allowing '-1' in expressions. E.g. we can now express "not empty" with ['-1', '-2'] ('-1' standing for empty and '-2' being the postfix operator for negation) searching e.g. for all issues that have any keyword (no matter which keyword).

Note that the multilink expression feature probably has never worked all those years with mysql. It was comparing strings (IDs are strings in the database) with integers and did never find anything.

I'm quite happy that the technical dept in that part of the code was finally cleaned up.

All existing tests pass and I've created a bunch of new tests that also pass for me. Lets see if the CI test also passes those tests.