I think we should jump forward to the latest Javassist (3.20.0), so that we have the recent bug fixes.

Once we upgrade to 3.20.0, should we stabilize on a 3.20.x branch? I'm currently helping to maintain the 3.18 branch (no plans to release any further 3.18.x release at this point). We could create a 3.20 branch and cherry pick fixes (from master) if you all want me to.