Can you elaborate on these incompatibilities, particularly generators? I work on the project and the only generator incompatibility I know of is that what PHP5 calls "Generator" we call "Continuation", but only since we haven't gotten around to renaming ours yet :) I'm also pretty sure HHVM supports 5.6 variadic functions in PHP code, or will very soon; the Hack typechecker currently does not, but that is strictly a missing feature, and something we do want to support.