It's not that the tyepchecker will miss things, it's that, for folks coming from other languages, it's not clear that there even exists a separate typechecker y

It's not that the tyepchecker will miss things, it's that, for folks coming from other languages, it's not clear that there even exists a separate typechecker you have to run! There isn't a mailing list right now. A few discussions are just in-person since we sit next to each other, but we're trying to make things as transparent as we can. Lots of stuff (and hopefully more moving forward) happen on #hhvm on Freenode or on GitHub issues -- basically the same channels as the HHVM project itself. There is also nontrivial discussion during code review, which is very unfortunately all internal right now; we hope to have that all moved external as soon as we can. (There are a lot of tricky integrations with internal tools that need to happen for that to work.)