I think it's mostly in HHVM, which is publicly available. It's not documented, but you can see it in the tests. E.g., https://github.com/facebook/hhvm/blob/ma

I think it's mostly in HHVM, which is publicly available. It's not documented, but you can see it in the tests. E.g., https://github.com/facebook/hhvm/blob/master/hphp/test/quick/hh_type_var.php https://github.com/facebook/hhvm/blob/master/hphp/test/quick...