1) Great question! Yes, it is super-common. Most of our type inferences on local variables are actually of form "Null-or-X". So we can emit code that skepticall

1) Great question! Yes, it is super-common. Most of our type inferences on local variables are actually of form "Null-or-X". So we can emit code that skeptically checks for Null, but then goes ahead and assumes X otherwise. 2) I'm not claiming we have all the answers here; but decent compile times has been a strong evolutionary pressure all along. 3) Yeah, we basically implement the "slightly relaxed" semantics described in that paper, and are addicted to the performance wins from doing so. WRT loosening semantics, though, in general you'd be surprised how close we come to quirk-for-quirk compatibility. The problem is that real PHP programs, including ours, end up depending, often unintentionally, on those quirks. I.e., nobody meant to rely on the order function params are evaluated in, but in practice we rely on it because param 0's evaluation has some side effect that param 1's evaluation needs to run correctly...