Feature

(there's probably already an issue about this...) There's a variety of obvious issues that cause false negatives/positives:

• escaped typevars (e.g. def f(x) -> T)
• unchecked code that isn't unreachable
• probably other things

I think we should add a flag which checks for those things.

Pitch

This would make it incredibly easy to find & eliminate (well, hopefully) wide classes of bugs!