[jmmcd]

Yes, of course, we sacrifice accuracy. I said so at the very start. But imagine a "go to definition" tool that works correctly for 99% of cases and barfs or goes to the wrong place on the others. If the alternative is no tool at all, that still sounds pretty useful!

I admit there are cases (refactoring is an example) where if you don't trust the tool 100% you won't use it. But many tools are not like that.

[seanmcdirmid]

Well, you'd be without feedback alot, state of the art in type recovery is nowhere near 99%. Your libraries can exploit the flexibiliy of a dynamic language (meta programming, crazy unions) and make the job much harder. Typescript seems very interesting in the way they handle type retrofits on libraries, but I haven't really absorbed it yet.