We definitely have gotten some useful reports there already since the blog post!

We are aware of a number of sources of false positives and actively trying to drive them down (prioritizing the patterns that are common in our codebase, but very much interested in making the tool useful to others too!).

Some sources of false positives are fundamental (any non-trivial type system will forbid some programs which are otherwise safe in ways that can't be proven statically), others need complex in-development features for the tool to understand (e.g. contracts, such as "foo(...) returns nil iff its third argument is nil"), and some are just a matter of adding a library model or similar small change and we just haven't run into it ourselves.

- You're confident that a flagged value is actually non-Nil?

- A value was Nil but you prefer it that way?