FYI: The RubyKaigi 2019 Progress Report on Ruby 3 Talk Slides have more (from the source) info. See the slides titled "Static Analysis" [1]

Ruby 3 static analysis will have four items:

1. Type signature format 2. Level-1 type checking tool 3. Type signature profiling / prototyping tool 4. Level-2 type checking tools

and so on. [1]: https://docs.google.com/presentation/d/1z_5JT0-MJySGn6UGrtda...