So it's something along the lines of Erlang / Elixir dialyzer. I don't like it much (dialyzer) because of the verbosity and unDRYness. Every function definition becomes two lines and the name of the function is repeated in both. Type names are longer than they should and are separated from argument names.

We had a discussion about Ruby's type annotations 4 years ago at https://news.ycombinator.com/item?id=9481186

I found it by googling the status of the work on Ruby 3 type annotations. The top comment is mine and I still agree with the past myself.