[Lio]

That's just documentation tool but the method is actually already statically typed by ruby.

I'm not sure why they don't link that in the documenation but I beleive it is something the RDoc mainer wants to do.

  def lines: (?string? separator, ?chomp: boolish) -> Array[String]
           | (?string? separator, ?chomp: boolish) { (String line) -> void } -> self

https://github.com/ruby/rbs/blob/master/core/string.rbs