Some of the highlights in RBS 3.6 are:

• Generics improvements (#1994)

You can install it with $ gem install rbs or using Bundler.

gem 'rbs', '~> 3.6.0'

Read the CHANGELOG for the details.

The upper bounds can be any type, not limited to class/module types, interface types, and singleton types.

interface _Foo[T < String?]
  def foo: () -> T
end

type foo = _Foo[String]     # OK
type bar = _Foo[Integer?]   # Error

The generic type parameters can have default types.

interface _Each[Element, Return = void]
  def each: { (Element) -> void } -> Return
end

type foo = _Each[Integer]    # Equivalent to _Each[Integer, void]

The default types allows adding generic parameters without breaking compatibilities.