FooBar = Struct.new(:foo, :bar)
class FooBar
  def my_foo
    foo
  end
end
def gen_foobar
  FooBar.new(1)
end
foobar = gen_foobar
foobar.foo = "str"
foobar.bar = :sym

__END__
# Classes
class Object
  private
  def gen_foobar: -> FooBar
end

class FooBar < Struct[untyped]
  attr_accessor foo(): Integer | String
  attr_accessor bar(): :sym?
  def my_foo: -> (Integer | String)
end