gistfile1.rb

class BasicObject
  def class
    classes = []
    ::ObjectSpace.each_object(::Class).each do |x|
      case self
        when x then classes << x
      end rescue nil
    end
    classes.reject! { |k| classes.any? { |c| c < k } }.first
  end
end

class S < BasicObject
end

puts S.new.class # => S