snippet.rb

class Object
  def self.method_added(name)
    const_set(:InstanceMethods, Module.new) unless defined?(self::InstanceMethods)
    meth = instance_method(name).bind(self.allocate)
    self::InstanceMethods.send(:define_method, name, &meth)
    remove_method(name)
    include self::InstanceMethods
  end
end

################## END MAGIC ##############

class X
  def x; "X" end
end

module Y
  def x; super + "Y" end
end

puts X.new.x
class X; include Y end
puts X.new.x


# output:
# X
# XY