ghost_class.rb

class A
  def self.hello
    p 'hello'
  end

  def class
    (class << self; self; end)
  end
end

module Foo
  def bar
    p 'bar'
  end
end

a = A.new
b = A.new

a.instance_eval do
  class << self
    extend Foo
  end
end

a.class.hello
a.class.bar

b.class.hello
b.class.bar