case.rb

class Pass
  def self.===(o)
    false
  end
end

module Case
  def test_it(test)
    case test
    when (self.kind_of?(Class) ? self : Pass)
      puts "Tested by my class"
    when self.class
      puts "Tested by an instance of my class"
    else
      puts "Tested by something else..."
    end
  end
end

class Tester
  class<<self
    include Case
  end
  include Case
end

t = Tester.new
Tester.test_it("A string")
Tester.test_it(t)
Tester.new.test_it(t)
Tester.new.test_it(Pass.new)