Crystal langでPhantom Typeっぽくやりたかったけど、なんか違う

play.cr

module A
  class State
  end

  class Ready < State
  end
  
  class NotReady < State
  end
end

class Data(T)
end

class Test
  include A
  
  def test(data : Data(Ready))
    print("test ok")
  end
  
  def self.start
    t = Test.new
    t.test(Data(Ready).new)
  end
end

Test.start

test.cr

class Data(T)
end

# こういう特殊化はできないっぽい
class Data(NotReady)
  def ready
    Data(Ready).new
  end
end

class Data(Ready)
  def say
    "Hello"
  end
end