This is the most simple approach where we treat the diode as an ideal element. By ideal we mean,  we will ignore the barrier potential of the diode that is 0.7v in case of silicon and 0.3 v in case of germanium when it is forward biased. So, when diode is forward biased, we will say infinite amount of current is flowing with zero voltage drop across its terminals. When it is reverse biased, we will say it has infinite resistance and hence zero leakage current across its terminal.