大般涅槃經義記 3.266

3.266

第五句中，「如來無常即為苦」者自立苦義，此乃名前如來捨身入於涅槃以為無常，無常故苦；亦可無常是其有為生滅行苦，故言是苦。「若是苦者，云何生樂」，難破佛樂，成佛為苦。

In the fifth clause, 'The Tathāgata being impermanent is indeed suffering' is one's own establishment of the meaning of suffering. This takes the earlier [statement that] the Tathāgata abandons the body and enters into nirvāṇa to be impermanence, and because it is impermanent, it is suffering; alternatively, impermanence is the conditioned suffering of conditioned arising and ceasing, and thus it says it is suffering. 'If it is suffering, how could bliss arise?' poses a challenge to refute the Buddha's bliss and establish that the Buddha is suffering.