俱舍論頌疏論本 28.38

28.38

喜即是喜受者，謂上座部等，於心所中別有喜體，非是喜受，謂簡彼故，故言「喜即是喜受」，謂初二定喜名喜受也。

Regarding "Joy is just the feeling of joy": in schools such as the Sthavira, among mental factors there is a distinct entity of joy that is not the feeling of joy. In order to distinguish from that, it is therefore said, 'joy is just the feeling of joy,' meaning that the joy of the first two dhyānas is named the feeling of joy.