29 Numerics library [numerics]

29.7 Mathematical functions for floating-point types [c.math]

29.7.5 Classification / comparison functions [c.math.fpclass]

The classification / comparison functions behave the same as the C macros with the corresponding names defined in the C standard library.
See also: ISO/IEC 9899:2018, 7.12.3, 7.12.4