30 Time library [time]

30.4 Time-related traits [time.traits]

30.4.4 Class template is_clock [time.traits.is.clock]

template<class T> struct is_clock;
is_clock is a Cpp17UnaryTypeTrait ([meta.rqmts]) with a base characteristic of true_type if T meets the Cpp17Clock requirements ([time.clock.req]), otherwise false_type.
For the purposes of the specification of this trait, the extent to which an implementation determines that a type cannot meet the Cpp17Clock requirements is unspecified, except that as a minimum a type T shall not qualify as a Cpp17Clock unless it meets all of the following conditions:
  • the qualified-ids T​::​rep, T​::​period, T​::​duration, and T​::​time_point are valid and each denotes a type ([temp.deduct]),
  • the expression T​::​is_steady is well-formed when treated as an unevaluated operand,
  • the expression T​::​now() is well-formed when treated as an unevaluated operand.
The behavior of a program that adds specializations for is_clock is undefined.