30 Time library [time]

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:

• (1.1)
  the qualified-ids T​::​rep, T​::​period, T​::​duration, and T​::​time_point are valid and each denotes a type ([temp.deduct]),
• (1.2)
  the expression T​::​is_steady is well-formed when treated as an unevaluated operand,
• (1.3)
  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.