cpp/chrono/is clock

If satisfies the  requirements, provides the member constant  equal. For any other type, is.

For the purpose of this trait, the extent to which an implementation determines that a type cannot meet the requirements is unspecified, except that a minimum  shall not qualify as a  unless it meets all of the following conditions:
 * The qualified-ids, , , and are all valid and each denotes a type;
 * The expressions and  are each well-formed when treated as an unevaluated operand.