21 Metaprogramming library [meta]

21.3 Metaprogramming and type traits [type.traits]

21.3.5 Unary type traits [meta.unary] General [meta.unary.general]

Subclause [meta.unary] contains templates that may be used to query the properties of a type at compile time.
Each of these templates shall be a Cpp17UnaryTypeTrait ([meta.rqmts]) with a base characteristic of true_type if the corresponding condition is true, otherwise false_type.