ISO/IEC TS 19571:2016

Since the extensions described in this ISO/IEC TS 19571:2016 are experimental and not part of the C++ standard library, they should not be declared directly within namespace std. Unless otherwise specified, all components described in this technical specification either: - modify an existing interface in the C++ Standard Library in-place, - are declared in a namespace whose name appends ::experimental::concurrency_v1 to a namespace defined in the C++ Standard Library, such as std, or - are declared in a subnamespace of a namespace described in the previous bullet, whose name is not the same as an existing subnamespace of namespace std. Each header described in this ISO/IEC TS 19571:2016 shall import the contents of std::experimental::concurrency_v1 into std::experimental as if by namespace std { namespace experimental { inline namespace concurrency_v1 {} } }