Defined in header <memory> constexpr std::allocator_arg_t allocator_arg = std::allocator_arg_t(); (since C++11)
std::allocator_arg is a constant of type std::allocator_arg_t used to disambiguate, at call site, the overloads of the constructors and member functions of allocator-aware objects, such as std::tuple, std::function, std::promise, and std::packaged_task.
See also
allocator_arg_t (C++11) tag type used to select allocator-aware constructor overloads (class)