struct piecewise_construct_t { };
constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
The struct piecewise_construct_t is an empty structure type used as a unique type to disambiguate constructor and function overloading. Specifically, pair has a constructor with piecewise_construct_t as the first argument, immediately followed by two tuple ([tuple]) arguments used for piecewise construction of the elements of the pair object.