bsl::pair::operator std::tuple<decltype(std::ignore)&, decltype(std::ignore)&>

This overload of `template <class PARAM_1, class PARAM_2> operator std::tuple<PARAM_1&, PARAM_2&>()` is called when the (template parameters) PARAM_1 (first element's type) and PARAM_2 (second element's type) are the type of std::ignore. Note that this method is defined within the class body intentionally to avoid build failure on MSVC 2015.