<mp-units/cartesian_vector.h>
template<
typename Arg,
typename... Args>
requires (sizeof...(Args) <= 2) && requires { typename std::common_type_t<Arg, Args...>; }
cartesian_vector<std::common_type_t<Arg, Args...>>(
Arg,
Args...) -> cartesian_vector<std::common_type_t<Arg, Args...>>;