template <class T> constexpr auto operator+(T a) -> std::enable_if_t<stlab::implementation::has_enabled_arithmetic<T>, T> (1) template <class T> constexpr auto operator+(T lhs, T rhs) -> std::enable_if_t<stlab::implementation::has_enabled_arithmetic<T>, T> (2) (1) template <class T> constexpr auto operator+(T a) -> std::enable_if_t<stlab::implementation::has_enabled_arithmetic<T>, T> Parameters T a (2) template <class T> constexpr auto operator+(T lhs, T rhs) -> std::enable_if_t<stlab::implementation::has_enabled_arithmetic<T>, T> MISSING Parameters T lhs T rhs