return std::visit([](auto&& arg) noexcept(!std::is_same_v<std::decay_t<decltype(arg)>, Concat>) -> ConstantValue { using T = std::decay_t<decltype(arg)>; if constexpr (std::is_same_v<T, std::monostate>) return ConstantValue(); else if constexpr (std::is_same_v<...