struct Color {
	int r, g, b;
};

class Environment {
public:
    Environment(double air_density, Color background_color) {}
private:
    static constexpr Color default_background_color = Color({ 210, 210, 210 });

    friend int main();
};

int main() {
    Environment environment(0.1, Environment::default_background_color); // OK
	return 0;
}