#include <concepts>
    #include <iostream>
    #include <numbers>
    
    template<class shape>
    concept Shape = requires(
        shape s,
        shape (&&circle)(double, double, double),
        shape (&&rectangle)(double, double, double, double)) {
        { s(circle, rectangle) } -> std::same_as<shape>; };
    
    auto Circle = [](double x, double y, double r) -> Shape auto {
        return [=](auto&& circle, auto&& rectangle) { return circle(x, y, r); }; };
    auto Rectangle = [](double x, double y, double w, double h) -> Shape auto {
        return [=](auto&& circle, auto&& rectangle) { return rectangle(x, y, w, h); }; };
    
    Shape auto exampleCircle = Circle(2.0, 1.4, 4.5);
    Shape auto exampleRectangle = Rectangle(1.3, 3.1, 10.3, 7.7);
    
    auto area = [](Shape auto shape) -> double { return shape(
        [](auto, auto, auto r) { return std::numbers::pi * r * r; },
        [](auto, auto, auto w, auto h) { return w * h; }); };
    
    int main() {
        std::cout << area(exampleCircle) << std::endl;
        std::cout << area(exampleRectangle) << std::endl;
    }