C++ 概念和提升几何图形

问：

我有一个运算符模板，它调用类型 的 boost 几何函数模板 ， .如果无法为类型实例化此函数模板，则我希望编译器选择回退运算符模板。如何使用 c++20 概念进行管理？T1T2T1T2

以下内容不编译：

#include <iostream>
#include <concepts>
#include <boost/geometry.hpp>

/*
 * Concept requiring that the template function "boost::geometry::within<T1, T2>"
 * can be instantiated for types T1, T2
 */
template<typename T1, typename T2>
concept WithinCanBeInstantiated = requires(const T1& t1, const T2& t2)
{
  boost::geometry::within<T1, T2>(t1, t2);
};

struct Within {
    /*
     * Operator template that should be used, if "boost::geometry::within<T1, T2>"
     * can be instantiated for types T1, T2
     */
    template <typename T1, typename T2>
    requires WithinCanBeInstantiated<T1, T2>
    bool operator()(const T1& t1, const T2& t2) const
    {
      return boost::geometry::within<T1, T2>(t1, t2);
    }
    
    /*
     * Operator template that should be used, if "boost::geometry::within<T1, T2>"
     * cannot be instantiated for types T1, T2
     */
    template <typename T1, typename T2>
    requires (not WithinCanBeInstantiated<T1, T2>)
    bool operator()(const T1& t1, const T2& t2) const
    {
      return false;
    }

};

int main() {
    typedef boost::geometry::model::d2::point_xy<double> point_type;
    typedef boost::geometry::model::polygon<point_type> polygon_type;

    polygon_type poly;
    boost::geometry::read_wkt(
        "POLYGON((2 1.3,2.4 1.7,2.8 1.8,3.4 1.2,3.7 1.6,3.4 2,4.1 3,5.3 2.6,5.4 1.2,4.9 0.8,2.9 0.7,2 1.3)"
            "(4.0 2.0, 4.2 1.4, 4.8 1.9, 4.4 2.2, 4.0 2.0))", poly);

    point_type p(4, 1);

    Within within;
    std::cout << "within exists: " << within(p, poly) << " no within: " << within(poly, p);
}

尝试编译

g++ -std=c++20 -O2 -Wall -pedantic -pthread main.cpp && ./a.out

我做错了什么？ 提前非常感谢！