primary-expression before decltype [closed]

I wrote a simple mymax which is a wrapper over std::max, but it fails to work with error primary-expression before decltype, The code is:

#include <functional>
#include <iostream>

using namespace std;

class car
{
    public:
        int cost;
        friend ostream& operator<<(ostream& os, car c);
        
};

ostream& operator<<(ostream& os, car c) {
    os << c.cost;
    return os;
}

template <typename T>
T mymax(T a, T b, function<T (T a, T b)> comp = less<T>())
{
    return std::max(a,b, comp);
}

int main()
{
    car bmw, audi;
    bmw.cost = 10;
    audi.cost = 9;
    auto carcomaprator = [](const car& l, const car& r) -> bool { return l.cost < r.cost;};
    cout<< mymax<car>(bmw, audi, decltype(carcomaprator)) << endl; // <------- fails here
    
    cout<< mymax<int>(8, 9) << endl;

    return 0;
}

why I am not able to pass car comparator and how to fix this and use lambda itself. This is only for learning purpose. I am not able to understand why decltype is not able to in this case.