Essential C++ : Calculating Factorials https://www.programming-books.io/essential/cpp/calculating-factorials-317f96c51a284c9ab5e0bf64625f5d70 (made with https://codeeval.dev)

main.cc

#include <iostream>
#include <type_traits>

template<long long n>
struct factorial :
  std::integral_constant<long long, n * factorial<n - 1>::value> {};

template<>
struct factorial<0> :
  std::integral_constant<long long, 1> {};

int main()
{
    std::cout << factorial<7>::value << std::endl;
}