Compile time factorial function implemenetation. It uses template specialization and variable template. Thus, it needs to be compiled above C++17.

compile_time_factorial.cpp

template<unsigned long long n>
constexpr unsigned long long Factorial = n * Factorial<n - 1>;

template<>
constexpr unsigned long long Factorial<0> = 1;

int main()
{
	constexpr auto fact_5 = Factorial<5>;   // 120
	constexpr auto fact_7 = Factorial<7>;   // 5040
	constexpr auto fact_20 = Factorial<20>; // 2432902008176640000
}