Factorial at compile time

constexpr-factorial.cpp

#include <concepts>

using ULL = unsigned long long int;

template <std::integral auto I>
struct Factorial {
  static constexpr ULL value = I * Factorial<I - 1>::value;
};

template <>
struct Factorial<0> {
  [[maybe_unused]]
  static constexpr ULL value = 1;
};

template <std::integral auto I>
inline constexpr auto FactorialV = Factorial<I>::value;

// or using consteval (C++20)

consteval auto consteval_factorial(std::integral auto n) -> std::integral auto {
  ULL result = 1;
  for (; n > 1; --n)
    result *= n;
  return result;
}