Showcases c++20 compile-time syntax using constexpr std::transform. Currently compiles with clang 10.0 experimental version.

cpp20_compiletime_expression.cpp

#include <array>
#include <algorithm>

constexpr int factorial(int n) {
    return n == 0 ? 1 : n * factorial(n - 1);
}

template <typename T, std::size_t N, typename F>
constexpr std::array<std::result_of_t<F(T)>, N>
transform(std::array<T, N> array, F f) {
    auto array_f = std::array<std::result_of_t<F(T)>, N>{};
    std::transform(array.begin(), array.end(), array_f.begin(), [&f](auto el){return f(el);});
    return array_f;
}

int main() {
    constexpr std::array<int, 4> ints{{1, 2, 3, 4}};
    constexpr std::array<int, 4> facts = transform(ints, factorial);
    static_assert(facts == std::array<int, 4>{{1, 2, 6, 24}}, "");
}