Solves the second ProjectEuler problem completely during compile time. Uses C++20 features (currently only in gcc head).

cpp20_compiletime_fibonacci_sum.cpp

#include <array>

using uint = unsigned int;

constexpr uint fib(std::size_t n) {
    return (n <=1)? 1 : fib(n-1) + fib(n-2);
}

template<typename T>
constexpr bool predicament(T num) {
    return num < 4000000 and num%2 == 0;
}

// This is basically constexpr std::accumulate with a condition
template<typename T, std::size_t N, class Function>
constexpr T compile_time_accumulator(std::array<T, N> const &A, Function&& f) {
  T sum(T(0));

  for(std::size_t i = 0; i < N; ++i) {
    if (f(A[i]))
        sum += A[i];
  }

  return sum;
}

// This is basically constexpr std::iota
template<typename T, std::size_t N, class Function, std::size_t... I>
constexpr std::array<T, N> make_array(Function&& func, std::index_sequence<I...>) {
    return { func(I)... };   
}

template<typename T, std::size_t N, class Function>
constexpr std::array<T, N> make_array(Function&& func) {
    return make_array<T,N>(std::forward<Function>(func), std::make_index_sequence<N>());
}

int main()
{
    constexpr auto N = 100;
    constexpr auto arr = make_array<uint, N>(fib);

    static_assert(compile_time_accumulator(arr, predicament<decltype(arr)::value_type>) == 4613732);
}