C++: What is constexpr evaluation (C++20)?

1// C++11: limited constexpr
2constexpr int factorial(int n) {
3    return (n <= 1) ? 1 : n * factorial(n - 1);
4}
5
6constexpr int f5 = factorial(5);  // 120 at compile time
7
8// C++20: constexpr dynamic allocation, virtual, try-catch
9constexpr std::vector<int> make_vec() {
10    std::vector<int> v;
11    for (int i = 0; i < 10; i++) {
12        v.push_back(i * i);
13    }
14    return v;
15}
16
17constexpr auto squares = make_vec();
18
19// consteval — must run at compile time
20consteval int square(int n) {
21    return n * n;
22}
23
24int x = square(5);     // OK
25// int y = square(x);   // Error — not compile-time
26
27// constexpr if
28template<typename T>
29auto process(T val) {
30    if constexpr (std::is_integral_v<T>) {
31        return val * 2;
32    } else {
33        return val + 0.5;
34    }
35}