decltype Never Runs Its Expression

1#include <iostream>
2 
3int main() {
4    int x = 5;
5    decltype(x++) a = 10;
6    decltype(++x) b = x;
7    std::cout << x << " " << a << " " << b << "\n";
8    b = 20;
9    std::cout << x << " " << a << " " << b << "\n";
10    a = a + x;
11    std::cout << x << " " << a << " " << b << "\n";
12    return 0;
13}