reference_wrapper: Is this UB?

我很好奇这段代码是否是UB。

https://wandbox.org/permlink/nU0iPLCrPXwQ7Kor

代码不会崩溃(在GCC和Clang中，无论是否进行优化(，这让我越来越困惑。。

#include <list>
#include <vector>
#include <iostream>
#include <utility>
#include <queue>
using namespace std;

int main() {
deque<int> l {1, 2, 3, 4, 5};
vector<reference_wrapper<int>> v (l.begin(), l.end());
// v now holds references to 1, 2, 3, 4, 5 in l
cout << v[3] << 'n'; // 4
l.pop_front();    // 1 popped
for (auto num : l) {
cout << num << ' '; // 2 3 4 5 
}
cout << 'n';    
cout << v[0] << 'n';  // I think this should crash because 1 is dangling reference
l.pop_front();  // 2 popped
for (auto num : l) {
cout << num << ' '; // 3 4 5
}
cout << 'n';    
cout << v[1] << 'n';  // I think it should crash but it still outputs 2
}