Why having both default destructor and vector member prevents class to be "nothrow movable constructible"?

Question

Given the following code:

#include <iostream>
#include <vector>
#include <type_traits>

class Test {
public:
    ~Test() = default;
    std::vector<int> m_vector;
};


int main() {
    std::cout << std::is_nothrow_move_constructible_v<Test> << std::endl;
    return 0;
}

It outputs 0, meaning the Test class can't be "nothrow moved". However, if I remove either ~Test() or m_vector then it returns 1.

How to explain this please?

For reference, I'm using clang++-7 with C++17.

Answer 1

When you define a user declared destructor, the class will not have an implicitly generated move constructor. Hence, moving will invoke copy constructor. The copy constructor of std::vector is potentially throwing, hence the implicit copy constructor of Test will also be potentially throwing.