Does std::lock_guard release the mutex after constructed with std::adopt_lock option?

I know my question is quite similar to this Why does std::lock_guard release the lock after using std::adopt_lock?, but the behavior I see is not. Here is my code:

#include <mutex>
#include <iostream>
using namespace std;

std::mutex m;
void fun2();
void fun1() {
    cout << "fun1" << endl;
    std::unique_lock<std::mutex> guard(m);
    cout << "lock mutex" << endl;
    fun2();
    if (guard.owns_lock()) {
        cout << "still holds mutex" << endl;
    }
    else {
        cout << "doesn't hold mutex" << endl;
    }
}
void fun2() {
    std::lock_guard<std::mutex> guard(m, std::adopt_lock);
    cout << "fun2" << endl;
}
int main(int argc, char* argv[]) {
    fun1();
    return 0;
}

And this is the result I get:

fun1
lock mutex
fun2
still holds mutex

Clearly, the unique_lock in fun1 still holds the mutex. So my question is “Does std::lock_guard release the mutex after constructed with std::adopt_lock option?”. Hope you all can help me clarify this situation. Thank you.

Answer

When you constructed a std::unique_lock to manage the mutex, you should stick to it unless you first break the association of the std::unique_lock with the mutex using std::unique_lock::release. In your sample, you touched the raw mutex when it’s still managed by a std::unique_lock and this is wrong.