Compiler gives wrong output

[U.Mutlu]

Hi,
on this page:
http://en.cppreference.com/w/cpp/thread/recursive_mutex/try_lock
they give the following C++11 example:

#include <iostream>
#include <mutex>

int main()
{
std::mutex test;
if (test.try_lock()==true) {
std::cout << "lock acquired" << std::endl;
test.unlock(); //now unlock the mutex
} else {
std::cout << "lock not acquired" << std::endl;
}

test.lock(); //to lock it again
if (test.try_lock()) { //true can be left out
std::cout << "lock acquired" << std::endl;
} else {
std::cout << "lock not acquired" << std::endl;
}
test.unlock();
}

and they say it would give this output:
Output:
lock acquired
lock not acquired

I wonder why my compiler (g++ 4.9.2 from Debian repo) gives a different output:
lock acquired
lock acquired

Which compiler is wrong, and why?

Besides that: doesn't the code contain the wrong mutex:
shouldn't it rather be "std::recursive_mutex" for a recursive mutex?

Thx

My compile switches:

$ g++ -Wall -O2 -std=c++11 t.cpp
$ ./a.out
lock acquired
lock acquired

$ g++ -Wall -O2 -std=gnu++11 t.cpp
$ ./a.out
lock acquired
lock acquired

[Richard Kettlewell]

AIUI, neither is wrong. The code violates the preconditions of
try_lock(), and therefore results in undefined behavior.

--
http://www.greenend.org.uk/rjk/

[U.Mutlu]

Mystery solved: when using such concurrency/thread objects,
then one has to link with pthread (-lpthread):
g++ -Wall -O2 -std=gnu++11 file.cpp -lpthread
Now my compiler gives the same result.