Can clang's std::string be made thread aware

While working with clang's thread sanitizer we noticed that std::string's copy-on-write technique isn't thread safe. We reduced the bug we were seeing to this code:

void test3() {
  std::unique_ptr<std::thread> thread;

  {
    auto output = make_shared<string>();
    std::string str = "test";
    thread.reset(new std::thread([str, output]() { *output += str; }));
    // The str string now goes out of scope but due to COW
    // the captured string may not have the copy of the content yet.
  }

  thread->join();
}

When compiled with thread sanitizer enabled:

clang++ -stdlib=libc++ -std=c++11 -O0 -g -fsanitize=thread -lpthread -o test main.cpp

or

clang++ -std=c++11 -O0 -g -fsanitize=thread -lpthread -o test main.cpp

And when run multiple times, it eventually produces this warning:

WARNING: ThreadSanitizer: data race (pid=30829)
  Write of size 8 at 0x7d0c0000bef8 by thread T62:
    #0 operator delete(void*) <null>:0
    ...

  Previous write of size 1 at 0x7d0c0000befd by thread T5:
    #0 std::__1::char_traits<char>::assign(char&, char const&) string:639
    ...

Can this be work arounded without changing the code (e.g. by passing some flags to the compiler) or is this a know bug?

Thanks for looking.