[Bug libstdc++/60521] New: std::lock_guard ignores adopt_lock strategy

[turchanov at farpost dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60521

            Bug ID: 60521
           Summary: std::lock_guard ignores adopt_lock strategy
           Product: gcc
           Version: 4.8.3
            Status: UNCONFIRMED
          Severity: blocker
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: turchanov at farpost dot com

lock_guard created with constructor lock_guard::lock_guard(mutex_type& m,
adopt_lock_t tag) must adopt already locked mutex. But current implementation
ignores completely this fact and tries to lock passed mutex unconditionally:

 template<typename _Mutex>
    class lock_guard {
...
      lock_guard(mutex_type& __m, adopt_lock_t) : _M_device(__m)
      { } // calling thread owns mutex
...
};

and given the fact that mutex type is not a recursive mutex this code sequence
produces a dead-lock:

mutex mtx;
mtx.lock();
lock_guard<mutex> guard(mtx, adopt_lock);  // <-- deadlocks!