Why are mutexes and condition variables trivially copyable?

Question

LWG 2424 discusses the undesirable status of atomics, mutexes and condition variables as trivially copyable in C++14. I appreciate that a fix is already lined up, but std::mutex

Answer 1

Not all implementations provide a nontrivial destructor for mutex. See libstdc++ (and assume that __GTHREAD_MUTEX_INIT has been defined):

  // Common base class for std::mutex and std::timed_mutex
  class __mutex_base
  {
  // […]
#ifdef __GTHREAD_MUTEX_INIT
    __native_type  _M_mutex = __GTHREAD_MUTEX_INIT;

    constexpr __mutex_base() noexcept = default;
#else
  // […]

    ~__mutex_base() noexcept { __gthread_mutex_destroy(&_M_mutex); }
#endif
  // […]
  };

  /// The standard mutex type.
  class mutex : private __mutex_base
  {
    // […]
    mutex() noexcept = default;
    ~mutex() = default;

    mutex(const mutex&) = delete;
    mutex& operator=(const mutex&) = delete;
  };

This implementation of mutex is both standard conforming and trivially copyable (which can be verified via Coliru). Similarly, nothing stops an implementation from keeping condition_variable trivially destructible (cf. [thread.condition.condvar]/6, although I couldn't find an implementation that does).

The bottom line is that we need clear, normative guarantees, and not clever, subtle interpretations of what condition_variable does or doesn't have to do (and how it has to accomplish that).