message: Don't return rvalue references
diff: @@ -48,7 +48,7 @@ public:
  non_empty_.notify();
  }
  - T&& pop_front()
  + T pop_front()
  {
  T next;
  @@ -66,7 +66,7 @@ public:
  queue_.pop_front();
  }
  - return std::move(next);
  + return next;
  }
  private:
  