An `std::function` capturing a `unique_ptr` cannot be enqueued

[dpuyda]

The following code snippet does not compile:

  moodycamel::BlockingConcurrentQueue<std::function<void()>> queue;
  auto foo = std::make_unique<int>();
  queue.enqueue([foo = std::move(foo)] {});

[dpuyda]

Note that it also doesn't compile with an ordinary std::queue:

  std::queue<std::function<void()>> queue;
  auto foo = std::make_unique<int>();
  queue.emplace([foo = std::move(foo)] {});

A problem here is that std::function requires the argument to be copy-constructible. Found a few explanations:

• https://codereview.stackexchange.com/questions/257663/emplace-a-lambda-which-captured-a-unique-ptr-into-container-like-queue
• https://stackoverflow.com/questions/8236521/how-to-capture-a-unique-ptr-into-a-lambda-expression