Thread pool c++ 17

A simple C++17 Thread Pool implementation supporting "internal" tasks without interlock.

Philosophy

1. A task is added to the thread pool.
2. The thread pool is running on the task list.
3. When the result of a task is required, if the task is not completed and has not been started by the thread pool, this task is executed thread-locally and removed from the thread pool.

One rule : The first arrived on the task, runs the task !

Exemple :

void main()
{
	auto & pool = thread_pool::instance(2);
	
	auto thA = pool.enqueue([&pool]
	{
		auto thA_1 = pool.enqueue([]{
		    // Do something awesome!
		});
		auto thA_2 = pool.enqueue([]{
		    // Do something crasy!
		});

		thA_1.get();
		thA_2.get();
	});
	
	auto thB = pool.enqueue([&pool]
	{

		auto thB_1 = pool.enqueue([] {
			// your dream here
		});

		auto thB_2 = pool.enqueue([] {
			// something
		});

		thB_1.get();
		thB_2.get();
	});


	thB.get();
	thA.get();
}