Set pendingTaskCountMax = 1.
Add only 1 worker into ThreadManager.
When adding task, set timeout > 0.
If there is no task, worker thread will sleep, because manager_->monitor_.wait(Line 259) is called in function ThreadManager::Worker::run.
Then suppose we have 2 tasks. We add the first task by calling ThreadManager::Impl::add, which will nofity monitor_. But before manager_->monitor_.wait() returns, we call ThreadManager::Impl::add again. If ThreadManager::Impl::add gets lock mutex_ successfully, ThreadManager::Worker::run will fall asleep again because it can not lock mutex_.
Now we have tasks_.size() == pendingTaskCountMax_, so ThreadManager::Impl::add will wait on maxMonitor_(Line 462). Becasue the worker cannot fetch a task(because it can not lock mutex_, it will not notify maxMonitor_), so ThreadManager::Impl::add will wait forever.
Now both the two threads wait for each other, result in deadlock.