More Task and async gotchas! This code will dead lock. 
Just a refresher, when you use a TaskCompletionSource, calling Set* or TrySet* will execute the continuation on that thread. Think of it like an event handler, you're triggering the event inline.
There's a new option that was introduced in .NET 4.6 and all of .NET Core that allows you to specify that the continuation should be dispatched to the thread pool.
When you use Task.WhenAll, it will continue execution based on the last task that completed. This means it can continue on some arbitrary thread.
In the above case, the second TaskCompletionSource didn't specify "RunContinuationsAsynchronously" and since it finishes last, WhenAll resumes when the second thread calls TrySetResult.
If you're calling TrySet* make sure you're either dispatching the calling code to the thread pool. If you're not doing this, make sure you know what the calling code can do and decide if that's ok. Ask yourself questions like "What happens if the caller blocks my thread?".
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