[nandos13]

Fixes #730

Both the generic and non-generic overloads for the AsUniTask method now have a switch on task.Status and returns a completed UniTask if the input task is in the Canceled, Faulted or RanToCompletion state.
Unit tests have been added for these already-completed cases. I also improved test coverage for some missing cases where a Task completes after the AsUniTask call. These tests pass both before and after my fix changes.

Note that this is a behavioral change, a completed Task no longer forces a yield. This might cause problems if code leaned on the yield, but I believe this is now correct behavior.

Thanks

[timcassell]

Wouldn't it be much simpler to just await it?

public static async UniTask<T> AsUniTask<T>(this Task<T> task, bool useCurrentSynchronizationContext = true)
    => await task.ConfigureAwait(useCurrentSynchronizationContext);

[nandos13]

@timcassell
Your suggestion looks simpler and does seem to fix the already-completed case, but I found it does change some behavior. When a Task is still pending at the call, the returned UniTask becomes single use.

• The current AsUniTask implementation returns a UniTask backed by UniTaskCompletionSource, which the README says "can await multiple times and await from many callers".
• The UniTask returned by an async UniTask method that had to wait can only be awaited once, and a second await throws.

This test passes on master and with this PR, but it fails with the await version:

[Fact]
public async Task AsUniTask_PendingTaskWithResult_CanBeAwaitedTwice()
{
    var source = new TaskCompletionSource<int>();

    UniTask<int> task = source.Task.AsUniTask();

    source.SetResult(42);

    Assert.Equal(42, await task);
    Assert.Equal(42, await task);
}

System.InvalidOperationException : Token version is not matched, can not await twice or get Status after await.

It's also worth noting that the README states "UniTask can not await twice" and doing so is generally unsupported anyway. Still, existing code that caches the result of AsUniTask and awaits it more than once works today and would start throwing, which I think is worth being aware of.

I'm happy to switch to the await version if preferred by the maintainers.