How to implement an efficient WhenEach that streams an IAsyncEnumerable of task results?

Question

I am trying to update my toolset with the new tools offered by C# 8, and one method that seems particularly useful is a version of Task.WhenAll that returns an IAsyncEnumera

Answer 1

Just for the fun of it, using System.Reactive and System.Interactive.Async:

public static async IAsyncEnumerable WhenEach(
    Task[] tasks)
    => Observable.Merge(tasks.Select(t => t.ToObservable())).ToAsyncEnumerable()