System.Collections.Generic

 namespace System.Collections.Generic {
+    public interface IAsyncEnumerable<out T> {
+        IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default(CancellationToken));
+    }
+    public interface IAsyncEnumerator<out T> : IAsyncDisposable {
+        T Current { get; }
+        ValueTask<bool> MoveNextAsync();
+    }
 }