Have you by any chance seen that I specifically mentioned version 7 of C#?
Which allows full control how async/await works, kind of like on the upcoming C++ co-routines, where async/await become syntactic sugar for blessed data structure.
EDIT: Should have provided the actual information.
The feature as introduced in C# 7 is called "Generalized async return types".
https://github.com/dotnet/docs/issues/1756