I'm not suggesting that C# makes a change, it is probably too late (although you could easily implement both models), I was just describing my preferred semantics (well, I prefer stackfull coroutines, but that's another story). I don't think it deviates much from the existing semantics, the minimum change is adding awaits automatically for any call to awaitable functions and requiring async annotations otherwise. This is a relatively minor change.

I don't think that the async/sync division is a minor thing, but thanks for the Kotlin reference, I'll take a look. Before watching the video I guess that they implement 'stackfull-like' behaviour in otherwise stackless coroutines by force-inlining any HOF so that the coroutine is again flattened. If that's the case, that's great! What happens if inlining can't happen? They reject the code or convert to stackfull coroutines? That's for me as always been the holy grail: stackfull semantics that optimize to stackless (i.e. bounded stack usage) when possible (i.e. when the compler can see all possible yield points).

I have been trying to figure out (in C++) the subset of the language such as the optimization is always guaranteed (at the very least you need first class and explicit continuations so that the compiler can track them and inlining must be possible). Possibly Kotlin has cracked it.