From a totally unnecessary blog (we beg to differ):

Stackless coroutines with Visual Studio 2015

by Paolo Severini

From the article:

I had been looking for some time now at the problem of implementing coroutines/resumable functions in order to have even in C++ something similar to what is provided by C# await and yield statements. It turns out that -- unbeknown to me -- this is quite a hot topic in the C++ community...

... stackful coroutines have a big disadvantage in the fact that fibers are very expensive... The new proposal (N4286) instead focuses mostly on a stackless implementation, and promises to be scalable to billions of concurrent coroutines...