Why will coroutines (as of now in the newest drafts for C++1z) be implemented as a core language feature (fancy keywords and all) as opposed to a library extension?
There already exist a couple of implementations for them (Boost.Coroutine, etc), some of which can be made platform independent, from what i have read. Why has the committee decided to bake it into the core language itself?
I'm not saying they shouldn't but Bjarne Stroustrup himself mentioned in some talk (don't know which one any more) that new features should be implemented in libraries as far as possible instead of touching the core language.
So is there a good reason to do so? What are the benefits?
While there are library implementation of coroutines, these tend to have specific restrictions. For example, a library implementation cannot detect what variables need to be maintained when a coroutine is suspended. It is possible to work around this need, e.g., by making the used variables explicit in some form. However, when coroutines should behave like normal functions as much as possible, it should be possible to define local variables.
I don't think any of the implementers of Boost coroutines thinks that their respective library interface is ideal. While it is the best which can be achieved in the currently language, the overall use can be improved.