> It does start in the fork, but the ultimate goal is for all changes to be merged upstream. Some aspects, like dynamicLookup[0], which allows dot syntax for methods resolved at runtime, are already in the language.

Personally, I feel like it would be unwise to merge in all of TensorFlow's requirements into Swift. Many features, such as the ones you mentioned, are actually useful to outside of the context of machine learning, so it does actually make sense to make them generally available. But compiler support for something as specific as @differentiable seems like an anti-goal for Swift.