I'm wondering how it accomplishes this as a practical matter. Does it compile a file twice? Once to create the interface info, and a second time to compile the implementation? Can they even parse enough of a C++ file to generate the interface in all cases?

That they chose not to mention the issue makes me nervous that they think they can just ignore the problem. Many previous languages have ignored the problem and said "don't do that." I hope this proposal does not go in that direction.