It continues to improve.

Better template support and error detection in C++ Modules with MSVC 2017 version 15.9

by Cameron DaCamara

From the article:

It has been a long time since we last talked about C++ Modules. We feel it is time to revisit what has been happening under the hood of MSVC for modules.

The Visual C++ Team has been dedicated to pushing conformance to the standard with a focus on making the overall compiler implementation more robust and correct with the rejuvenation effort. This rejuvenation effort has given us the ability to substantially improve our modules implementation. We’ve mostly done this work transparently over the past few months until now. We are proud to say the work has reached a point where talking about it would hopefully provide developers with even more reasons to use C++ Modules with MSVC!