the latest MSVC version (shipped with VS 2019 16.4.0) doesn't compile boost/proto/generate.hpp since there is an old MSVC-fix, which is no longer needed, and now even produces errors.
The "develop" branch of proto contains a fix for that since 2015 (!) but it never made it to the master branch. (https://github.com/boostorg/proto/issues/20)
Could some please merge it over to make it into release 1.72 - would be really great!