[awson]

AFAUI, this extension uses proprietary C++ front-end commercially licensed from EDG.

The same front-end powers their Visual Studio IDE (not Code).

Also, AFAIK, there exist an alternative `clangd`-based C++ extension, which is fully open-source (but I could be wrong).

[gpderetta]

clangd is fully open source. You can build it yourself from llvm sources. It works fine with emacs LSP support. I never used the proprietary extension so I can't compare the functionality, stability nor speed.