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Or on Linux or Windows: gcc is now written in C++, and I'm pretty sure MSVC itself is in C++.

Yes, both MSVC's front-end C1XX and back-end C2 (aka UTC) are written in C++. Even our CRT is mostly C++ now.

Do you update it to follow the latest standards or is it early 2003-ish C++?

C1XX uses new features fairly aggressively. For example, it uses lambdas, auto, range-for, etc.