Thanks. For us compiler newbs who need to understand what compilers are used for what versions on what platforms, is there a CPython reference I can use?

I think the choice is made by setuptools, configurable by your project. For Windows it will be MSVC. For Linux and macOS it will be GCC or Clang.

For CPython supported platforms and compilers, see PEP 11. It's basically:

Windows: MSVC
Everything else: GCC or Clang

GCC vs. Clang doesn't really matter much. Clang tries hard to be GCC compatible. Now that Clang is popular, GCC even tries to be Clang compatible.

Thanks, this has been incorporated into #2019.