CMAKE_CXX_SIMULATE_ID not documented, except in a few obscured pages online, but not in Kitware
CMAKE_CXX_SIMULATE_ID is reportedly a way to detect clang-cl vs. clang on Windows in Cmake. I got lucky and found the two pages on the whole Internet explaining how to use this.
But the next day, I couldn't find them again at first. I tried Kitware's own documentation, but it's absent entirely.
It seems to work. I have used it to fix a build (Vulkan Loader) which isn't building on Windows because it mistakenly thinks clang is Unixy. and then proceeds to use lots of -D and -std options all over the place, breaking clang-cl, which expects forward-slashy arguments (/std:c++newest, etc) mainly and some other weird conventions. So I edited the CMakeLists.txt of that project to make the logic branch when detecting that while CMAKE_CXX_COMPILER_ID might be Clang, CMAKE_CXX_SIMULATE_ID is MSVC, so do weird stuff.
It'd be nice to have official documentation on this.