[cfe-dev] Duplicated header between Clang and MSVC - why?

[Gaier, Bjoern via cfe-dev]

Hello Clang-Experts,

I'm using Clang-CL in Visual Studio - everything is working fine! Hooray!
But recently I noticed that some header files related to the C-Library are duplicated on my hard drive. For example: stddef.h - so I searched for it and noticed, that Visual Studio has like... 5 of them, but all related to different versions of MSVC. That made sense to me, but then: surprise! I also have that file under "C:\Program Files\LLVM9\lib\clang\9.0.0\include" being obviously related to Clang. I also noticed that there are many files duplicated between Clang and MSVC.

Now I wonder... If I run Clang-CL, should I use the most up to date MSVC header or the Clang one?
Why are these files duplicated anyway?

I feel like, that this is a silly question, but I seriously have no idea.

Kind greetings
Björn
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