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I'm using the clang-cl driver for Visual Studio with cmake for many projects. It generally works great. But recently I've found that when compiling cmake itself, it is still using the cl.exe compiler instead of clang-cl.exe.

For most projects, I just set CC="clang-cl.exe" and CXX="clang-cl.exe", then run cmake like before, and everything "magically" works. Since this seemed not to work for cmake, I've tried to add -DCMAKE_C_COMPILER=clang-cl.exe -DCMAKE_CXX_COMPILER=clang-cl.exe, but also that does not help.

Here is the output I get:

#> C:\BDA\bld\XTJyb_7r\0\cmake \
    -DCMAKE_PREFIX_PATH=C:/data/ci-dst-C7Ha32c192930137c1300/Tools;C:/data/ci-dst-C7Ha32c192930137c1300/Tools/lib/cmake \
    -DCMAKE_INSTALL_PREFIX=C:\data\ci-dst-C7Ha32c192930137c1300\Tools \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_FIND_USE_PACKAGE_REGISTRY=OFF \
    -DCMAKE_POSITION_INDEPENDENT_CODE=ON \
    -DCMAKE_VERBOSE_MAKEFILE=ON \
    -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
    -DCMAKE_CUDA_COMPILER=nvcc.exe \
    -DCMake_BUILD_DEVELOPER_REFERENCE=OFF \
    -DCMake_BUILD_LTO=ON \
    -DCMAKE_USE_SYSTEM_LIBARCHIVE=OFF \
    -DCMAKE_USE_SYSTEM_CURL=OFF \
    -DCMAKE_USE_SYSTEM_EXPAT=OFF \
    -DCMAKE_USE_SYSTEM_ZLIB=OFF \
    -DCMAKE_USE_SYSTEM_BZIP2=OFF \
    -DCMAKE_USE_SYSTEM_ZSTD=OFF \
    -DCMAKE_USE_SYSTEM_LIBLZMA=OFF \
    -DCMAKE_USE_SYSTEM_FORM=OFF \
    -DCMAKE_USE_SYSTEM_JSONCPP=OFF \
    -DCMAKE_USE_SYSTEM_LIBRHASH=OFF \
    -DCMAKE_USE_SYSTEM_LIBUV=OFF \
    -DCMAKE_USE_SYSTEM_KWIML=OFF \
    -DCMAKE_USE_OPENSSL=ON \
    -DBUILD_CursesDialog=OFF \
    -DCMAKE_C_COMPILER=clang-cl.exe \
    -DCMAKE_CXX_COMPILER=clang-cl.exe \
    -DCMAKE_CXX_STANDARD=17
Re-run cmake no build system arguments
-- Building for: Visual Studio 16 2019
-- Selecting Windows SDK version 10.0.19041.0 to target Windows 10.0.19044.
-- The C compiler identification is MSVC 19.29.30137.0
-- The CXX compiler identification is MSVC 19.29.30137.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.29.30133/bin/Hostx64/x64/cl.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.29.30133/bin/Hostx64/x64/cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
[...]

Why is the compiler selected as cl.exe? Am I doing something wrong?

I would like to use some clang-cl optimizations, so it would be nice if I could pick clang-cl. Or is it not supported for compiling cmake?