CMAKE_<LANG>_COMPILER_FRONTEND_VARIANT
New in version 3.14.
Identification string of the compiler frontend variant.
Some compilers have multiple, different frontends for accepting command line options. (For example Clang originally only had a frontend compatible with the GNU compiler but since its port to Windows (Clang-Cl) it now also supports a frontend compatible with MSVC.) When CMake detects such a compiler it sets this variable to what would have been the CMAKE_<LANG>_COMPILER_ID for the compiler whose frontend it resembles.
Note
In other words, this variable describes what command line options and language extensions the compiler frontend expects.
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https://cmake.org/cmake/help/v3.23/variable/CMAKE_LANG_COMPILER_FRONTEND_VARIANT.html