The ARMCC (ARM toolchain) support seems to be broken in CMake 3.13.4. The reason is probably that ARMCC generates the predefined marco `__GNU__` and the CMakeCompilerIdDetection.cmake script recognizes ARMCC as a GNU Compiler. changing Line 73: ```cmake list(APPEND ordered_compilers SCO AppleClang Clang GNU MSVC ADSP IAR ARMCC ) ``` by ```cmake list(APPEND ordered_compilers SCO ARMCC AppleClang Clang GNU MSVC ADSP IAR ) ``` in the `Modules/CMakeCompilerIdDetection.cmake` fixes the problem for me Regards, Francois Lion