In order to build a Fortran project with CMake CMAKE_Fortran_Compiler variable is set with appropriate path. This variable only accept windows like paths unlike CMAKE_CXX_COMPILER which can also accept unix like path escaped with an extra '\'. For example: -DCMAKE_CXX_COMPILER=c:\\work\\llvm-dev\\build\\bin\\clang-cl.exe -DCMAKE_CXX_COMPILER=c:\\work\\llvm-dev\\build\\bin\\clang-cl.exe works out of the box but: -DCMAKE_Fortran_COMPILER=c:\\work\\llvm-dev\\build\\bin\\flang_new.exe does not work, however following path works: -DCMAKE_Fortran_COMPILER="c:/work/llvm-dev/build/bin/flang-new.exe" Apparently CMake is calling TO_CMAKE_PATH on C/CXX compilers but not for Fortran compilers.