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Currently BOOST_COMP_NVCC is never set but always BOOST_COMP_NVCC_EMULATED because the NVCC preprocessor emulates the host compiler being used (gcc/clang/msvc/...) which are detected earlier. The nvcc compilation process is somewhat special as can be read here: https://docs.nvidia.com/cuda/cuda-compiler-driver-nvcc/index.html#cuda-compilation-trajectory The nvcc compiler precompiles the input two times. Once for the device code being compiled by the cicc device compiler and once for the host code compiled by the real host compiler. NVCC uses gcc/clang/msvc/... depending on the host compiler being set on the command line. Predef (as a preprocessor only lib) detects the one doing the preprocessing as compiler and expects it to be the one doing the real compilation. This is not true for NVCC which is only doing the preprocessing and which is using another compiler for parts of its work. So for NVCC it should be allowed to set BOOST_COMP_NVCC additionally to the already detected host compiler because both is true: It is gcc/clang/... compiling the code, but it is also NVCC doing th preprocessing and adding some other quirks you may want to detect. This behavior is similar to what boost config is doing in `select_compiler_config.hpp`. There the NVCC detection is not handled as a real compiler (part of the #if-#elif) but as additional option before the real compiler.