Protobuf incompatible with nvcc

[kmorel]

When you compile MGARD with CUDA and a recent version of protobuf, you get an error like this:

[ 99%] Building CUDA object CMakeFiles/mgard-library.dir/src/mgard-x/RuntimeX/DeviceAdapters/DeviceAdapterCuda.cu.o
/home/software/spack/[padded-to-256-chars]/linux-x86_64_v3/cuda-12.9.1-pdix7w7d3ugenere6g6rwm7yvkxtzeqb/bin/nvcc -forward-unknown-to-host-compiler -ccbin=/usr/bin/g++ -DMGARD_ENABLE_CUDA -DMGARD_ENABLE_OPENMP -DMGARD_ENABLE_SERIAL -DMGARD_TIMING -DMGARD_ZSTD -Dmgard_library_EXPORTS --options-file CMakeFiles/mgard-library.dir/includes_CUDA.rsp --expt-relaxed-constexpr -w --generate-line-info -O3 -DNDEBUG -std=c++17 "--generate-code=arch=compute_90,code=[compute_90,sm_90]" -Xcompiler=-fPIC -DPROTOBUF_USE_DLLS -Wnon-virtual-dtor -DNOMINMAX -MD -MT CMakeFiles/mgard-library.dir/src/mgard-x/RuntimeX/DeviceAdapters/DeviceAdapterCuda.cu.o -MF CMakeFiles/mgard-library.dir/src/mgard-x/RuntimeX/DeviceAdapters/DeviceAdapterCuda.cu.o.d -x cu -rdc=true -c /tmp/root/spack-stage/spack-stage-mgard-1.6.0-7kee2kkxudejqwlbzpntujj6okwu2426/spack-src/src/mgard-x/RuntimeX/DeviceAdapters/DeviceAdapterCuda.cu -o CMakeFiles/mgard-library.dir/src/mgard-x/RuntimeX/DeviceAdapters/DeviceAdapterCuda.cu.o
/home/software/spack/[padded-to-256-chars]/linux-x86_64_v3/protobuf-33.1-mc3sumwwz6wj7h3optmympl4n6fhrkwz/include/google/protobuf/message_lite.h(298): error: variable cannot have incomplete type "google::protobuf::internal::EnumTraitsImpl::Undefined"
    static Undefined value;
    ^

The problem is that recent versions of protobuf rely on inline variables and relaxed rules around static data members to make this particular code legal without a full definition. This is completely legal in C++17 (which the host compiler fully supports), but this is a part of the C++17 spec not supported by the nvcc wrapper.

This problem was observed when adding the 1.6.0 release to mgard.

The best advice is to avoid using protobuf in code compiled with nvcc. However, the protobuf use seems most likely coming from Metadata.hpp, and that seems like a core feature.