[ 21%] Building CXX object src/mongocxx/CMakeFiles/mongocxx_shared.dir/gridfs/downloader.cpp.o
cd /home/ubuntu/mongo-cxx-driver/build/src/mongocxx && /usr/bin/c++   -DMONGOCXX_EXPORTS -DMONGO_CXX_DRIVER_COMPILING -I/include -I/home/ubuntu/mongo-cxx-driver/src/mongocxx/.. -I/home/ubuntu/mongo-cxx-driver/build/src/mongocxx/.. -isystem /usr/local/include/libmongoc-1.0 -isystem /usr/local/include/libbson-1.0 -I/home/ubuntu/mongo-cxx-driver/src/bsoncxx/.. -I/home/ubuntu/mongo-cxx-driver/build/src/bsoncxx/..  -O3 -DNDEBUG -fPIC -fvisibility=hidden -fvisibility-inlines-hidden   -std=c++11 -o CMakeFiles/mongocxx_shared.dir/gridfs/downloader.cpp.o -c /home/ubuntu/mongo-cxx-driver/src/mongocxx/gridfs/downloader.cpp
/home/ubuntu/mongo-cxx-driver/src/mongocxx/gridfs/downloader.cpp: In member function 'void mongocxx::v_noabi::gridfs::downloader::close()':
/home/ubuntu/mongo-cxx-driver/src/mongocxx/gridfs/downloader.cpp:80:24: error: ambiguous overload for 'operator=' (operand types are 'boost::optional<mongocxx::v_noabi::cursor>' and '<brace-enclosed initializer list>')
     _get_impl().chunks = {};
                        ^
In file included from /home/ubuntu/mongo-cxx-driver/src/mongocxx/../bsoncxx/stdx/optional.hpp:42:0,
                 from /home/ubuntu/mongo-cxx-driver/src/mongocxx/../mongocxx/gridfs/downloader.hpp:23,
                 from /home/ubuntu/mongo-cxx-driver/src/mongocxx/gridfs/downloader.cpp:15:
/usr/include/boost/optional/optional.hpp:922:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(const boost::optional<T>&) [with T = mongocxx::v_noabi::cursor]
     optional& operator= ( optional const& rhs )
               ^
/usr/include/boost/optional/optional.hpp:930:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(boost::optional<T>&&) [with T = mongocxx::v_noabi::cursor]
     optional& operator= ( optional && rhs ) 
               ^
/usr/include/boost/optional/optional.hpp:959:15: note: candidate: boost::optional<T>& boost::optional<T>::operator=(boost::none_t) [with T = mongocxx::v_noabi::cursor]
     optional& operator= ( none_t none_ ) BOOST_NOEXCEPT
               ^
src/mongocxx/CMakeFiles/mongocxx_shared.dir/build.make:686: recipe for target 'src/mongocxx/CMakeFiles/mongocxx_shared.dir/gridfs/downloader.cpp.o' failed
make[2]: *** [src/mongocxx/CMakeFiles/mongocxx_shared.dir/gridfs/downloader.cpp.o] Error 1
make[2]: Leaving directory '/home/ubuntu/mongo-cxx-driver/build'
CMakeFiles/Makefile2:691: recipe for target 'src/mongocxx/CMakeFiles/mongocxx_shared.dir/all' failed
make[1]: *** [src/mongocxx/CMakeFiles/mongocxx_shared.dir/all] Error 2
make[1]: Leaving directory '/home/ubuntu/mongo-cxx-driver/build'
Makefile:138: recipe for target 'all' failed
make: *** [all] Error 2