This likely a fallout because you're defining both -DCMAKE_CXX_STANDARD=17 and USE_CXXSTD= c++17 See also: https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html
(In reply to Daniel Engberg from comment #0) Please indicate where the fallout is. https://portsfallout.com/port?port=devel%2Fgrpc&maintainer=
Just look at the build log, "[ 4% 85/1796] /usr/bin/c++ -Dgrpc_EXPORTS -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/include -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1 -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/third_party/address_sorting/include -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/src/core/ext/upb-gen -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/src/core/ext/upbdefs-gen -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/third_party/upb -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/third_party/utf8_range -I/wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/third_party/xxhash -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -isystem /usr/local/include -std=c++17 -O2 -pipe -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -isystem /usr/local/include -std=c++17 -DNDEBUG -std=gnu++17 -fPIC -pthread -Wno-float-conversion -Wno-implicit-float-conversion -Wno-implicit-int-float-conversion -Wno-unknown-warning-option -DNOMINMAX -MD -MT CMakeFiles/grpc.dir/src/core/ext/filters/deadline/deadline_filter.cc.o -MF CMakeFiles/grpc.dir/src/core/ext/filters/deadline/deadline_filter.cc.o.d -o CMakeFiles/grpc.dir/src/core/ext/filters/deadline/deadline_filter.cc.o -c /wrkdirs/usr/ports/devel/grpc/work/grpc-1.62.1/src/core/ext/filters/deadline/deadline_filter.cc" https://pdr2.bofh.network/data/140-diizzy/2024-04-08_00h13m17s/logs/grpc-1.62.1,2.log