[c++] Use C++20

[johnkerl]

Context:

• Core libtiledb is already built using C++20 as of 2023: C++20 TileDB-Inc/TileDB#4034
• We would like to also build libtiledbsoma using C++20
  • General currency with the community
  • Currency with core
  • Access to newer language features
• PR: [c++/python/r] C++20 experiment [WIP] #3153
• [sc-57301]
• Build failure: fmt::format in non-constexpr function fmtlib/fmt#4179

Issues:

• [c++/python/r] C++20 experiment [WIP] #3153
• Works on Linux fine
• On MacOS: https://github.com/single-cell-data/TileDB-SOMA/actions/runs/11256492779/job/31298502792#step:10:36

using C++ compiler: ‘g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0’
[19](https://github.com/single-cell-data/TileDB-SOMA/actions/runs/11256492779/job/31298502792#step:10:20)using C++20
[20](https://github.com/single-cell-data/TileDB-SOMA/actions/runs/11256492779/job/31298502792#step:10:21)g++ -std=gnu++20 -I"/usr/share/R/include" -DNDEBUG -I. -I../inst/include/ -I/__w/TileDB/TileDB/dist/include -I/usr/local/include -I/usr/local/include/tiledbsoma    -I'/usr/lib/R/site-library/Rcpp/include' -I'/usr/lib/R/site-library/RcppSpdlog/include' -I'/usr/lib/R/site-library/RcppInt64/include' -I'/usr/lib/R/site-library/nanoarrow/include'     -fpic  -g -O2 -ffile-prefix-map=/build/r-base-2y82rL/r-base-4.4.1=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2   -c RcppExports.cpp -o RcppExports.o
[21]In file included from ./rutilities.h:6,
[22]                 from ../inst/include/tiledbsoma_types.h:25,
[23]                 from RcppExports.cpp:4:
[24]/usr/lib/R/site-library/RcppSpdlog/include/spdl.h: In instantiation of ‘void spdl::trace(const char*, Args&& ...) [with Args = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >}]’:
[25]./rutilities.h:43:24:   required from here
[26]/usr/lib/R/site-library/RcppSpdlog/include/spdl.h:34:92:   in ‘constexpr’ expansion of ‘fmt::v10::basic_format_string<char, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >(fmt)’
[27]/usr/lib/R/site-library/RcppSpdlog/include/spdl.h:34:92: error: ‘fmt’ is not a constant expression
[28]   34 |     inline void trace(const char* fmt, Args&&... args ) { RcppSpdlog::log_trace(fmt::format(fmt, std::forward<Args>(args)... ).c_str()); }
[29]      |                                                                                 ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[30]make: *** [/usr/lib/R/etc/Makeconf:204: RcppExports.o] Error 1
[31]ERROR: compilation failed for package ‘tiledbsoma’
[32]* removing ‘/tmp/RtmpCicerS/Rinst1de8576f09f4/tiledbsoma’
[33]      -----------------------------------
[34]ERROR: package installation failed
[35]

I was able to repro this on my Mac. Using latest RcppSpdlog which as of this writing is 0.0.18.

I also tried:

• Git-pull latest https://github.com/fmtlib/fmt and check out latest tag 11.0.2
• Git-pull latest https://github.com/eddelbuettel/rcppspdlog
  • Copy the 11.0.2 fmt headers into local rcppspdlog's inst/include/spdlog/fmt/bundled
  • Do a local R install
  • Problem remains

This suggests that we will have a genuine bug report on RcppSpdlog.

Update from googling a bit more: fmtlib/fmt#4179 (comment)

[johnkerl]

Re-opening for tracking/visibility as we've got multiple CI/feedstock issues in flight

[johnkerl]

This needs to remain open. Currently we have C++20 on main and C++17 on release-1.15, with blocking issues including

• [c++/python] Update minimum macOS version to 13.3 #3361
• [python] CI broken on main (mitigated: needs follow-up) #3398
• Nightly feedstock build failed TileDB-Inc/tiledbsoma-feedstock#230
• Build r-tiledbsoma with C++ 20 TileDB-Inc/tiledbsoma-feedstock#231

[johnkerl]

@jdblischak @XanthosXanthopoulos what are the remaining PRs to merge?

[jdblischak]

I can only comment on the feedstock perspective. I think we are ready to support C++20. In the pre-check and then actual update PRs for a C++20 supported release, you can follow the instructions I detailed at TileDB-Inc/tiledbsoma-feedstock#246 (comment) to migrate the feedstock to C++20 (you'll also need to run conda smithy rerender afterwards).

Lingering doubts:

• The work to fix the dynamic linking of libspdlog is a WIP ([c++] Support dynamic linking to libspdlog #3585, Dynamically link to conda-installed libspdlog TileDB-Inc/tiledbsoma-feedstock#252). However, my view is that we can migrate to C++20 first and then improve the libspdlog linking in future versions
• We can't actually upload any new conda binaries to anaconda.org at the moment, so there is no rush for a new SOMA release (Nightly feedstock build failed TileDB-Inc/tiledbsoma-feedstock#254)

[johnkerl]

@jdblischak there will be a 1.15.4 today (heads up) with spatial updates but I've no desire to get C++20 mods into it

And for today it will suffice to get PyPI artifacts; Conda 1.15.4 can wait.

[johnkerl]

I can only comment on the feedstock perspective

@jdblischak the feedstock issues were the only remaining blocker -- so we are good

I need to tag a 1.15.4 today, as-is -- I'll follow your above guidance for 1.15.5 or 1.16.0 whichever comes first :)