[R-package] CRAN warnings about use of 'char_traits<fmt::detail::char8_type>'

[jameslamb]

Description

I received the following email from CRAN today. They found a warning raised when {lightgbm} is compiled with clang-18, and are giving us 14 days to fix it or risk the project being archived.

Please see the problems shown on
https://cran.r-project.org/web/checks/check_results_lightgbm.html.

Please correct before 2024-01-24 to safely retain your package on CRAN.

Do remember to look at the 'Additional issues'.

The CRAN Team

That is about this compilation warning:

/usr/local/clang-trunk/bin/clang++ -std=gnu++17 -I"/data/gannet/ripley/R/R-clang-trunk/include" -DNDEBUG -I./include -DEIGEN_MPL2_ONLY -DEIGEN_DONT_PARALLELIZE -DMM_PREFETCH=1 -DMM_MALLOC=1 -DUSE_SOCKET -DLGB_R_BUILD  -isystem /usr/local/clang-trunk/include -I/usr/local/clang/include   -fopenmp -pthread -fpic  -O3 -Wall -pedantic -frtti -Wp,-D_FORTIFY_SOURCE=3  -c treelearner/serial_tree_learner.cpp -o treelearner/serial_tree_learner.o
In file included from treelearner/linear_tree_learner.cpp:5:
In file included from treelearner/linear_tree_learner.h:8:
In file included from /usr/local/clang-trunk/bin/../include/c++/v1/string:625:
In file included from /usr/local/clang-trunk/bin/../include/c++/v1/string_view:213:
/usr/local/clang-trunk/bin/../include/c++/v1/__fwd/string_view.h:22:41: warning: 'char_traits<fmt::detail::char8_type>' is deprecated: char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 19, so please migrate off of it. [-Wdeprecated-declarations]
   22 | template <class _CharT, class _Traits = char_traits<_CharT> >
      |                                         ^
./include/LightGBM/fmt/core.h:344:26: note: in instantiation of default argument for 'basic_string_view<fmt::detail::char8_type>' required here
  344 | template <typename Char> using std_string_view = std::basic_string_view<Char>;
      |                          ^~~~~
./include/LightGBM/fmt/core.h:435:50: note: in instantiation of template type alias 'std_string_view' requested here
  435 |                                       S, detail::std_string_view<Char>>::value)>
      |                                                  ^
./include/LightGBM/fmt/format.h:777:57: note: in instantiation of template class 'fmt::basic_string_view<fmt::detail::char8_type>' requested here
  777 | inline auto compute_width(basic_string_view<char8_type> s) -> size_t {
      |                                                         ^
/usr/local/clang-trunk/bin/../include/c++/v1/__string/char_traits.h:81:8: note: 'char_traits<fmt::detail::char8_type>' has been explicitly marked deprecated here
   81 | struct _LIBCPP_DEPRECATED_(
      |        ^
/usr/local/clang-trunk/bin/../include/c++/v1/__config:933:53: note: expanded from macro '_LIBCPP_DEPRECATED_'
  933 | #      define _LIBCPP_DEPRECATED_(m) __attribute__((__deprecated__(m)))
      |                                                     ^

(R CMD check logs from CRAN)

Reproducible example

This is happening on CRAN's builds with clang-18: https://www.stats.ox.ac.uk/pub/bdr/clang18/lightgbm.log

I'll try to add CI here that reproduces it. CRAN's "Additional Issues" documentation (link) says the following for clang-18:

Tests as for the main fedora-clang checks but using LLVM pre-18
including flang-new as the Fortran compiler.
Other details as for 
https://www.stats.ox.ac.uk/pub/bdr/Rconfig/r-devel-linux-x86_64-fedora-clang .

Environment info

LightGBM version or commit hash: v4.2.0

Additional Comments

Trying to reproduce this might benefit from #6116.

Related conversations about this warning:

• Deprecated: char_traits<T> in LLVM 18 jtv/libpqxx#738 (comment)
• https://reviews.llvm.org/D138307
• Velox build failing on macOS Sonoma (Xcode 15) with deprecated warnings as errors facebookincubator/velox#7235

[jameslamb]

Assigned this to myself. I'm investigating it right now.

Anyone else reading this... please do post if you have ideas!

[jameslamb]

Decided to leave this open until we see a new release to CRAN pass checks.

[jameslamb]

Happy to say v4.3.0, which contains the fix from #6265, has been accepted to CRAN.

It's passing all but one checks (just waiting for r-patched-linux-x86_64 to run), and the clang18 warning is no longer shown there.

https://cran.r-project.org/web/checks/check_results_lightgbm.html

This can be closed.