https://github.com/cpp-netlib/cpp-netlib/blob/552ce94bd91c055f11ba524adf0ca0712063d711/libs/network/src/server_request_parsers_impl.cpp#L21C1-L21C8

In libc++ 19.0.0 release notes (https://releases.llvm.org/19.1.0/projects/libcxx/docs/ReleaseNotes.html):

The base template forstd::char_traits has been removed in LLVM 19. If you are usingstd::char_traits with types other thanchar,wchar_t,char8_t,char16_t,char32_t or a custom character type for which you specializedstd::char_traits, your code will stop working. The Standard does not mandate that a base template is provided, and such a base template is bound to be incorrect for some types, which could currently cause unexpected behavior while going undetected.

In NixOS, this is creating build failures for us when buildingcpp-netlib-0.13.0-final with clang:

In file included from /tmp/nix-build-cpp-netlib-0.13.0-final.drv-0/source/libs/network/src/server_request_parsers_impl.cpp:10:In file included from /tmp/nix-build-cpp-netlib-0.13.0-final.drv-0/source/boost/network/protocol/http/server/impl/parsers.ipp:5:In file included from include/boost/spirit/include/qi.hpp:16:In file included from include/boost/spirit/home/qi.hpp:14:......In file included from /nix/store/kx521w0j19ssvmcw8gfziiszjgm2bf8x-libcxx-19.1.7-dev/include/c++/v1/__format/parser_std_format_spec.h:39:/nix/store/kx521w0j19ssvmcw8gfziiszjgm2bf8x-libcxx-19.1.7-dev/include/c++/v1/string:2538:5: error: implicit instantiation of undefined template 'std::char_traits<unsigned int>'

but I note that in yourmain it has been relocated to the above file with minimal changes so you're likely to have the same issues.