Here’s the warnings counts (by standards level) from Boost. 1.90.0: (Apple clang 17): logs/clang11-build.txt: 34 logs/clang14-build.txt: 40 logs/clang17-build.txt: 40 logs/clang20-build.txt: 44 logs/clang23-build.txt: 48 logs/clang2c-build.txt: 48 Here are the same numbers from Boost 1.91.0 (Apple clang 21) logs/clang11-build.txt: 79 logs/clang14-build.txt: 91 logs/clang17-build.txt :91 logs/clang20-build.txt: 99 logs/clang23-build.txt: 103 logs/clang2c-build.txt: 3167 Most of the bugs fall into three bins: https://github.com/boostorg/proto/issues/37 https://github.com/bfgroup/b2/issues/566 And several libraries (at least regex and spirit) instantiate std::char_traits types that are not ‘characters’ Example: /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__fwd/string.h:45:41: warning: 'char_traits<unsigned char>' 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 a future release, so please migrate off of it. [-Wdeprecated-declarations] 45 | template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT> > | ^ ./boost/xpressive/traits/cpp_regex_traits.hpp:321:18: note: in instantiation of default argument for 'basic_string<char_type>' required here 321 | typedef std::basic_string<char_type> string_type; | ^~~~~~~~~~~~~~~~~~~~~~~ ./boost/xpressive/traits/cpp_regex_traits.hpp:673:22: note: in instantiation of template class 'boost::xpressive::cpp_regex_traits<unsigned char>' requested here 673 | inline unsigned char cpp_regex_traits<unsigned char>::hash(unsigned char ch) | ^ And /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__fwd/string.h:45:41: warning: 'char_traits<unsigned int>' 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 a future release, so please migrate off of it. [-Wdeprecated-declarations] 45 | template <class _CharT, class _Traits = char_traits<_CharT>, class _Allocator = allocator<_CharT> > | ^ ./boost/spirit/home/support/utf8.hpp:24:18: note: in instantiation of default argument for 'basic_string<ucs4_char>' required here 24 | typedef std::basic_string<ucs4_char> ucs4_string; | ^~~~~~~~~~~~~~~~~~~~~~~ — Marshall