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Because of LWG 467, std::char_traits<char>::lt compares the values cast to unsigned char rather than char, so even when char is signed we get unsigned comparision. std::char_traits<char>::compare uses __builtin_memcmp and that works the same, but during constexpr evaluation we were calling __gnu_cxx::char_traits<char_type>::compare. As char_traits::lt is not virtual, __gnu_cxx::char_traits<char_type>::compare used __gnu_cxx::char_traits<char_type>::lt rather than std::char_traits<char>::lt and thus compared chars as signed if char is signed. This change fixes it by inlining __gnu_cxx::char_traits<char_type>::compare into std::char_traits<char>::compare by hand, so that it calls the right lt method. 2021-02-23 Jakub Jelinek <jakub@redhat.com> PR libstdc++/99181 * include/bits/char_traits.h (char_traits<char>::compare): For constexpr evaluation don't call __gnu_cxx::char_traits<char_type>::compare but do the comparison loop directly. * testsuite/21_strings/char_traits/requirements/char/99181.cc: New test.
file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.