undefined template 'std::char_traits<unsigned char>' #46
Description
Attempting to build dptf (revision 00b9ac1) with recent libc++ fails with errors like the following:
In file included from /tmp/dptf/DPTF/Sources/Libraries/ConfigurationFileContent/ConfigurationFileContent.cpp:21:
/tmp/dptf/DPTF/Linux/Libraries/../../Sources/ThirdParty/nlohmann_json/json.hpp:6221:19: error: implicit instantiation of undefined template 'std::char_traits<unsigned char>'
6221 | typename std::char_traits<char_type>::int_type get_character()
| ^
/tmp/dptf/DPTF/Linux/Libraries/../../Sources/ThirdParty/nlohmann_json/json.hpp:23789:19: note: in instantiation of template class 'nlohmann::detail::iterator_input_adapter<std::__wrap_iter<const unsigned char *>>' requested here
23789 | auto ia = detail::input_adapter(std::forward<InputType>(i));
| ^
/tmp/dptf/DPTF/Sources/Libraries/ConfigurationFileContent/ConfigurationFileContent.cpp:137:21: note: in instantiation of function template specialization 'nlohmann::basic_json<>::from_bson<const std::vector<unsigned char> &>' requested here
137 | auto data = json::from_bson(segment);
| ^
/usr/bin/../include/c++/v1/__fwd/string.h:23:29: note: template is declared here
23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
| ^
A specialization of char_traits for char is required to exist, but not for unsigned char. Previously, libc++ had a default implementation of char_traits for arbitrary types, but this has been deprecated for some time and was removed in https://reviews.llvm.org/D157058. As a result, char_traits<unsigned char> now fails to compile.