writetest fails to compile with clang-19

[paparodeo]

/nix/store/y101iwcfapmp9i0nrmlkpda500xyb2r8-libcxx-19.1.3-dev/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits'
820 | static_assert(is_same<_CharT, typename traits_type::char_type>::value,
| ^
/tmp/nix-build-rapidjson-unstable-2024-04-09.drv-0/source/test/unittest/writertest.cpp:394:36: note: in instantiation of template class 'std::basic_string' requested here
394 | EXPECT_FALSE(writer.String(s));
| ^
/nix/store/y101iwcfapmp9i0nrmlkpda500xyb2r8-libcxx-19.1.3-dev/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
| ^

https://releases.llvm.org/19.1.0/projects/libcxx/docs/ReleaseNotes.html

The base template for std::char_traits has been removed in LLVM 19. If you are using std::char_traits with types other than char, wchar_t, char8_t, char16_t, char32_t or a custom character type for which you specialized std::char_traits, your code will stop working.