template<>structValue2StringFormatter<uint8_t>{template<typename T>static std::basic_string<uint8_t>format(const T& val)    {        std::basic_ostringstream<char> ss;        ss << val;returnreinterpret_cast<constuint8_t*>(ss.str().c_str());    }static std::basic_string<uint8_t>format(const utf16string& val)    {returnformat(utility::conversions::utf16_to_utf8(val));    }};

Here it assumes that the standard lib will accomodate instantiating a std::basic_string<uint8_t> or any other T for that matter, but that isn't the case as only the 'regular' char types are specialized for char_traits in the standard.

Current libstdc++ and older libc++ (up to at least libc++18) will accept this code, but not in recent libc++ where only the stipulated types are supported.

This can be isolated further with the following snippet:

Snippet:

// tst.cc#include<cstdint>#include<string>intmain() {auto s1 = std::basic_string<char>{};auto s2 = std::basic_string<uint8_t>{};}

msvc: OK
gcc: OK
clang+libstdc++: OK
clang+old-libc++: OK
clang+libc++-20: FAIL

$ dpkg -l | egrep 'libc\+\+-[[:digit:]]{2}-dev'ii  libc++-20-dev:amd64                           1:20~++20241203111125+9a4c5a59d4ec-1~exp1~20241203111142.2504 amd64        LLVM C++ Standard library (development files)$ clang++ -v -stdlib=libc++  tst.ccDebian clang version 20.0.0 (++20241203111125+9a4c5a59d4ec-1~exp1~20241203111142.2504)Target: x86_64-pc-linux-gnuThread model: posixInstalledDir: /usr/lib/llvm-20/binFound candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14Candidate multilib: .;@m64Selected multilib: .;@m64 "/usr/lib/llvm-20/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-obj -dumpdir a- -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name tst.cc -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=all -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -funwind-tables=2 -target-cpu x86-64 -tune-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/deb -v -fcoverage-compilation-dir=/home/deb -resource-dir /usr/lib/llvm-20/lib/clang/20 -internal-isystem /usr/lib/llvm-20/bin/../include/c++/v1 -internal-isystem /usr/lib/llvm-20/lib/clang/20/include -internal-isystem /usr/local/include -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/14/../../../../x86_64-linux-gnu/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -ferror-limit 19 -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -fcolor-diagnostics -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /tmp/tst-398935.o -x c++ tst.ccclang -cc1 version 20.0.0 based upon LLVM 20.0.0 default target x86_64-pc-linux-gnuignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/14/../../../../x86_64-linux-gnu/include"ignoring nonexistent directory "/include"#include "..." search starts here:#include <...> search starts here: /usr/lib/llvm-20/bin/../include/c++/v1 /usr/lib/llvm-20/lib/clang/20/include /usr/local/include /usr/include/x86_64-linux-gnu /usr/includeEnd of search list.In file included from tst.cc:2:/usr/lib/llvm-20/bin/../include/c++/v1/string:831:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned char>'  831 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,      |                                          ^tst.cc:6:13: note: in instantiation of template class 'std::basic_string<unsigned char>' requested here    6 |   auto s2 = std::basic_string<uint8_t>{};      |             ^/usr/lib/llvm-20/bin/../include/c++/v1/__fwd/string.h:23:29: note: template is declared here   23 | struct _LIBCPP_TEMPLATE_VIS char_traits;      |                             ^1 error generated.

Fixes#1812.

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