[WIP] Fails to build on recent libc++ (20 tested)

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

I tracked down this source file Release/include/cpprest/streams.h

template<>
struct Value2StringFormatter<uint8_t>
{
    template<typename T>
    static std::basic_string<uint8_t> format(const T& val)
    {
        std::basic_ostringstream<char> ss;
        ss << val;
        return reinterpret_cast<const uint8_t*>(ss.str().c_str());
    }

    static std::basic_string<uint8_t> format(const utf16string& val)
    {
        return format(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>

int main() {
  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.cc
Debian clang version 20.0.0 (++20241203111125+9a4c5a59d4ec-1~exp1~20241203111142.2504)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-20/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
Candidate multilib: .;@m64
Selected 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.cc
clang -cc1 version 20.0.0 based upon LLVM 20.0.0 default target x86_64-pc-linux-gnu
ignoring 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/include
End 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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[yuxiang-wen]

收到，谢谢