Incompatibility between clang++ 14 and libstdc++ 12

[berolinux]

This sample code:

#include <iomanip>
#include <iostream>

int main(int argc, char **argv) {
        std::tm tm = {};
        std::stringstream ss("2022-03-18 01:02:03");
        ss >>std::get_time(&tm, "%Y-%m-%d %H:%M:%S");
        std::cout << std::asctime(&tm);
}

works perfectly when built with gcc 12.0, or when built with clang 14 using libc++ or libstdc++ versions prior to 12.0 -- but when using clang with libstdc++ 12.0 (20220313 snapshot), it results in

In file included from test.cc:1:
In file included from /usr/bin/../lib64/gcc/x86_64-openmandriva-linux-gnu/12.0.0/../../../../include/c++/12.0.0/iomanip:43:
In file included from /usr/bin/../lib64/gcc/x86_64-openmandriva-linux-gnu/12.0.0/../../../../include/c++/12.0.0/locale:41:
In file included from /usr/bin/../lib64/gcc/x86_64-openmandriva-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/locale_facets_nonio.h:2069:
/usr/bin/../lib64/gcc/x86_64-openmandriva-linux-gnu/12.0.0/../../../../include/c++/12.0.0/bits/locale_facets_nonio.tcc:1477:18: error: reference to non-static member function must be called
      if ((void*)(this->*(&time_get::do_get)) == (void*)(&time_get::do_get))
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/bin/../lib64/gcc/x86_64-openmandriva-linux-gnu/12.0.0/../../../../include/c++/12.0.0/iomanip:433:20: note: in instantiation of member function 'std::time_get<char>::get' requested here
              __mg.get(_Iter(__is.rdbuf()), _Iter(), __is,
                   ^
test.cc:7:5: note: in instantiation of function template specialization 'std::operator>><char, std::char_traits<char>>' requested here
        ss >>std::get_time(&tm, "%Y-%m-%d %H:%M:%S");
           ^
1 error generated.

[philnik777]

This looks like a clang issue and not a libc++ issue to me. Am I correct @berolinux?

[berolinux]

yes, libc++ isn't involved, this happens when using libstdc++ from gcc 12.
It's also only triggered with -fgnuc-version set to something newer than 5.0.

clang doesn't implement a language extension implemented in gcc and icc, which libstdc++ assumes is there if GNUC >= 5

[philnik777]

I don't know exactly which label should be used for clang. I hope someone who knows it adds the correct one.

[ms178]

Just for additional context, the gcc folks implemented a workaround on their side for this recently: gcc-mirror/gcc@e63ac86

[efriedma-quic]

Could you give some context about why you're using -fgnuc-version=?

[berolinux]

Workaround for legacy cruft doing the likes of

#if __GNUC__ > (some number between 5 and 12)
// do something same that would work with any fairly recent compiler
#else
// do something else that is much slower but happens to work with gcc 4.x or other compilers from that time frame
#endif

gcc 5 was released in 2015 -- lots of progress has been made in both gcc and clang since then. And a lot of things never assumed there would be other compilers than gcc, so they use #if __GNUC__ > ... even where they should be using __cplusplus or something.

[efriedma-quic]

I think the last time the default was discussed was https://discourse.llvm.org/t/whats-the-rationale-to-continue-declare-compatibility-with-gcc-4-2-1/52530 . See also https://discourse.llvm.org/t/rfc-bump-up-clangs-gnuc-minor/23084 ; in particular, clang still doesn't implement __builtin_va_arg_pack(), which is probably still a blocker for messing with the default here.