error with compiling clang 19, c++20 and format header

[gvollant]

I've installed clang-19 on a fresh ubuntu 24.04, using llvm.sh from https://apt.llvm.org/

I've an error compiling a c++ simple source using format header

gvollant@gv-w11x64:/mnt/y/avir$ cat format.cpp
#include <format>
#include <iostream>

int main()
{
    return 0;
}

gvollant@gv-w11x64:/mnt/y/avir$
gvollant@gv-w11x64:/mnt/y/avir$
gvollant@gv-w11x64:/mnt/y/avir$ clang++-19  -std=c++20 -Wall format.cpp
In file included from format.cpp:1:
In file included from /usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/format:58:
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/unicode.h:807:4: error: expression is not assignable
  807 |           ++this;
      |           ^ ~~~~
1 error generated.
gvollant@gv-w11x64:/mnt/y/avir$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 24.04 LTS
Release:        24.04
Codename:       noble
gvollant@gv-w11x64:/mnt/y/avir$ clang++-19 --version
Ubuntu clang version 19.1.0 (++20240805101653+d033ae172d1c-1~exp1~20240805101708.18)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin

no problem using c++17 instead c++20

[keinflue]

This is a known bug in GCC's libstdc++ (which you are using as standard library implementation). It has been resolved for GCC 14.2 (you are using 14.1), see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115119.

For why Clang 19 fails on this while other versions didn't, see #92439 and especially the release note at https://github.com/llvm/llvm-project/blob/430b2545032db9de7898444502915f89e20f7c4c/clang/docs/ReleaseNotes.rst#c-specific-potentially-breaking-changes.

[gvollant]

this mean ubuntu must update their header

[keinflue]

Yes, they have it already on 24.10 (https://packages.ubuntu.com/oracular/libstdc++-14-dev), but not yet backported to 24.04 (https://bugs.launchpad.net/ubuntu/+source/gcc-14/+bug/2073390).

Or you compile with libc++ instead of libstdc++ by adding -stdlib=libc++ (assuming it is installed).

[gvollant]

we can install libc++ using apt pakcage with

apt install libc++-19-dev

@helicos441

[gvollant]

test made October 19, 2024 with updated ubuntu 24.04.1 and claing 19.1.2, problem fixed

unicode header is fixed