emacs28-nativecomp breaks clang++

[TerrorJack]

Repro in clean ubuntu:22.04 container:

apt update
apt install -y software-properties-common
add-apt-repository -y ppa:kelleyk/emacs
apt install -y emacs28-nativecomp clang

Try to compile a minimal C++ file like this using clang++ -c foo.cpp -o foo.o:

#include <string>

std::string foo(std::string x) { return x; }

Results in an error:

foo.cpp:1:10: fatal error: 'string' file not found
#include <string>
         ^~~~~~~~
1 error generated.

[dajohans]

That sounds strange. I also use 22.04 and emacs-nativecomp for C++ without issues, regardless of gcc or clang. Since you’re on a clean install, are you certain that you have installed the C++ standard library? The error message about file string not found sounds like the C++ standard library is missing. Clang doesn’t contain its own standard library so you have to install it separately, which is easiest to do by installing g++.

[TerrorJack]

Even after installing g++, same error occurs. But using g++ to compile it yields no error.

[dajohans]

Wait now I remember that I had a similar problem recently, but with clangd and not clang. But clangd uses clang, so I suppose the problem probably was with clang. I had installed the packages called gcc and g++, which defaults to gcc-11 and g++-11. But clangd was looking for include files in the place where g++-12 would have put its standard library, not where g++-11 would have put it. So what I did was to specifically install g++-12 and then clangd found the include files. Perhaps you could try that too. This is really stupid, but the c++ ecosystem is trash so I guess this is the kinds of things we have to deal with.

I don't remember which command I used to check clang's include paths(and I don't have a terminal available atm, so I can't test). But when I checked that, I remember seeing that it does know about both the path to the g++-11 and g++-12 library include paths(even before g++-12 was installed) and the command said afterwards that clang chose to use the path to the g++-12 library, even though g++-12 wasn't installed at the time.

Another problem I had yesterday was that the g++-12 which was intalled with apt was not the latest one that Ubuntu offers. I don't know why this happened. I could only install g++ 12.0.1, but g++ 12.1 is available. But by adding this ppa (which I believe is Canonical's own ppa) I could install the newer g++ 12.1. For example, one problem I had with g++ 12.0.1 is that std::expected didn't exist in C++23. But g++-12 is reported to be the only c++ compiler which has implemented std::expected, so it should work. When upgrading to g++ 12.1 from the ppa, then std::expected did indeed exist.