bug#49350: gcc-toolchain compilation error with -nostdinc++

[Sarah Morgensen via Bug reports for GNU Guix <bug-guix@gnu.org>]

Hello,

Thanks for the report.

Ontje Lünsdorf <oluensdorf@mailbox.org> writes:

> Hi all,
>
>
> I've problems compiling LLVM manually with the gcc-toolchain.
> sanitizer_posix_libcdep.cpp is build with -nostdinc++ and fails to find
> bits/c++config.h.
>
> I think the issue can be reproduced with this example test.cpp:
>
> #include <stdlib.h>
> int main() { return 0; }

I am able to repro with this test case on Guix System, both in and
outside of a guix environment --pure.

>
> c++ from archlinux compiles that file, while c++ from gcc-toolchain doesn't:
>
> $ c++ -nostdinc++ test.cpp
> $ guix environment --ad-hoc gcc-toolchain -- c++ -nostdinc++ test.cpp
> In file included from /gnu/store/nh9r04xhzmbk3hjsj92yhlhmjsp521g4-profile/include/c++/stdlib.h:36,
>                  from test.cpp:1:
> /gnu/store/nh9r04xhzmbk3hjsj92yhlhmjsp521g4-profile/include/c++/cstdlib:41:10: fatal error: bits/c++config.h: No such file or directory
>    41 | #include <bits/c++config.h>
>       |          ^~~~~~~~~~~~~~~~~~
> compilation terminated.
>
> Is this expected and I'm doing something wrong?

The issue seems to be with the (automatically set by Guix)
CPLUS_INCLUDE_PATH environment variable. The following works as
expected:

  guix environment --pure --ad-hoc gcc-toolchain -- CPLUS_INCLUDE_PATH= c++ -nostdinc++ test.cpp

It seems that -nostdinc++ controls whether GCC decides to add the C++
std include paths internally, but it still looks in the paths indicated
by CPLUS_INCLUDE_PATH. Guix sets CPLUS_INCLUDE_PATH and friends because
there is no system-wide /include. I'm not sure how expected this
specific behavior is, though...

>
> Thanks & best regards,
> Ontje

Hope that helps,
Sarah