/tmp/guix-build-gcc-10.1.0.drv-0/build/prev-x86_64-unknown-linux-gnu/libstdc++-v3/config.log
reads this:

--8<---------------cut here---------------start------------->8---
configure:19924: checking for float trig functions
configure:19948:  /tmp/guix-build-gcc-10.1.0.drv-0/build/./gcc/xgcc -shared-libgcc -B/tmp/guix-build-gcc-10.1.0.drv-0/build/./gcc -nostdinc++ -L/tmp/guix-build-gcc-10.1.0.drv-0/build/x86_64-unknown-linux-gnu/libstdc++-v3/src -L/tmp/guix-build-gcc-10.1.0.drv-0/build/x86_64-unknown-linux-gnu/libstdc++-v3/src/.libs -L/tmp/guix-build-gcc-10.1.0.drv-0/build/x86_64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs -B/gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/bin/ -B/gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/lib/ -isystem /gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/include -isystem /gnu/store/jrzxs91zhpf6yr5fxisn3jjj7xai8zlk-gcc-10.1.0/x86_64-unknown-linux-gnu/sys-include   -fno-checking -c -fno-builtin -D_GNU_SOURCE  conftest.cpp >&5
In file included from /gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include/c++/math.h:36,
                 from conftest.cpp:122:
/gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include/c++/cmath:41:10: fatal error: bits/c++config.h: No such file or directory
   41 | #include <bits/c++config.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.
configure:19948: $? = 1
configure: failed program was:

[...]

| /* end confdefs.h.  */
| #include <math.h>
| int
| main ()
| {
| acosf (0); asinf (0); atanf (0); cosf (0); sinf (0); tanf (0); coshf (0); sinhf (0); tanhf (0);
|   ;
|   return 0;
| }
configure:19962: result: no
--8<---------------cut here---------------end--------------->8---

At this point, we have:

--8<---------------cut here---------------start------------->8---
export CPLUS_INCLUDE_PATH=\
"/gnu/store/61pv34q6kad3cii1pngyairvxbxgdm1n-isl-0.22.1/include\
:/gnu/store/35afkywncrr5xsb4cxcljf6rpjcb7f61-gmp-6.2.0/include\
:/gnu/store/5jf395qa3v4amdi60850rz2a15zlsrza-mpfr-4.0.2/include\
:/gnu/store/lgrnkwh7w5yawgqaglwj1pls5vwz1nz7-mpc-1.1.0/include\
:/gnu/store/243algr6h60j46spn5dqhjc4mhkd0a0p-libelf-0.8.13/include\
:/gnu/store/rykm237xkmq7rl1p0nwass01p090p88x-zlib-1.2.11/include\
:/gnu/store/i8h2pcxqdq07ijm3ibkka8f4smn1w48v-bzip2-1.0.8/include\
:/gnu/store/9860f1abqj8wjjnwl8a9v54pdcc3bhgf-xz-5.2.4/include\
:/gnu/store/60g7r3l01fd7c58yjbm6krgcwj1jkpwg-file-5.38/include\
:/gnu/store/swqdvwri9dbv6zssg6v0by7l05hd6wxp-gawk-5.0.1/include\
:/gnu/store/hm40bxnv8jxmbc1lpb7zfimii4xm9m81-make-4.3/include\
:/gnu/store/m1z7cdbqsqyp9xnjw5cvlb4a7gkcg3m4-binutils-2.34/include\
:/gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include/c++\
:/gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include\
:/gnu/store/fa6wj5bxkj5ll1d7292a70knmyl7a0cr-glibc-2.31/include\
:/gnu/store/gfapkk5c6hvl1d94m4sqnhn7f9l5gqyh-linux-libre-headers-5.4.20/include"
--8<---------------cut here---------------end--------------->8---

but <bits/c++config.h> is in a directory not listed here:

--8<---------------cut here---------------start------------->8---
$ find /gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0 -name c++config.h
/gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/include/c++/x86_64-unknown-linux-gnu/bits/c++config.h
--8<---------------cut here---------------end--------------->8---

‘gcc-final’ doesn’t have this problem because it depends on ‘libstdc++’
(separate package) where:

--8<---------------cut here---------------start------------->8---
$ find /gnu/store/v507xkc5flnzqa49yp41w5y611p4lqbg-libstdc++-7.5.0 -name c++config.h
/gnu/store/v507xkc5flnzqa49yp41w5y611p4lqbg-libstdc++-7.5.0/include/bits/c++config.h
--8<---------------cut here---------------end--------------->8---

So somehow the problems seems to be that ‘xgcc’ doesn’t search
‘gcc-7.5.0/include/c++/x86_64-unknown-linux-gnu’.  We could add it to
CPLUS_INCLUDE_PATH manually, but it seems to me we’re missing something.

Thoughts?

Thanks,
Ludo’.