[Octave-bug-tracker] [bug #65327] octave 10 stores sets CXX wrong

[Markus Mützel]

Follow-up Comment #5, bug#65327 (group octave):

IIUC, that is an issue with the autoconf macro AC_PROG_CXX. Specifically, it's
caused by a "feature" that was added here:
https://lists.gnu.org/archive/html/autoconf/2013-01/msg00016.html

That change added some code in a feature test for C++11 that is incompatible
with C++20:
https://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=blob;f=lib/autoconf/c.m4;h=d8eab7bbd6775b89c06b1ba19349ba3946cd7095;hb=HEAD#l2642

Thus, `-std=gnu++11` is added to your `CXX` variable, and it goes downhill
from there.

Tbh, I don't understand the logic for why that was added in the first place.

I also don't know if there is an option to ask autoconf for a working C++
compiler without it (arbitrarily) adding some unnecessary (and in this case
detrimental) flags.

We are already using the macro `AX_CXX_COMPILE_STDCXX` for getting potentially
necessary `-std` flags (which after the first test result adds another
`-std=gnu++17` in your case).

Maybe, try and report the issue to the autoconf developers?



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