Re: configure adds -std=gnu++11 to CXX variable

[Jakub Jelinek <jakub@redhat.com>]

On Tue, May 28, 2024 at 07:35:43AM -0700, Paul Eggert wrote:
> On 2024-05-28 01:20, Jonathan Wakely wrote:
> > I am not aware of any distro ever changing the default -std setting for g++
> > or clang++. Are you attempting to solve a non-problem, but introducing new
> > ones?
> 
> If it's a non-problem for C++, why does Autoconf upgrade to C++11 when the
> default is C++98? Autoconf has done so since Autoconf 2.70 (2020), with
> nobody complaining as far as I know.
> 
> Was the Autoconf 2.70 change done so late that it had no practical effect,
> because no distro was defaulting to C++98 any more? If so, it sounds like

That seems to be the case.
Dunno about clang++ defaults, but GCC defaults to
-std=gnu++98 (before GCC 6), or
-std=gnu++14 starting with GCC 6 (April 2016), or
-std=gnu++17 starting with GCC 11 (April 2021).
So, if autoconf in 2020 c++98 default to c++11, bet it didn't affect almost
anything.  RHEL 7 uses GCC 4.8, which partially but not fully supports c++11
(e.g. GCC which is written in C++11 these days can build using GCC 4.8.5
as oldest compiler, has to use some workarounds because the C++11 support
isn't finished there, the library side even further from that).  And trying
to enable C++11 on GCC 4.4 (RHEL 6) wouldn't be a good idea, too much was
missing there.
With C++20 in GCC 14, from core most of the features are in except that
modules still need further work, for library side at least cppreference
says it is complete too but I think the ABI hasn't been declared stable yet,
so C++17 is still the default, dunno if it will change in GCC 15 or not.

> Autoconf should go back to its 2.69 behavior and not mess with the C++
> version as that's more likely to hurt than help.

Yes.

> For background on that Autoconf 2.70 change, see this 2013 thread:
> 
> https://lists.gnu.org/r/autoconf/2013-01/msg00016.html

From what I can see, the change was proposed when the C++11 support wasn't
complete and didn't expect compilers will actually change their defaults
when the support is sufficiently stable.
Note, even for C GCC updates the default, -std=gnu99 default was changed to
-std=gnu11 in GCC 5 (April 2015) and -std=gnu17 in GCC 8 (May 2018).
-std=gnu23 support is still incomplete even in GCC 14.

	Jakub