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

[Florian Weimer <fweimer@redhat.com>]

* Paul Eggert:

> On 2024-05-27 03:35, Florian Weimer wrote:
>> Does this turn on experimental language modes by default?  That's
>> probably not what we want.
>
> What do C++ developers want these days? Autoconf should have a
> reasonable default, and C++11 is surely not a good default anymore.

It's still a good default for GCC 5.

GCC developers will correct me, but I think the default C++ dialect is
updated to a newer version once the implementation is reasonably
complete and bugs have been ironed out.

This is different from the C front end, where it took close to 40 years
(from the introduction of void * into C) to activate type checking for
pointer types by default.

>> It would be better to have an option to raise the C++ mode to at least a
>> certain revision, and otherwise use the default.
>
> That option is already available. For example, a builder who doesn't
> want C++23 can use './configure ac_cv_prog_cxx_cxx23=no', and a
> developer can discourage C++23 by putting ':
> ${ac_cv_prog_cxx_cxx23=no}' early in configure.ac.

But that is not the same thing.  If a project uses C++14 constructs,
wouldn't it make sense to tell configure to try to get (likely
experimental) support for it if the compiler does not enable C++14 by
default?  And if the system is already at C++17, leave it at that?

Setting C++14 unconditionally could be incompatible with used system
libraries, which assume C++17 support because the distribution is aware
that the system compiler supports C++17.

Thanks,
Florian