Re: [sr #110286] Make it possible to request a specific (non-latest) version of a language standard

[Dima Pasechnik]

On Mon, Jul 27, 2020 at 02:31:38PM -0400, Zack Weinberg wrote:
> URL:
>   <https://savannah.gnu.org/support/?110286>
> 
>                  Summary: Make it possible to request a specific (non-latest)
> version of a language standard
>                  Project: Autoconf
>             Submitted by: zackw
>             Submitted on: Mon 27 Jul 2020 06:31:37 PM UTC
>                 Category: None
>                 Priority: 5 - Normal
>                 Severity: 1 - Wish
>                   Status: None
>                  Privacy: Public
>              Assigned to: None
>         Originator Email: 
>              Open/Closed: Open
>          Discussion Lock: Any
>         Operating System: None
> 
>     _______________________________________________________
> 
> Details:
> 
> Feedback on the 2.69b beta indicates that users find the new behavior of
> AC_PROG_CC and AC_PROG_CXX, automatically selecting the latest supported
> language standard, problematic.  Quoting
> https://lists.gnu.org/archive/html/autoconf/2020-07/msg00010.html :
> 
> > One issue we [PostgreSQL] would like to point out is that
> > the new scheme of automatically checking for the latest
> > version of the C and C++ standards (deprecating AC_PROG_CC_C99
> > etc.) is problematic...
> > 
> > [W]e have set C99 as the project standard. So checking for
> > C11 is (a) useless, and (b) bad because we don't want
> > developers to accidentally make use of C11 features and have
> > the compiler accept them silently.
> >
> > It looks like it's not easy to disable these tests even
> > with low-level hackery. It would be better if there were
> > still a way to set the preferred level of C and C++ in
> > an official way.
> 
> and https://lists.gnu.org/archive/html/autoconf/2020-07/msg00012.html :
> 
> > Particularly as applies to C++, it is useful to be able
> > to set an upper bar (and lower bar) for the C++ version
> > requested because C++11 is in many ways not similar to
> > older C++. I am already aware that some of my own software
> > will not compile with C++11 because of removed deprecated
> > features which I have not attended to yet in the code.
> > Furthermore, the ABI often changes between C++ versions
> > and it is useful to be able to specify the C++ version
> > for purposes of the ABI.

In regard of C++, we (in SageMath, https://sagemath.org) are using 
AX_CXX_COMPILE_STDCXX from
 https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
to do version detection of C++.

Perhaps it can be promoted to the core AC_ macros.

Just in case,
Dmitrii
> 
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> 
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