Re: isl 0.26 C++17 issues when bootstrapping with GCC

[Iain Sandoe <idsandoe@googlemail.com>]

This 

> On 30 Aug 2023, at 00:32, Ben Boeckel via Gcc <gcc@gcc.gnu.org> wrote:
> 
> On Tue, Aug 29, 2023 at 18:57:37 +0200, Richard Biener wrote:
>> I suppose for bootstrapping we could disable ISL during stage1 since
>> it enables an optional feature only.  Other than that GCC only
>> requires a C++11 compiler for building, so ISL breaks that constraint
>> with requiring C++17.
> 
> Note that it doesn't *require* it per sé; the tests that try it are
> compiled if C++17 support was detected at all. The headers seem to just
> have optional header-only `std::any`-using APIs if C++17 is around.
> `isl` supporting a flag to disable the tests would also work, but that
> doesn't fix 0.26. It also doesn't mean it won't start requiring C++17 at
> some point in the future.

Perhaps, in the short-term (i.e. before it requires C++ > 11) we can
solve this by ensuring that we pass -std=c++11 to the configure stages
as well as to the build.  ISTM that configure is finding C++17-capability
(because we do not, I think, force C++11 for the configure) and then 
the build takes it away by forcing -std=c++11.

[ will try this out ]

> In light of that, I feel that skipping it for bootstrap is probably the
> right solution here. Alas, my skill with autotools is closer to the
> caveman-with-club level rather than that of a surgeon.

I am not sure we have an easy way to exclude a host module from
stage1 only (but ICBW).

Iain

> 
> --Ben