disable using c++ 20 iota-view and always use boost

[tjhei]

gcc-10 does not contain the iterator typedef, which makes the usage
somewhat incompatible to what we want. For now, disable the c++20
support.

see #10426

[tamiko]

I agree that this should be the way forward for now. I had multiple compilation failures (not just step-69) with our use of std::iota_view in the library. On second inspection - I had only step-69 failing so far.

[tamiko]

I think the main issue we faced is the fact that iota_view<...>::end() does return an object of type iota_view<...>::sentinel and not iota_view<...>::iterator.

[masterleinad]

std::iota_view::iterator is exposition-only and private in a reference implementation, see https://github.com/cplusplus/draft/releases/download/n4861/n4861.pdf [[range.iota.overview]]. I don't think it was necessary to disable it here, it will just not change in the future.

[bangerth]

We can revisit the whole issue in a couple of years when there are good C++20 implementations around.

[bangerth]

That said, do we ever use the iterator type? I thought #10426 explicitly got rid of it.

[masterleinad]

That said, do we ever use the iterator type? I thought #10426 explicitly got rid of it.

Correct, that was the only thing not working with gcc-10 (which makes sense if I read the standard correctly). If we really want this class to be forward-compatible, I would prefer to actually try to use std::ranges::iota_view if we can.

[bangerth]

And we were -- it worked, with the exception of that one place which you fixed :-)