GitHub-CI: test CPP17

[peterrum]

As discussed with @tjhei.

Tempted by #13497 and #13502.

[tjhei]

Good idea!

[marcfehling]

We could also use the CMAKE functionality. Is there a difference? Might be just portability.
I'm actually undecided what I prefer personally :-)

Suggested change

	-D DEAL_II_CXX_FLAGS='-Werror -std=c++17' \
	-D CMAKE_CXX_STANDARD=17 \
	-D DEAL_II_CXX_FLAGS='-Werror' \

[peterrum]

I have changed this!

[bangerth]

This just switches one of the testers to C++17. Ideally, or course, we would add a separate CI step that adds a C++17 tester, but that increases the wait time we have (or exceeds whatever limit we are allotted). How did you choose this particular tester to switch to C++17? I supposed this leaves enough testers who continue to check C++14.

[peterrum]

This just switches one of the testers to C++17. Ideally, or course, we would add a separate CI step that adds a C++17 tester, but that increases the wait time we have (or exceeds whatever limit we are allotted). How did you choose this particular tester to switch to C++17? I supposed this leaves enough testers who continue to check C++14.

This is how we normally do it. If we see that something is not tested (in the past: python bindings, 64-bit indices), we just modify one test. We still have enough tests with C++14. Adding a new test would be an option, but would mean longer queue times.

[masterleinad]

I don't think setting CMAKE_CXX_STANDARD works. I still see

#        C++ language standard:  C++14

in the respective CI run. We should rather do DEAL_II_CXX_FLAGS=-std=c++17.

[marcfehling]

I don't think setting CMAKE_CXX_STANDARD works. I still see

#        C++ language standard:  C++14

in the respective CI run. We should rather do DEAL_II_CXX_FLAGS=-std=c++17.

Interesting. I think in addition to CMAKE_CXX_STANDARD we would need to set CXX_STANDARD_REQUIRED=ON, see also here. Or we can specify the flags for the gnu compiler directly.

[masterleinad]

I think in addition to CMAKE_CXX_STANDARD we would need to set CXX_STANDARD_REQUIRED=ON, see also here. Or we can specify the flags for the gnu compiler directly.

That isn't enough for me. I think I remember that #10307 ignores the CMAKE_CXX_STANDARD completely and only relies on flags we pass and the default behavior of the compiler to decide what kind of language version support we have.

[peterrum]

CI says now: C++ language standard: C++17!

[bangerth]

Nice! Now we just need a tester for C++20 ;-)

[peterrum]

Nice! Now we just need a tester for C++20 ;-)

@bangerth Couldn't we just simply convert the C++17 to a C++20 test? What would we miss?

[bangerth]

We probably could, but I don't think we use any C++20 features yet (even with the appropriate #ifdef). So I think there is, at least for now, not a lot of use for this.

But that might change in the future, of course. Do we have a tester that already has a compiler that's new enough? I think we'd need GCC 11 at least.