Update EKAT to require C++20

[tcclevenger]

Update EKAT to require C++20

Motivation

Needed to update to Kokkos 5.0.

Testing

CI already uses C++20.

[bartgol]

Asking the obv question: are we sure that E3SM will build everywhere with C++20 required? If not, this my hold back any ekat update in e3sm.

[rljacob]

It will not build with all of our current default compilers. In particular, the "intel" compiler on Chrysalis. Possibly others.

[bartgol]

Looks good. Only possible concern is whether e3sm is ready for c++20 across all machines.

[tcclevenger]

@bartgol @rljacob Good to know. I created this PR while I'm testing E3SM on the target machines, but since we already know there will be some that are not compatible, we will need to find a different way to handle this.

Kokkos 5 is going to require C++20, what would the process be to eventually update compilers on Chrysalis (and maybe others) so that we can upgrade? Or is this going to be a bigger roadblock? I guess the first step is for me to test exactly which compilers do not work.

[bartgol]

One solution would be to mis with a cmake2.0 way of specifying the std, via something like

if (NOT CMAKE_CXX_STANDARD)
  target_compile_features(ekat_core PUBLIC cxx_std_20)
endif()

Then, in the mach files for old compilers we keep lines like

set (CMAKE_CXX_STANDARD 17)

which will force ekat to honor the required standard. Of course, for machines where we do set 17 in the mach files, we won't be able to build anything that needs kokkos 5.0 (e.g. any F case, since EAM still use kokkos for the SL transport). This means that we cannot switch to kokkos 5 until we can abandon compilers that only support c++17.

[tcclevenger]

On Chrysalis:

• gnu: default compiler is compatible
• oneapi-ifx: version is technically compatible, but we get a build error about not being able to find "concepts" file. Output indicates oneAPI is not correctly configured.
• intel: Kokkos no longer supports INTEL's classic compilers (as of Kokkos 4.6, we currently use Kokkos 4.5). Maybe they are C++20 compliant, but we will need to change anyways to update Kokkos.

[bartgol]

• oneapi-ifx: version is technically compatible, but we get a build error about not being able to find "concepts" file. Output indicates oneAPI is not correctly configured.

Ugh, that's underwhelming. I wonder if this is on the chrys IT folks' radar...

[tcclevenger]

Ugh, that's underwhelming. I wonder if this is on the chrys IT folks' radar...

Yeah, I want to try and make a small reproducer outside of E3SM. Make sure it's not something we are doing.