[EPIC] Move forward to C++20

[untereiner]

What is the requested feature?
I want to open discussions to move forward to c++20

Is your request related to a specific problem?
Below is a list of desirable features that are standardized in C++20.
I also put the min (compiler/stdlib) versions where they are implemented

	clang	gcc
standardized text format (in replacement of fmt)	17	13
range library	15	10
concepts library	13	10
span	7	10
litterals as template params	12	9

I would like to discuss the feasibility of this change on the different architectures that geos is compiled on..

Describe the solution you'd like
n.a.

Describe alternatives you've considered
n.a.

Additional context
n.a.

[CusiniM]

We are mostly ready for this. It's part of the reason why we started compiling with cuda12. The only oustanding issues/questions are:

• do all target platforms have cuda12 and compilers that are new enough for novel c++20 features? I am not sure how to get this answer.
  • TotalEnergies @herve-gross ?
  • LLNL yes
  • Chevron @paveltomin ?
• do the target AMD systems support c++20? I think the answer is yes but @wrtobin can probably confirm this.
• the geos cuda12 builds on rockylinux have a compilation error that we have not been able to solve (see

  GEOS/.github/workflows/ci_tests.yml

  Line 297 in a7ddc53

  # compiler error in ElasticFirstOrderWaveEquationSEMKernel::StressComputation::launch in call to FE_TYPE::computeFirstOrderStiffnessTermX

  ). This should be resolved before fully moving to c++20.
• we should upate our CI images to make sure they all have adequate compilers and cuda versions (this is the easiest part).

[untereiner]

Thanks @CusiniM !
The question raised today in the team while implementing with string literals and templates.

For TTE maybe @Bubusch can also answer ?

[paveltomin]

• do all target platforms have cuda12 and compilers that are new enough for novel c++20 features?
  • Chevron @paveltomin ?

CC @drmichaeltcvx
I think we do, I've been compiling with gcc 13 recently and it works fine but there are tons of warnings which would be nice to fix

[Bubusch]

@CusiniM @untereiner

Not yet on Pangea 3, I just asked to install cuda12 and gcc12 so we can make the switch.
Pangea 4 is OK.

FYI @sframba @herve-gross