Update to C++20

[kab163]

No description provided.

[kab163]

It seems there are some C++20 compatibility issues in the current version of fmt, so will see how updating to 11.1.0 works

[adayton1]

It seems there are some C++20 compatibility issues in the current version of fmt, so will see how updating to 11.1.0 works

I would suggest updating to 12.1.0.

[kab163]

On matrix, these commands work:
$ ml cmake/3.30.5
$ ml cuda/12.9.1
$ ml clang/19.1.3-magic
$ cmake -DENABLE_CUDA=On -DCMAKE_CXX_COMPILER=clang++ -DCMAKE_C_COMPILER=clang -DCMAKE_CUDA_ARCHITECTURES=90 -DBLT_CXX_STD=c++20 -DENABLE_OPENMP=On -DCUDA_TOOLKIT_ROOT_DIR=/usr/tce/packages/cuda/cuda-12.9.1 -DCMAKE_CUDA_COMPILER=/usr/tce/packages/cuda/cuda-12.9.1/bin/nvcc ../

So to make C++20 work, we need an updated default cmake version - at least 3.25+
cmake/3.25.2 + cuda/12.2.2 + clang/14.0.6 also works

[kab163]

Waiting until after the Feb 3rd RAJA discussion (regarding C++20) for merging....
Will merge when ready!

[rhornung67]

Waiting until after the Feb 3rd RAJA discussion (regarding C++20) for merging....

@kab163 I think our plan is to require C++20 for the next RAJA Suite release (mid-summer) and we currently test with it. So go ahead and merge as you wish.