Fix small issues in preparation for C++20

[phdum-a]

GCC 16 will set C++20 at the default and is expected to be released soon. Currently compiling main with C++20 causes compiler failures, due to small issues (changes in how fmt deals with typing, ADL ambiguity). This PR fixes these issues in a way that is compatible with both C++17 and C++20, so that people can compile with both. This PR does not actually enable C++20 or any C++20 simplifications.

[hughcars]

LGTM, one missed to_array that possibly also needs fixing.

[phdum-a]

Ok, I found one more issues with ADL on format since C++20 now adds std::format. Different compilers include <format> automatically, so it was not showing up on clang on mac for me. I now qualified all format as fmt::format.

[Sbozzolo]

Would it make sense to have one test that compiles with C++20 (to check we stay compliant)?

[phdum-a]

Would it make sense to have one test that compiles with C++20 (to check we stay compliant)?

Yes, probably good to have one test on a modern "reference" compiler if that is easy to do. And we can only do some quick tests, since the compilation is the important part.

Different compilers (clang, gcc) have different ways they deal with language changes (like what standard library headers are included by default), so we really want a full test suite like in #654, but that needs a cleaned up compiler support matrix.

[Sbozzolo]

I have not tested this, but this might work for the spack.yml:

  build-x64-gcc-c++20:
    needs: filter
    if: needs.filter.outputs.test == 'true'
    runs-on: [self-hosted, x64]
    steps:
      - uses: actions/checkout@v6
      - uses: ./.github/actions/palace-ci
        with:
          toolchain: gcc
          variant: "cxxstd=20"
          run-regression-tests: 'false'