[wdconinc]

Briefly, what does this PR introduce? Please link to any relevant presentations or discussions.

This PR bumps the C++ standard where possible to 23. There are a few interlinked dependency trees that change in lock step due to their inclusion of headers. Upcoming changes that we want to prepare for:

• podio will make C++20 deprecated soon,
• acts will require C++23 in the next release.

Motivation is also the availability of some generic algorithms on containers that would be useful to take advantage of. In particular, iteration over two coordinated containers (e.g. hits and links) using zip becomes available for use in EICrecon.

What is the urgency of this PR?

• High (please describe reason below)
• Medium
• Low

What kind of change does this PR introduce?

• Bug fix (issue #__)
• New feature (issue: C++23 features)
• Optimization (issue #__)
• Updated documentation
• other: __

Please check if any of the following apply

• This PR introduces breaking changes. Please describe changes users need to make below.
• This PR changes default behavior. Please describe changes below.
• AI was used in preparing this PR. Please describe usage below.

AI propagated the changes.

[Copilot]

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🟡 Changes recommended

The change has an active documentation issue: the packages.yaml example in docs/spack-environment.md is out of sync with the actual spack-environment/packages.yaml, making the updated guidance misleading until corrected.

1 open issue

• Documented packages.yaml is out of sync with actual environment · New

What changed in this PR

Updates the Spack environment configuration to prefer/build key packages with C++23, aligning the software stack with upcoming upstream requirements.

Changes:

• Bump cxxstd constraints to 23 for multiple packages in spack-environment/packages.yaml, while retaining documented exceptions where C++23 is not yet supported.
• Update the Spack environment documentation example to reflect the newer ROOT version and cxxstd=23 usage.

File	Description
spack-environment/​packages.yaml	Raises per-package Spack cxxstd requirements to 23 for several core dependencies and documents exceptions that remain on C++20.
docs/​spack-environment.md	Updates the documentation’s example packages.yaml snippet to reflect the current ROOT version and cxxstd=23 settings.

_{Review effort: Balanced}