build(macos): configure C++ standard and ICU root

[martona]

Description

Pass the project C++ 23 standard explicitly so fetched dependencies build against modern headers. C++23 was chosen because it's already used for the sunshine target in cmake/targets/common.cmake and by the Homebrew formula in packaging/sunshine.rb, but those don't apply early enough to dependencies built through scripts/macos_build.sh. This can break when Boost falls back to FetchContent.

Point CMake at Homebrew's icu4c@78 prefix to match the dependency installed by the macOS build path. Without this, CMake can pick up inconsistent ICU headers/configuration from the Apple SDK or system paths while Boost.Locale is built from source.

Mirror the manual macOS build script in CI so the two paths stay aligned, as requested in the script comments.

Screenshot

Issues Fixed or Closed

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[ReenigneArcher]

I'm confused about setting CXX_STANDARD here. We already set it in cmake itself, and we always use fetch content in our CI builds, so I'm not sure this is actually an issue.

[martona]

The existing CXX_STANDARD setting applies only to the sunshine target, and it is set after dependencies/common.cmake has already added the FetchContent targets.

In my local build, Homebrew had Boost 1.90 installed, while the project requested exactly 1.89. CMake rejected the newer Homebrew Boost and fell back to FetchContent, so Boost.Locale was built as part of the project. That fetched Boost target then compiled against Homebrew’s ICU headers without a C++17+ default and failed on ICU headers using constructs like auto non-type template parameters and std::is_same_v.

Passing -DCMAKE_CXX_STANDARD=23 at configure time makes the project default visible before FetchContent targets are created. I used 23 specifically because that is already the standard requested for the Sunshine target and the Homebrew packaging path.

That said, if you prefer, setting the project-wide C++ standard near the top-level CMake before dependencies are loaded would be cleaner than passing it from the macOS scripts.

I also wouldn't be offended if you chose to reject as it is very much an edge case and easy to work around with exports in .zshrc but I doubt I'm the only one to ever run into this.

[ReenigneArcher]

I think this can probably affect Linux as well, so probably we could just set it in the top level CMakelists.txt as well as in the tests.

[martona]

Right, it probably does. Do you want me to take a crack at it?

[ReenigneArcher]

Right, it probably does. Do you want me to take a crack at it?

Yes please!