C++26 Reflection

[stephenberry]

C++26 P2996 Reflection Support

This PR adds experimental support for C++26 static reflection (P2996) to Glaze.

Overview

When compiled with -DGLZ_REFLECTION26=1 and a P2996-compatible compiler, Glaze uses std::meta::nonstatic_data_members_of for automatic struct reflection instead of the traditional aggregate-based approach. This enables reflection of:

• Non-aggregate types (classes with constructors)
• Types with private members
• Any class type without requiring glz::meta specializations

Key Changes

The specified Trait

Introduced glz::specified<T> trait and glz::is_specified<T> concept to mark types with custom Glaze serialization. This prevents P2996 from automatically reflecting types that already have explicit to/from implementations, avoiding template ambiguity.

// Mark a type as having custom serialization
template <>
struct glz::specified<MyType> : std::true_type {};

Types marked as specified:

• Standard library: std::tuple, std::variant, std::reference_wrapper, std::complex, std::bitset
• Chrono types: duration, time_point, year_month_day, hh_mm_ss
• Thread-safe containers: async_vector, async_string, shared_async_vector, shared_async_map, async
• Eigen types: Matrix, Transform, Ref

Simplified reflectable Concept

The P2996 version of reflectable is cleaner since P2996 handles the complexity:

template <class T>
concept reflectable =
   std::is_class_v<std::remove_cvref_t<T>> &&
   !(is_no_reflect<T> || glaze_t<T> || meta_keys<T> ||
     range<T> || pair_t<T> || null_t<T> || str_t<T> || bool_t<T> ||
     tuple_t<T> || func_t<T> || is_specified<T>);

CI Integration

Added GitHub Actions workflow using the vsavkov/clang-p2996 Docker image for automated testing.

Other Fixes

• Fixed std::is_trivial_v deprecation warning for C++26
• Extended bool_t concept to handle std::vector<bool>::const_reference
• Adjusted tests for P2996's type naming differences (returns variant<...> instead of std::variant<...>)

Building with P2996

cmake -B build \
  -DCMAKE_CXX_FLAGS='-std=c++26 -freflection -fexpansion-statements' \
  -Dglaze_ENABLE_REFLECTION26=ON

Known Limitations

• jsonrpc_test is excluded due to a libc++ std::expected constraint recursion bug in the experimental compiler
• Type names from P2996 reflection may differ from traditional Glaze (no std:: prefix)

[packit-as-a-service]

One of the tests failed for 932ef26. @admin check logs None, packit dashboard https://dashboard.packit.dev/jobs/copr/3128523 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/9977010/

[packit-as-a-service]

One of the tests failed for d620209. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-2217/fedora-rawhide-ppc64le/09977031-glaze/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/copr/3128544 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/9977031/

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[packit-as-a-service]

One of the tests failed for f7618dd. @admin check logs https://download.copr.fedorainfracloud.org/results/packit/stephenberry-glaze-2217/srpm-builds/09980686/builder-live.log, packit dashboard https://dashboard.packit.dev/jobs/srpm/526779 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/9980686/

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[packit-as-a-service]

One of the tests failed for dc878ec. @admin check logs None, packit dashboard https://dashboard.packit.dev/jobs/copr/3342323 and external service dashboard https://copr.fedorainfracloud.org/coprs/build/10198419/