X3 -> X4 transition

[saki7]

Note: This library is currently under heavy development.

We will not accept unrelated PRs until this transition is complete. If you have any idea for supporting the migration process, please contact us by posting a comment on this issue.

History: https://lists.boost.org/archives/list/boost@lists.boost.org/thread/K3EQLEQJHEUBROB6ODUKTZHFP2FDUS2E/?sort=date

X4 Release Plan

Estimated release date: Q2 2026

We will officially release X4 when all of the following conditions are satisfied. This list is provided as-is, and may change in the future.

• The policy and the status quo of the development is fully described in our new README.
• All the essential long-standing bugs and unimplemented features from the X3 era is resolved.
• No controversial semantic ambiguities remain in both the codebase and the new documentation.
• All public customization point shall be placed inside namespace extension, or each one should be clarified (by code comments) whether it is allowed to specialize.
• Basic documentation is available to help beginners learn and use X4 in a new application.
  • We are intentionally not aiming for comprehensive documentation in the initial release; the effort would be impractical.
• X4 has zero Boost dependencies, i.e., we successfully replace Boost.Variant, Boost.Fusion, and Boost.Endian.

After the above conditions are met, I will contact the core member of the Boost Organization, then our repository boostorg/spirit_x4 will be added to the boostorg/boost superproject.

Dependency PRs

• Implement full CMake support #31
• Make config.hpp toplevel #34
• Migrate to Catch2 #48
• Replace boost::fusion -- @yaito3014 is working on the fork
  • Drop fusion associative sequence support #54
  • ... more coming soon ...
• Replace boost::variant -- We (@saki7 and @yaito3014) have finished implementing the standalone implementation on spring-summer 2025, repo at yaito3014/rvariant; pending PR.
• Remove boost::endian dependency -- Remove binary parser #74

Transition / Deprecation PRs

• [X3] C++23 era #4
• [CI] Quick fix to remove obsolete components #18
• Quick fix README to decribe the status of X4 #19
• Remove pre-X3 components #28
• Remove x3 example #29
• Remove deprecated API #30
• Revise library structure, rename X3 -> X4 #32
  • Follow-up: Refactor include guards #36
• Change coding style #40
  • Follow-up: Fix typo in has_attribute #66
• Update meta info #41
• Prevent side effect on the failed branch of alternative #50
  • Follow-up: Remove TLS optimization in alternative #56
• Remove unused bjam features #55
• Remove confix_directive #61
• Remove _pass(ctx), return bool directly from semantic action #68
• Remove _where context #72
• Simplify attribute traits #71
• Rename *_val contexts to *_var #75
• Rename list to delimited_list

Core Feature / Core Optimization PRs

• Remove RContext #37
  • Follow-up: Make rcontext only when required #57
• Improve context to make it capable of holding both lvalue/rvalue #42
• Make numeric policies stateless #46
• Implement as<T>(...) directive #53
  • Follow-up: #include as.hpp in global header #69
• Implement x4::with_local<T>[...] and x4::with_local<T, ID>[...] #73

Debugging PRs

• Make get_info::operator() static
  • Remove result_type
• Improve debug print for char32_t
• Add NatVis support

QoL PRs

• Include only minimal headers #33
• Reduce the number of identifiers in top-level x4:: scope #44
  • Follow-up: Suppress unused variable warning on CPOs #47
• Relax expectation failure context requirement #52
• Make x4::attr(...) capable of holding constexpr string #58
• Remove redundant constructors #65
• CI: Use full toolchain version for cache key #67
• Add [[maybe_unused]] to all inline variables #70
• Reconsider entities defined in traits:: and extension::, reconsider naming of concepts
• Add .clang-tidy
• Add cppwarningnotifier[bot]

Documentation / Semantic Clarification PRs

• Clarify behavior of iterator/attribute on failed parse #64
• Revise README (fix [X4] X4 needs a modern README #21)
• Revise documentation (fix X4 documentation should be more visible #2)