Allow usage of stateful visitors inside rfl::Variant

[FrancescoMerenda]

This PR allows the usage of stateful visitors inside rfl::Variant.

Fixes #595

[gemini-code-assist]

Summary of Changes

Hello @FrancescoMerenda, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the rfl::Variant class by introducing support for stateful visitors. The modification involves adjusting the parameter type for visitors within the internal visitation logic, enabling them to alter their state during execution. This resolves a limitation where visitors were previously treated as immutable, expanding the flexibility and utility of the Variant's visitation mechanism.

Highlights

• Stateful Visitors: The rfl::Variant class now supports stateful visitors by modifying the internal do_visit helper functions to accept visitors by non-const reference (F&) instead of const reference (const F&).
• Visitor Modification: This change allows visitors to modify their internal state during the visitation process, which was previously prevented by the const qualifier.

Changelog

• include/rfl/Variant.hpp
  • Modified the visit_one lambda capture for the visitor parameter _f from const F& to F& in do_visit_no_result (both const and non-const overloads).
  • Modified the visit_one lambda capture for the visitor parameter _f from const F& to F& in do_visit_with_result (both const and non-const overloads).
  • Modified the visit_one lambda capture for the visitor parameter _f from const F& to F& in do_visit_with_reference (both const and non-const overloads).

Activity

• This pull request addresses and fixes issue Inconsistency in rfl::Variant visit overloads prevents stateful visitors #595, which likely concerned the inability to use stateful visitors with rfl::Variant.

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[gemini-code-assist]

Code Review

This pull request correctly enables the use of stateful visitors with rfl::Variant by changing the visitor parameter in internal helper functions from a const reference to a non-const reference. The changes are consistent across all relevant do_visit_* methods and their const overloads.

The suggestions to refactor the visit_one lambdas to use captures instead of parameters are valid. This approach simplifies the lambda signatures and calls, improving code readability by avoiding parameter shadowing.

[liuzicheng1987]

@FrancescoMerenda thank you for the contribution!