Made sure TaggedUnion can be used with rfl::NoExtraFields; fixes #567

[liuzicheng1987]

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[gemini-code-assist]

Summary of Changes

Hello @liuzicheng1987, 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 addresses an incompatibility issue where rfl::TaggedUnion could not be used effectively with the rfl::NoExtraFields policy. The core change involves streamlining the internal parsing mechanism for tagged unions to ensure consistent application of necessary field transformations. This fix guarantees that tagged unions behave as expected when strict field policies are enforced, and a new test case has been introduced to confirm the correct behavior.

Highlights

• Parsing Logic Simplification: The conditional if constexpr (no_field_names_) block within the Parser_tagged_union.hpp file has been removed, standardizing the parsing path for TaggedUnion types.
• Consistent Field Handling: The parsing logic now consistently uses tagged_union_wrapper_no_ptr_t and applies the get_fields transformation, ensuring proper handling of fields regardless of the no_field_names_ setting.
• New Test Case: A new test file, test_tagged_union_with_no_extra_fields.cpp, has been added to verify that rfl::TaggedUnion correctly functions when combined with the rfl::NoExtraFields policy.

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[gemini-code-assist]

Code Review

This pull request addresses an issue where rfl::TaggedUnion was incompatible with rfl::NoExtraFields. The change in include/rfl/parsing/Parser_tagged_union.hpp refactors the parsing logic to always use a wrapper type that includes the discriminator. This simplifies the code by removing a conditional branch and correctly handles processors that check for extraneous fields. A new test case in tests/json/test_tagged_union_with_no_extra_fields.cpp is added to verify the fix. The changes are correct and improve the robustness of the library. The implementation is clean and I have no suggestions for improvement.