Added rfl::DefaultVal, resolves #534

[liuzicheng1987]

No description provided.

[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 introduces the rfl::DefaultVal mechanism, significantly enhancing the rfl library's deserialization capabilities by allowing struct fields to be initialized with default values when they are absent in the input data. This feature addresses the common need to provide fallback values for optional fields, improving robustness and reducing boilerplate code. The changes involve adding the DefaultVal wrapper type, updating the parsing logic to respect these default values, and introducing new compile-time traits for introspection. The has_tag_v trait was also modernized using C++20 concepts.

Highlights

• New rfl::DefaultVal Type: Introduced a new C++ template struct rfl::DefaultVal<T> that allows fields in reflected structs to specify a default value, which will be used if the field is missing during deserialization.
• Updated Parsing Logic: The NamedTupleParser has been enhanced to correctly handle rfl::DefaultVal fields. These fields are no longer considered 'required' if absent in the input, and their default values are automatically applied.
• C++20 Concepts for has_tag_v: The has_tag_v trait, used for tagged unions, has been refactored to leverage C++20 concepts, improving compile-time checks and code clarity.
• New Internal Traits: Two new internal traits, is_default_val_v and has_default_val_v, were added to enable compile-time detection of DefaultVal types and structs containing them, respectively.
• Comprehensive Testing: New test cases have been added for JSON serialization and deserialization, covering both standard field-named objects and stripped-field-name arrays, to thoroughly validate the behavior of rfl::DefaultVal.

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[gemini-code-assist]

Code Review

This pull request introduces rfl::DefaultVal, a useful feature for providing default values for fields during parsing. The implementation is mostly solid, with good test coverage for both standard and no-field-name scenarios. I've identified a few areas for improvement in rfl::DefaultVal.hpp, including a bug in a move assignment operator, some inefficiencies in constructors, and opportunities to modernize the code using C++20 concepts for better consistency and readability. My detailed comments and suggestions are provided below.

[gemini-code-assist]

This unconstrained converting copy constructor is redundant with the constrained version on lines 42-44. Having both can lead to ambiguity during template resolution. It would be safer to remove this one and rely solely on the constrained version.