Made sure the Description and Validated retain optional; fixes #503

[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 resolves an issue where the JSON schema generation logic failed to correctly identify optional types when they were nested within rfl::Description or rfl::Validated wrappers. The core change modifies the is_optional utility function to recursively check for optionality through these wrapper types, ensuring that the generated schemas accurately represent the intended optional status of fields. A new comprehensive test case has been introduced to confirm the correct behavior and prevent regressions.

Highlights

• Fix for Optional Type Detection: The is_optional function in src/rfl/json/to_schema.cpp has been updated to correctly identify types as optional even when they are wrapped by rfl::Description or rfl::Validated types. Previously, these wrappers could obscure the underlying optionality.
• Improved JSON Schema Generation: This change ensures that the generated JSON schemas accurately reflect the optionality of fields, especially for described or validated optional types, by correctly propagating the optional status through Description and Validated wrappers.
• New Test Case: A new test file, tests/json/test_json_schema5.cpp, has been added to validate this fix. It demonstrates that fields like rfl::Description<std::optional<std::string>> and rfl::Validator<std::optional<unsigned int>> are now correctly recognized as optional in the generated schema.

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[gemini-code-assist]

Code Review

This pull request correctly extends the is_optional function to recursively check for optional types within Description and Validated wrappers. This ensures that fields using these wrappers with an optional underlying type are not incorrectly marked as 'required' in the generated JSON schema. The new test case test_json_schema5 effectively validates this fix.

The implementation is clean and uses if constexpr for compile-time branching, which is efficient. I have one minor suggestion to improve the code.

On a related note, I observed that the schema generated for the age field in the new test, which is a Validator over an std::optional, does not seem to include null as a possible type. This might be a separate issue worth investigating to ensure the generated schema fully reflects the optionality of validated fields.

[gemini-code-assist]

The lambda capture [&] is not necessary here as the lambda body does not access any variables from the enclosing scope. The recursive call to is_optional is a call to a free function and does not require a capture. It's good practice to use a non-capturing lambda [] when no captures are needed as it can sometimes lead to better compiler optimizations.

Suggested change

	return _t.variant_.visit([&](const auto& _v) -> bool {
	using T = std::remove_cvref_t<decltype(_v)>;
	return std::is_same<T, parsing::schema::Type::Optional>();
	};
	return rfl::visit(handle, _t.variant_);
	if constexpr (std::is_same_v<T, parsing::schema::Type::Description>) {
	return is_optional(*_v.type_);
	} else if constexpr (std::is_same_v<T, parsing::schema::Type::Validated>) {
	return is_optional(*_v.type_);
	} else {
	return std::is_same_v<T, parsing::schema::Type::Optional>;
	}
	});
	return _t.variant_.visit([](const auto& _v) -> bool {
	using T = std::remove_cvref_t<decltype(_v)>;
	if constexpr (std::is_same_v<T, parsing::schema::Type::Description>) {
	return is_optional(*_v.type_);
	} else if constexpr (std::is_same_v<T, parsing::schema::Type::Validated>) {
	return is_optional(*_v.type_);
	} else {
	return std::is_same_v<T, parsing::schema::Type::Optional>;
	}
	});