generic support

[ManuelLeitner]

add support for generic JsonConverter, toJson and fromJson with arbitrary generic arguments

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[gemini-code-assist]

Summary of Changes

Hello @ManuelLeitner, 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 significantly enhances json_serializable's capabilities by introducing robust support for generic types in both custom JsonConverter implementations and direct toJson/fromJson functions. The changes ensure that the code generation process correctly interprets and applies generic arguments, leading to more accurate and flexible serialization/deserialization logic for complex data structures.

Highlights

• Generic Function Type Handling: The _convertData function in type_helper_ctx.dart has been updated to correctly identify and instantiate generic function types, ensuring that toJson and fromJson methods with generic arguments are properly processed.
• Generic JsonConverter Support: The _compatibleMatch function in json_converter_helper.dart now supports generic JsonConverter implementations by instantiating the converter's class type with its generic arguments, allowing for more flexible custom converters.
• Analyzer Package Imports: New imports from the analyzer package (package:analyzer/src/dart/constant/value.dart and package:analyzer/dart/element/nullability_suffix.dart) were added to facilitate advanced type analysis for generic types.

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[gemini-code-assist]

Code Review

This pull request adds support for generic arguments in JsonConverter, toJson, and fromJson functions. The changes to support generic JsonConverters look good. However, to support generic toJson and fromJson functions, the implementation relies on internal APIs from the analyzer package, which is a significant maintenance risk. I've added a high-severity comment about this, along with a couple of medium-severity suggestions for code improvements.

[gemini-code-assist]

These changes rely on internal APIs from the analyzer package (DartObjectImpl, FunctionState), which are imported via package:analyzer/src/dart/constant/value.dart. These APIs are not stable and can break this package on any analyzer update, creating a significant maintenance risk.

Could you investigate if there's an alternative approach using only public APIs to retrieve the generic type arguments for the function? If this is the only way, the dependency on analyzer's internals should be clearly documented as a known risk.

[gemini-code-assist]

This condition can be expressed more idiomatically using isNotEmpty.

Suggested change

	if (((state as FunctionState).typeArguments?.length ?? 0) > 0) {
	if ((state as FunctionState).typeArguments?.isNotEmpty == true) {

[gemini-code-assist]

The converterClassType variable is already assigned constantValue.type as InterfaceType on line 285. You can reuse it here to avoid the redundant cast and access.

Suggested change

	final generics = (constantValue.type as InterfaceType).typeArguments;
	final generics = converterClassType.typeArguments;

[kevmoo]

This is NOT ideal...

[kevmoo]

at a minimum we need a changelog entry and tests here!