[BUG][DART] Fields named json get shadowed by ToJson and become unserializable due to cyclic pointer reference

Created by: 0xNF

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
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Description

Here's a repo for the issue: https://github.com/0xNF/openapi_gen_dart_json_shadow

The Dart SDK generator will incorrectly shadow field variables named json in the generated toJson() method, resulting in a cyclic pointer loop when running the toJson() method.

openapi-generator version

jar name: openapi-generator-cli-6.0.0-20220412.074015-127.jar jar version: 6.0.0-SNAPSHOT

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  version: "1.0"
  title: Dart Shadow Json Demo
servers:
  - url: 'localhost'
    variables:
      host:
        default: localhost
components:
  schemas:
    ItemWithFieldNamedJson:
      type: object
      properties:
        json:
          type: object
          additionalProperties: {}
paths:
  /:
    get:
      operationId: shadow
      responses:
        '200':
          description: produces a shadowed json field when generated in dart
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ItemWithFieldNamedJson'

Generation Details

See: https://github.com/0xNF/openapi_gen_dart_json_shadow/blob/master/shadowed/lib/model/item_with_field_named_json.dart

class ItemWithFieldNamedJson {
  ItemWithFieldNamedJson({
    this.json = const {},
  });

  Map<String, Object> json;

  Map<String, dynamic> toJson() {
    final json = <String, dynamic>{};
      json[r'json'] = json;
    return json;
  }

Steps to reproduce

Run the generator command

java -jar openapi-generator-cli-6.0.0-20220412.074015-127.jar generate -i .\shadowspec.yaml -g dart -o shadowed

Examine the shadowed/lib/model/item_with_field_named_json.dart
See the shadowed variable on line 34, and the infinite reference on line 35

Related issues/PRs

Use another parameter name to stop variable shadowing. related to https://github.com/OpenAPITools/openapi-generator/pull/10263

Suggest a fix

The hard fix:

For any variables that are generated by the Generator, use static analysis to determine if there are any conflicting variable names in the namespace in the generated functions, and then modify the desired generated variable name appropriately, repeating as many times as necessary to ensure truly unique variable names within the functions scope.

The easier fix:

The Dart generator already knows about reserved keywords. For fields that are named int, generated fields are named int_ with an underscore. Extend this idea to json. This is a half measure though, as if I name my field variable json_, then we're back to being shadowed.