[BUG] [Dart] additional properties of type: object doesn't generate with mapCastOfType

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

The additionalProperties field, which specified a Map<T1, T2> in Dart, doesn't generate properly by fields defined by

schema:
   type: string
   additionalProperties:
      type: object

openapi-generator version

6.0.0

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  version: "1.1"
  title: Dart MappedValue Fail
servers:
  - url: 'localhost'
    variables:
      host:
        default: localhost
components:
  schemas:
    ItemGood:
      type: object
      properties:
        mappedProps:
          type: string
          additionalProperties:
            type: string
    ItemBad:
      type: object
      properties:
        mappedProps:
          type: string
          additionalProperties:
            type: object
paths:
  /items:
    get:
      operationId: GetItems
      responses:
        "200":
          description: A list of Items
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ItemGood'

Generation Details

java -jar ..\openapi-generator\modules\openapi-generator-cli\target\openapi-generator-cli.jar generate -i .\shadowspec.yaml -g dart -o mapped

Steps to reproduce

After generation, see the ./mapped/lib/model/item_good.dart and ./mapped/lib/model/item_bad.dart files.

ItemGood, defined as

    ItemGood:
      type: object
      properties:
        mappedProps:
          type: string
          additionalProperties:
            type: string

generates correctly with

      return ItemGood(
        mappedProps: mapCastOfType<String, String>(json, r'mappedProps') ?? const {},
      );

However, ItemBad, defined as

    ItemBad:
      type: object
      properties:
        mappedProps:
          type: string
          additionalProperties:
            type: object

generates incorrectly, using mapValueOfType instead of mapCastOfType:

      return ItemBad(
        mappedProps: mapValueOfType<Map<String, Object>>(json, r'mappedProps') ?? const {},
      );

Related issues/PRs

Suggest a fix

Maps should try to generate with mapValueOfType in as many situations as possible