[BUG] OpenAPI Generator Dart-Dio: Enum Discriminators in Discriminated Unions Cause Compilation Errors

Bug Report Checklist

• [ x] Have you provided a full/minimal spec to reproduce the issue?
• [x ] Have you validated the input using an OpenAPI validator?
• [x ] Have you tested with the latest master to confirm the issue still exists?
• [x ] Have you searched for related issues/PRs?
• [ x] What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When using enum properties as discriminator fields in OpenAPI discriminated unions, the generated Dart code contains compilation errors:

1. Incorrect discriminator assignment: The generator assigns enum values directly instead of string literals
2. Missing string conversion: Enum discriminators are not properly converted to strings for comparison
3. Type mismatch errors: Generated code expects string discriminators but receives enum values

openapi-generator version

OpenAPI generator version 7.12.0

OpenAPI declaration file content or url

# Problematic discriminated union using enum as discriminator
MerchantDetailsDto:
  type: object
  properties:
    uuid:
      type: string
      format: uuid
    merchantType:
      $ref: '#/components/schemas/MerchantTypeEnum'
  required:
    - uuid
    - merchantType
  discriminator:
    propertyName: merchantType
    mapping:
      TYPE_A: '#/components/schemas/MerchantDetailsTypeADto'
      TYPE_B: '#/components/schemas/MerchantDetailsTypeBDto'
      TYPE_C: '#/components/schemas/MerchantDetailsTypeCDto'
      TYPE_D: '#/components/schemas/MerchantDetailsTypeDDto'
      TYPE_E: '#/components/schemas/MerchantDetailsTypeEDto'
      TYPE_F: '#/components/schemas/MerchantDetailsTypeFDto'

# Another problematic discriminated union
QrCodeDto:
  type: object
  properties:
    type:
      $ref: '#/components/schemas/QrCodeTypeEnum'
    name:
      type: string
    code:
      type: string
    terminalId:
      format: int64
      type: integer
  required:
    - type
    - code
    - terminalId
  discriminator:
    propertyName: type
    mapping:
      STATIC_TYPE: '#/components/schemas/StaticQrCodeDto'
      DYNAMIC_TYPE: '#/components/schemas/DynamicQrCodeDto'

# The enum definitions that cause the problem
MerchantTypeEnum:
  type: string
  enum:
    - TYPE_A
    - TYPE_B
    - TYPE_C
    - TYPE_D
    - TYPE_E
    - TYPE_F

QrCodeTypeEnum:
  type: string
  enum:
    - DYNAMIC_TYPE
    - STATIC_TYPE

Generation Details

Generator: dart-dio with built_value serialization
OpenAPI Generator Version: 7.12.0
Input Spec: merchants-v2.1.yaml (contains the problematic discriminated unions)
Output Directory: ../ (relative to config directory)
Package Name: merchant_app
Discriminator Behavior: legacyDiscriminatorBehavior: false (uses new discriminator logic)
Enum Handling: enumUnknownDefaultCase: true (allows unknown enum values)

Steps to reproduce

1. Use an OpenAPI spec with discriminated union using enum discriminator
2. Generate Dart code using dart-dio template
3. Attempt to compile generated code
4. Observe compilation errors related to discriminator field assignments

Suggest a fix

1. Fix the dart-dio Template Logic
   The core issue is in the dart-dio template's discriminator handling. The template should:
   Detect when discriminator properties are enum types (not just strings)
   Generate proper enum conversion code instead of direct assignment
   Handle null discriminator values gracefully

2. Enhance Discriminator Property Type Detection
   The generator should:
   Parse the discriminator property's schema reference to determine if it's an enum
   Extract the enum class name from the schema reference
   Generate appropriate conversion logic based on the property type

3. Add Configuration Option for Enum Discriminator Handling
   Introduce a new configuration option:

additionalProperties:
  enumDiscriminatorHandling: "auto"  # or "manual", "legacy"

4. Improve Error Messages and Validation
   Add validation that:
   Warns when enum discriminators are detected but not properly handled
   Provides clear error messages about what needs to be fixed
   Suggests configuration changes to resolve issues

Start with the dart-dio template's discriminator handling and the enum schemas in the supplied OpenAPI YAML. Generate code with version 7.12.0, legacyDiscriminatorBehavior disabled, and enumUnknownDefaultCase enabled, then compile the output to reproduce the errors. Done means generated Dart for enum discriminated unions compiles and handles discriminator values correctly.