fix(dart-dio): use enum type for discriminator when property is enum

When discriminator property references an enum type, use that enum type
instead of String for discriminatorValue getter/extension. This ensures
type safety and proper enum handling in generated Dart code.

Changes:
- Updated class_discriminator.mustache to handle enum discriminators
- Use enum.valueOf() for enum discriminators, throw error instead of returning null
- Added test case for enum discriminator scenario
- regenerated sample client with correct enum discriminator handling

Fixes #21570

Author: Lutra-Fs

modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/class.mustache

@@ -13,9 +13,8 @@ part '{{classFilename}}.g.dart';
 }
 {{#discriminator}}{{#hasDiscriminatorWithNonEmptyMapping}}
 {{>serialization/built_value/class_discriminator}}
-
 {{/hasDiscriminatorWithNonEmptyMapping}}{{/discriminator}}
 {{>serialization/built_value/class_serializer}}{{#vendorExtensions.x-is-parent}}
 
 {{>serialization/built_value/class_concrete}}{{/vendorExtensions.x-is-parent}}
-{{>serialization/built_value/class_inline_enums}}
+{{>serialization/built_value/class_inline_enums}}

modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/built_value/class_discriminator.mustache

@@ -1,3 +1,4 @@
+{{^discriminator.isEnum}}
 extension {{classname}}DiscriminatorExt on {{classname}} {
     String? get discriminatorValue {
         {{#mappedModels}}
@@ -17,4 +18,27 @@ extension {{classname}}BuilderDiscriminatorExt on {{classname}}Builder {
         {{/mappedModels}}
         return null;
     }
-}
+}
+{{/discriminator.isEnum}}
+{{#discriminator.isEnum}}
+extension {{classname}}DiscriminatorExt on {{classname}} {
+    {{discriminator.propertyType}} get discriminatorValue {
+        {{#mappedModels}}
+        if (this is {{modelName}}) {
+            return {{discriminator.propertyType}}.valueOf(r'{{mappingName}}');
+        }
+        {{/mappedModels}}
+        throw UnsupportedError('Invalid discriminator value for {{classname}}');
+    }
+}
+extension {{classname}}BuilderDiscriminatorExt on {{classname}}Builder {
+    {{discriminator.propertyType}} get discriminatorValue {
+        {{#mappedModels}}
+        if (this is {{modelName}}Builder) {
+            return {{discriminator.propertyType}}.valueOf(r'{{mappingName}}');
+        }
+        {{/mappedModels}}
+        throw UnsupportedError('Invalid discriminator value for {{classname}}Builder');
+    }
+}
+{{/discriminator.isEnum}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/dio/DartDioClientCodegenTest.java

@@ -115,4 +115,32 @@ public void verifyDartDioGeneratorRuns() throws IOException {
         TestUtils.ensureContainsFile(files, output, "README.md");
         TestUtils.ensureContainsFile(files, output, "lib/src/api.dart");
     }
+
+    @Test
+    public void verifyEnumDiscriminatorUsesEnumType() throws IOException {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("dart-dio")
+                .setInputSpec("src/test/resources/3_0/dart-dio/enum-discriminator.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"))
+                .addAdditionalProperty(CodegenConstants.SERIALIZATION_LIBRARY, DartDioClientCodegen.SERIALIZATION_LIBRARY_BUILT_VALUE)
+                .addAdditionalProperty(CodegenConstants.LEGACY_DISCRIMINATOR_BEHAVIOR, false)
+                .addAdditionalProperty(CodegenConstants.ENUM_UNKNOWN_DEFAULT_CASE, true);
+
+        ClientOptInput opts = configurator.toClientOptInput();
+
+        Generator generator = new DefaultGenerator().opts(opts);
+        List<File> files = generator.generate();
+        files.forEach(File::deleteOnExit);
+
+        File modelFile = new File(output, "lib/src/model/merchant_details_dto.dart");
+        TestUtils.ensureContainsFile(files, output, "lib/src/model/merchant_details_dto.dart");
+
+        String contents = Files.readString(modelFile.toPath(), StandardCharsets.UTF_8);
+        Assert.assertTrue(contents.contains("MerchantTypeEnum get discriminatorValue"));
+        Assert.assertTrue(contents.contains("MerchantTypeEnum.valueOf(r'TYPE_A')"));
+        Assert.assertFalse(contents.contains("String? get discriminatorValue"));
+    }
 }

modules/openapi-generator/src/test/resources/3_0/dart-dio/enum-discriminator.yaml

@@ -0,0 +1,51 @@
+openapi: 3.0.3
+info:
+  title: Dart Dio Enum Discriminator
+  version: 1.0.0
+paths:
+  /merchant:
+    get:
+      responses:
+        '200':
+          description: ok
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/MerchantDetailsDto'
+components:
+  schemas:
+    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'
+    MerchantDetailsTypeADto:
+      allOf:
+        - $ref: '#/components/schemas/MerchantDetailsDto'
+        - type: object
+          properties:
+            aField:
+              type: string
+    MerchantDetailsTypeBDto:
+      allOf:
+        - $ref: '#/components/schemas/MerchantDetailsDto'
+        - type: object
+          properties:
+            bField:
+              type: string
+    MerchantTypeEnum:
+      type: string
+      enum:
+        - TYPE_A
+        - TYPE_B

samples/openapi3/client/petstore/dart-dio/oneof_polymorphism_and_inheritance/lib/src/model/fruit.dart

@@ -46,25 +46,25 @@ abstract class Fruit implements Built<Fruit, FruitBuilder> {
 }
 
 extension FruitDiscriminatorExt on Fruit {
-    String? get discriminatorValue {
+    FruitType get discriminatorValue {
         if (this is Apple) {
-            return r'APPLE';
+            return FruitType.valueOf(r'APPLE');
         }
         if (this is Banana) {
-            return r'BANANA';
+            return FruitType.valueOf(r'BANANA');
         }
-        return null;
+        throw UnsupportedError('Invalid discriminator value for Fruit');
     }
 }
 extension FruitBuilderDiscriminatorExt on FruitBuilder {
-    String? get discriminatorValue {
+    FruitType get discriminatorValue {
         if (this is AppleBuilder) {
-            return r'APPLE';
+            return FruitType.valueOf(r'APPLE');
         }
         if (this is BananaBuilder) {
-            return r'BANANA';
+            return FruitType.valueOf(r'BANANA');
         }
-        return null;
+        throw UnsupportedError('Invalid discriminator value for FruitBuilder');
     }
 }