Fix generation of an empty data class when using kotlinx.serialization (#22350)

* Fix generation of an empty data class

* Add a test

* Delete polymorphicJacksonSerialization artifacts on exit

Author: diastremskii

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java

@@ -989,6 +989,12 @@ public Map<String, ModelsMap> postProcessAllModels(Map<String, ModelsMap> objs)
                         additionalProperties.discriminatorValue = mappedModel.getMappingName();
                         // Remove the discriminator property from the derived class, it is not needed in the generated code
                         getAllVarProperties(mappedModel.getModel()).forEach(list -> list.removeIf(prop -> prop.name.equals(discriminator.getPropertyName())));
+
+                        // If model has no properties after removing discriminator, mark it as empty
+                        // so it generates as a class instead of an empty data class
+                        if (mappedModel.getModel().vars.isEmpty() && !mappedModel.getModel().isEnum && !mappedModel.getModel().isAlias) {
+                            mappedModel.getModel().setHasVars(false);
+                        }
                     }
 
                 }

modules/openapi-generator/src/test/java/org/openapitools/codegen/kotlin/KotlinClientCodegenModelTest.java

@@ -573,7 +573,7 @@ public void polymorphicKotlinxSerialization() throws IOException {
     @Test(description = "generate polymorphic jackson model")
     public void polymorphicJacksonSerialization() throws IOException {
         File output = Files.createTempDirectory("test").toFile();
-//        output.deleteOnExit();
+        output.deleteOnExit();
 
         final CodegenConfigurator configurator = new CodegenConfigurator()
                 .setGeneratorName("kotlin")
@@ -644,6 +644,36 @@ public void testIntArrayToEnum() throws IOException {
       TestUtils.assertFileContains(modelKt, "enum class DaysOfWeek(val value: kotlin.Int)");
   }
 
+    @Test(description = "convert an empty model to object")
+    public void emptyModelKotlinxSerializationTest() throws IOException {
+        final Schema<?> schema = new ObjectSchema()
+                .description("an empty model");
+        final DefaultCodegen codegen = new KotlinClientCodegen();
+        codegen.processOpts();
+
+        OpenAPI openAPI = TestUtils.createOpenAPIWithOneSchema("EmptyModel", schema);
+        codegen.setOpenAPI(openAPI);
+
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("kotlin")
+                .setAdditionalProperties(new HashMap<>() {{
+                    put(CodegenConstants.MODEL_PACKAGE, "model");
+                    put(SERIALIZATION_LIBRARY, "kotlinx_serialization");
+                }})
+                .setInputSpec("src/test/resources/3_0/kotlin/empty-model.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        generator.opts(clientOptInput).generate();
+
+        final Path modelKt = Paths.get(output + "/src/main/kotlin/model/EmptyModel.kt");
+        TestUtils.assertFileNotContains(modelKt, "data class EmptyModel");
+    }
+
     private static class ModelNameTest {
         private final String expectedName;
         private final String expectedClassName;

modules/openapi-generator/src/test/resources/3_0/kotlin/empty-model.yaml

@@ -0,0 +1,42 @@
+openapi: 3.0.0
+info:
+  title: Test Empty Model
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    Response:
+      type: "object"
+      properties:
+        result_or_error:
+          nullable: true
+          discriminator:
+            propertyName: "_type"
+            mapping:
+              result: "#/components/schemas/EmptyModel"
+              error: "#/components/schemas/Error"
+          oneOf:
+            - $ref: "#/components/schemas/EmptyModel"
+            - $ref: "#/components/schemas/Error"
+    EmptyModel:
+      type: object
+      properties:
+        _type:
+          required:
+            - "_type"
+          type: string
+          default: "result"
+      additionalProperties: false
+    Error:
+      type: object
+      required:
+        - "code"
+      properties:
+        code:
+          type: integer
+        _type:
+          required:
+            - "_type"
+          type: string
+          default: "error"
+      additionalProperties: false