fix: ensure superclass model is generated when using discriminator without properties

Previously, models acting as superclasses with only a discriminator and no defined properties were incorrectly treated as free-form objects, preventing class generation.
This fix updates the free-form object detection logic to correctly handle such cases, ensuring base models are generated when a discriminator is present.

Closes #7638

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -881,7 +881,7 @@ public class ModelUtils {
             } else if (schema.getAdditionalProperties() instanceof JsonSchema) {
                 return true;
             } else if (schema.getTypes() != null) {
-                if (schema.getTypes().size() == 1) { // types = [object]
+                if (schema.getTypes().size() == 1 && schema.getDiscriminator() == null) { // types = [object]
                     return SchemaTypeUtil.OBJECT_TYPE.equals(schema.getTypes().iterator().next());
                 } else { // has more than 1 type, e.g. types = [integer, string]
                     return false;

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

@@ -4815,6 +4815,25 @@ public class DefaultCodegenTest {
         assertEquals(openAPI, codegen.openAPI);
     }
 
+    @Test
+    public void testParentModelGeneratedEvenWithoutProperties() throws Exception {
+        File output = Files.createTempDirectory("test_7638_").toFile();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("java")
+                .setInputSpec("src/test/resources/3_1/issue_7638.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        final ClientOptInput clientOptInput = configurator.toClientOptInput();
+        DefaultGenerator generator = new DefaultGenerator();
+        List<File> files = generator.opts(clientOptInput).generate();
+
+        TestUtils.ensureContainsFile(files, output, "src/main/java/org/openapitools/client/model/Pet.java");
+        TestUtils.ensureContainsFile(files, output, "src/main/java/org/openapitools/client/model/Dog.java");
+
+        output.deleteOnExit();
+    }
+
     @Test
     public void testReferencedEnumType() {
         final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/issue-5676-enums.yaml");

modules/openapi-generator/src/test/resources/3_1/issue_7638.yaml

@@ -0,0 +1,34 @@
+{
+  "openapi": "3.1.0",
+  "info": {
+    "title": "Parent Model Generated Even Without Properties",
+    "version": "1.0.0-SNAPSHOT"
+  },
+  "servers": [ ],
+  "paths": { },
+  "components": {
+    "schemas": {
+      "Pet": {
+        "type": "object",
+        "discriminator": {
+          "propertyName": "petType"
+        }
+      },
+      "Dog": {
+        "allOf": [
+          {
+            "$ref": "#/components/schemas/Pet"
+          },
+          {
+            "type": "object",
+            "properties": {
+              "legs": {
+                "type": "string"
+              }
+            }
+          }
+        ]
+      }
+    }
+  }
+}