fix: set isOverridden from the model's own properties rather than only when unset

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

@@ -6331,8 +6331,12 @@ protected void addVars(IJsonSchemaValidationProperties m, List<CodegenProperty>
                     cp = fromProperty(key, prop, mandatory.contains(key));
                 }
 
-                if (cm != null && cm.allVars == vars && cp.isOverridden == null) { // processing allVars and it's a parent property
-                    cp.isOverridden = true;
+                if (cm != null && cm.allVars == vars) {
+                    // Processing allVars: a property is overridden when it comes from the parent rather than
+                    // being declared by this model. fromProperty caches CodegenProperty instances, so the flag
+                    // must be assigned rather than only filled in when unset - a cached instance may already
+                    // carry a value from whichever model happened to be processed first.
+                    cp.isOverridden = !varsMap.containsKey(key);
                 }
 
                 vars.add(cp);

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

@@ -2362,6 +2362,23 @@ public void testIsOverriddenProperty() {
         Assertions.assertEquals(cp1.isOverridden, false);
     }
 
+    @Test
+    public void testOverrideSetterWhenChildIsDeclaredBeforeParent() {
+        final Path output = newTempFolder();
+        OpenAPI openAPI = new OpenAPIParser()
+                .readLocation("src/test/resources/3_0/allOf_composition_discriminator_child_first.yaml", null, null)
+                .getOpenAPI();
+
+        JavaClientCodegen codegen = new JavaClientCodegen();
+        codegen.setOutputDir(output.toString());
+        codegen.setLibrary(JavaClientCodegen.APACHE);
+
+        new DefaultGenerator().opts(new ClientOptInput().openAPI(openAPI).config(codegen)).generate();
+
+        assertThat(output.resolve("src/main/java/org/openapitools/client/model/Cat.java")).content()
+                .contains("  @Override\n  public Cat petType(@javax.annotation.Nonnull String petType) {");
+    }
+
     @Test
     public void testForJavaApacheHttpClientOverrideSetter() {
         final Path output = newTempFolder();

modules/openapi-generator/src/test/resources/3_0/allOf_composition_discriminator_child_first.yaml

@@ -0,0 +1,52 @@
+openapi: 3.0.2
+info:
+  title: OAI Specification example for Polymorphism
+  version: 1.0.0
+paths:
+  /pet:
+    get:
+      responses:
+        '200':
+          description: desc
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Pet'
+components:
+  schemas:
+    MyPets:
+      oneOf:
+        - $ref: '#/components/schemas/Cat'
+        - $ref: '#/components/schemas/Dog'
+      discriminator:
+        propertyName: petType
+    Pet:
+      type: object
+      required:
+        - petType
+      properties:
+        petType:
+          type: string
+      discriminator:
+        propertyName: petType
+    Cat:
+      allOf:
+        - $ref: '#/components/schemas/Pet'
+        - type: object
+          properties:
+            name:
+              type: string
+            characteristics:
+              $ref: '#/components/schemas/Characteristics'
+    Dog:
+      allOf:
+        - $ref: '#/components/schemas/Pet'
+        - type: object
+          properties:
+            bark:
+              type: string
+    Characteristics:
+      type: object
+      properties:
+        canHunt:
+          type: boolean