[core] Restore sibling example for allOf with a single $ref (#23335)

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

@@ -4307,6 +4307,13 @@ public CodegenProperty fromProperty(String name, Schema p, boolean required, boo
             if (original.getTitle() != null) {
                 property.setTitle(original.getTitle());
             }
+            // the example was computed above against the inner (allOf/$ref) schema, which does
+            // not carry the example declared as a sibling of the allOf/$ref. Restore it here so
+            // that e.g. `allOf: [ $ref ]` with a sibling `example` keeps the declared example
+            // instead of falling back to the literal "null".
+            if (original.getExample() != null) {
+                property.example = toExampleValue(original);
+            }
         }
 
         // override defaultValue if it's not set and defaultToEmptyContainer is set

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

@@ -2105,6 +2105,22 @@ public void testTitleProperty() {
         assertEquals("Ref-Property-Title", codegen.fromProperty("refProperty", (Schema) testProperties.get("refProperty")).title);
     }
 
+    @Test
+    public void testAllOfSingleRefSiblingExample() {
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/property-title.yaml");
+        new InlineModelResolver().flatten(openAPI);
+        final DefaultCodegen codegen = new DefaultCodegen();
+        codegen.setOpenAPI(openAPI);
+
+        final Map testProperties = Collections.unmodifiableMap(openAPI.getComponents().getSchemas().get("ModelWithTitledProperties").getProperties());
+
+        // a plain property keeps its example
+        assertEquals("Simple-Property-Example", codegen.fromProperty("simpleProperty", (Schema) testProperties.get("simpleProperty")).example);
+        // an `allOf: [ $ref ]` property must keep the example declared as a sibling of the allOf,
+        // instead of falling back to the literal "null" computed against the inner $ref schema
+        assertEquals("Ref-Property-Example", codegen.fromProperty("refProperty", (Schema) testProperties.get("refProperty")).example);
+    }
+
     @Test
     public void testDeprecatedRef() {
         final OpenAPI openAPI = TestUtils.parseSpec("src/test/resources/3_0/model-deprecated.yaml");

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

@@ -19,9 +19,11 @@ components:
         simpleProperty:
           type: string
           title: Simple-Property-Title
+          example: Simple-Property-Example
         refProperty:
           type: string
           title: Ref-Property-Title
+          example: Ref-Property-Example
           allOf:
             - $ref: '#/components/schemas/RefObject'
       type: object