fix(java): don't emit unqualified enum default for discriminator ref-to-property

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

@@ -7155,7 +7155,25 @@ public void updateCodegenPropertyEnum(CodegenProperty var) {
                 }
             }
             if (enumName != null) {
-                var.defaultValue = toEnumDefaultValue(var, enumName);
+                if (var.isEnum || !languageSpecificPrimitives.contains(varDataType)) {
+                    var.defaultValue = toEnumDefaultValue(var, enumName);
+                } else {
+                    // Not an inline nested enum (var.isEnum) and varDataType is a raw
+                    // language-specific primitive (e.g. "String"): there's no enum class at
+                    // this use site to qualify the value with (e.g. a discriminator property
+                    // that is a `$ref` into another schema's property rather than a ref to the
+                    // enum schema itself). allowableValues/enum matching still ran, but
+                    // emitting a bare, unqualified token like `String.ARCHIVE` would not
+                    // compile. Drop the default instead (see #24874).
+                    //
+                    // Note: this check is intentionally based on languageSpecificPrimitives
+                    // rather than referencedSchema.isPresent() — referencedSchema's name match
+                    // against the raw dataType string is brittle for codegens whose dataType is
+                    // namespace/package-qualified (e.g. PHP's `OpenAPI\Server\Model\Foo`), which
+                    // would never match `toModelName(...)` and incorrectly drop valid enum
+                    // defaults for genuine named-enum refs.
+                    var.defaultValue = null;
+                }
             }
         }
     }

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

@@ -1869,6 +1869,27 @@ public void testObjectDefaultWithEnumProperty_issue24298() {
                 .contains("new OutputFormat().order(OutputFormat.OrderEnum.SIMILARITY).limit(10)");
     }
 
+    @Test
+    public void testDiscriminatorPropertyRefToEnumDoesNotEmitInvalidDefault_issue24874() {
+        final Path output = newTempFolder();
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName(JAVA_GENERATOR)
+                .setInputSpec("src/test/resources/bugs/issue_24874.yaml")
+                .setAdditionalProperties(Map.of(MODEL_NAME_PREFIX, "Stock"))
+                .setOutputDir(output.toString().replace("\\", "/"));
+
+        Map<String, File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate().stream()
+                .collect(Collectors.toMap(File::getName, Function.identity()));
+
+        // CategoryEvent.category is a discriminator property that is a `$ref` to another schema's
+        // (CategorySource) inline enum property, not to a named enum schema and not an inline enum
+        // itself. There is no enum type at this use site to qualify the discriminator mapping value
+        // with, so no default must be emitted here; previously this rendered the uncompilable
+        // `this.category = String.ARCHIVE;` (see #24874).
+        JavaFileAssert.assertThat(files.get("StockArchiveCategoryEvent.java"))
+                .fileDoesNotContain("String.ARCHIVE");
+    }
+
     @Test
     public void testWebClientJsonCreatorWithNullable_issue12790() {
         final Path output = newTempFolder();

modules/openapi-generator/src/test/resources/bugs/issue_24874.yaml

@@ -0,0 +1,35 @@
+openapi: "3.1.0"
+info:
+  title: Shared category discriminator
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    CategorySource:
+      type: object
+      required:
+        - category
+      properties:
+        category:
+          type: string
+          enum:
+            - ARCHIVE
+    CategoryEvent:
+      type: object
+      required:
+        - category
+      properties:
+        category:
+          description: Event category
+          $ref: "#/components/schemas/CategorySource/properties/category"
+      discriminator:
+        propertyName: category
+        mapping:
+          ARCHIVE: "#/components/schemas/ArchiveCategoryEvent"
+    ArchiveCategoryEvent:
+      allOf:
+        - $ref: "#/components/schemas/CategoryEvent"
+        - type: object
+          properties:
+            value:
+              type: string