[core] Allow oneOf members that declare x-implements (#23577) (#24076)

OneOfImplementorAdditionalData.addToImplementor used putIfAbsent + List.add
on the model's x-implements vendor extension. When a oneOf member schema
already declares x-implements in the spec, the parsed value is a scalar
string or an immutable list, so appending the oneOf interface threw
java.lang.UnsupportedOperationException during model post-processing.

Normalize the existing value into a fresh mutable list (preserving any
user-supplied interfaces) before appending. No behavior change for models
without a pre-existing x-implements.

Author: seonwooj0810

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

@@ -103,11 +103,20 @@ public void addFromInterfaceModel(CodegenModel cm, List<Map<String, String>> mod
      */
     @SuppressWarnings("unchecked")
     public void addToImplementor(CodegenConfig cc, CodegenModel implcm, List<Map<String, String>> implImports, boolean addInterfaceImports) {
-        implcm.getVendorExtensions().putIfAbsent(X_IMPLEMENTS, new ArrayList<String>());
+        // The model may already declare x-implements in the spec, in which case the parsed value can be
+        // a scalar string or an immutable list. Normalize it to a fresh mutable list (preserving any
+        // existing entries) so the oneOf interfaces below can be appended without failing.
+        Object existing = implcm.getVendorExtensions().get(X_IMPLEMENTS);
+        List<String> impl = new ArrayList<>();
+        if (existing instanceof Collection) {
+            impl.addAll((Collection<String>) existing);
+        } else if (existing instanceof String && !((String) existing).isEmpty()) {
+            impl.add((String) existing);
+        }
+        implcm.getVendorExtensions().put(X_IMPLEMENTS, impl);
 
         // Add implemented interfaces
         for (String intf : additionalInterfaces) {
-            List<String> impl = (List<String>) implcm.getVendorExtensions().get(X_IMPLEMENTS);
             impl.add(intf);
             if (addInterfaceImports) {
                 // Add imports for interfaces

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

@@ -7995,6 +7995,19 @@ void oneOf_issue_23577() throws IOException {
                 .fileContains("String type;");
     }
 
+    @Test
+    void oneOf_issue_23577_userDefinedXImplements() throws IOException {
+        // Default oneOf-interface generation (without REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING):
+        // a member schema that already declares its own x-implements must still be able to
+        // receive the oneOf interface, i.e. the user-supplied x-implements value must remain mutable.
+        Map<String, File> files = generateFromContract("src/test/resources/3_0/oneOf_issue_23577.yaml", SPRING_BOOT,
+                Map.of(GENERATE_MODEL_DOCS, false, GENERATE_APIS, false, INTERFACE_ONLY, true));
+        JavaFileAssert.assertThat(files.get("CreatedEvent.java"))
+                .implementsInterfaces("com.example.Notification", "Event");
+        JavaFileAssert.assertThat(files.get("UpdatedEvent.java"))
+                .implementsInterfaces("Event");
+    }
+
     @Test
     void oneof_polymorphism_and_inheritance() throws IOException {
         Map<String, File> files = generateFromContract("src/test/resources/3_0/oneof_polymorphism_and_inheritance.yaml", SPRING_BOOT,