[core] Fix UnsupportedOperationException for oneOf members declaring x-implements (#23577)

[seonwooj0810]

Fixes #23577

Problem

When a schema is a member of a oneOf and also declares its own x-implements in the spec, generation crashes with java.lang.UnsupportedOperationException during model post-processing:

java.lang.UnsupportedOperationException
    at java.util.AbstractList.add(AbstractList.java:155)
    at org.openapitools.codegen.utils.OneOfImplementorAdditionalData.addToImplementor(OneOfImplementorAdditionalData.java:111)
    at org.openapitools.codegen.DefaultCodegen.postProcessAllModels(DefaultCodegen.java:577)
    ...

addToImplementor did putIfAbsent(X_IMPLEMENTS, new ArrayList<>()) and then List.add(...) on the stored value. When the model already carries x-implements from the spec, that value is the parser-supplied object (a scalar string or an immutable list), so putIfAbsent is a no-op and the subsequent add fails.

This is the default oneOf-interface code path. The existing oneOf_issue_23577 test only covers the REPLACE_ONE_OF_BY_DISCRIMINATOR_MAPPING normalizer path, which converts oneOf to inheritance and never reaches this code — so the default path remained broken.

Fix

Normalize the existing x-implements value into a fresh mutable list (preserving any user-supplied interfaces, whether stored as a scalar string or a list) before appending the oneOf interface. Models without a pre-existing x-implements are unaffected — the result is identical to the previous putIfAbsent(new ArrayList<>()) behavior.

Test evidence

Added SpringCodegenTest#oneOf_issue_23577_userDefinedXImplements, which generates the reporter's spec on the default oneOf-interface path (no normalizer) and asserts the member implements both its declared interface and the oneOf parent:

• CreatedEvent implements com.example.Notification and Event
• UpdatedEvent implements Event

Confirmed the new test fails with UnsupportedOperationException before the change and passes after. The pre-existing oneOf_issue_23577 and OneOfImplementorAdditionalDataTest still pass.

Tests run: 3, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS

No generated samples change (the previously-crashing combination produced no committed samples; the no-x-implements path is byte-for-byte unchanged), so no sample regeneration is included.

Verification done: (1) no in-flight PR (searched open PRs for x-implements/OneOfImplementor/#23577); (2) no claim comments on the issue; (3) code fix in .java only; (4) reproduced the crash on upstream/master via a failing test, then confirmed the fix; (5) no parent epic.

---

Summary by cubic

Fixes a crash when a oneOf member also declares its own x-implements. We now normalize existing x-implements to a mutable list before adding the oneOf interface.

• Bug Fixes
  • Convert pre-existing x-implements (string or immutable list) into a new mutable list before appending oneOf interfaces; models without x-implements are unchanged.
  • Add test oneOf_issue_23577_userDefinedXImplements to verify members implement both the user-defined interface(s) and the oneOf parent.

^{Written for commit bac76e7. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[wing328]

tested locally to confirm no longer get the UnsupportedOperationException.