[swift5/swift6] Fix oneOf decoding when enumUnknownDefaultCase is enabled

[jasperpatterson]

When enumUnknownDefaultCase=true is used with oneOf types, the generated Swift decoding logic always matches the first variant and never tries subsequent ones, effectively breaking oneOf discrimination.

Root Cause

Two features interact badly:

1. enumUnknownDefaultCase=true adds CaseIterableDefaultsLast conformance to every generated enum, making enum decoding never throw — unknown values silently fall back to .unknownDefaultOpenApi.
2. The oneOf decoder uses try? container.decode(Variant.self) to attempt each variant in order. Since inner enum properties never fail to decode, the first variant always succeeds.

Fix

Introduces an UnknownCaseCheckable protocol that lets the oneOf decoder reject a variant whose enum properties only matched due to the unknown-case fallback:

• Models.mustache — Defines the UnknownCaseCheckable protocol with a default implementation returning false (only when enumUnknownDefaultCase is enabled)
• model.mustache — Generates conformance extensions only on types that have enum properties ({{#hasEnums}}):
• modelOneOf.mustache — Adds a guard to the non-discriminator decode path:

  if let value = try? container.decode(Variant.self),
     (value as? UnknownCaseCheckable)?.containsUnknownDefaultOpenApiCase != true {

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---

Summary by cubic

Fixes Swift oneOf decoding when enumUnknownDefaultCase=true by rejecting variants that only decode due to .unknownDefaultOpenApi, including values inside arrays. Restores correct oneOf matching without discriminators in Swift 5 and 6.

• Bug Fixes
  • Add UnknownCaseCheckable protocol with a default false implementation (Swift5/Swift6 Models.mustache).
  • Generate conformance for enums and models with enum fields; skip oneOf interfaces. Detect unknown-defaults in single enum fields, arrays of enums, and NullEncodable-wrapped enums; map/dictionary enum fields are ignored.
  • Guard the non-discriminator oneOf decode path to skip values where containsUnknownDefaultOpenApiCase == true (Swift5/Swift6 modelOneOf.mustache).
  • Add Swift6 test to verify the guard/protocol and update Petstore samples for Swift5/Swift6.
  • No change when enumUnknownDefaultCase is disabled.

^{Written for commit 58dacc1. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

4 issues found across 23 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="modules/openapi-generator/src/main/resources/swift6/model.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/swift6/model.mustache:35">
P2: Unknown-case detection skips enums inside containers, so oneOf decoding can still accept a variant even when enum elements fell back to `.unknownDefaultOpenApi`.</violation>
</file>

<file name="modules/openapi-generator/src/main/resources/swift5/model.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/swift5/model.mustache:39">
P2: containsUnknownDefaultOpenApiCase skips enum containers, so unknown enum values inside arrays/maps are not detected and oneOf decoding can still accept the wrong variant.</violation>
</file>

<file name="samples/client/petstore/swift6/resultLibrary/PetstoreClient/Classes/OpenAPIs/Models/EnumArrays.swift">

<violation number="1" location="samples/client/petstore/swift6/resultLibrary/PetstoreClient/Classes/OpenAPIs/Models/EnumArrays.swift:47">
P2: UnknownCaseCheckable ignores arrayEnum, so unknown enum values in array_enum won’t be detected and oneOf discrimination can accept a variant based on unknown-enum fallback.</violation>
</file>

<file name="samples/client/petstore/swift5/resultLibrary/PetstoreClient/Classes/OpenAPIs/Models/EnumArrays.swift">

<violation number="1" location="samples/client/petstore/swift5/resultLibrary/PetstoreClient/Classes/OpenAPIs/Models/EnumArrays.swift:50">
P2: UnknownCaseCheckable ignores arrayEnum, so unknown enum defaults inside arrayEnum are not detected.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[Dustin4444]

Fixes #23501

[wing328]

thanks for the PR

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11) @dydus0x14 (2023/06)

[4brunu]

Hi @jasperpatterson thanks for your PR. Can you please take a look at the CI errors? Thanks

[jasperpatterson]

Hi @jasperpatterson thanks for your PR. Can you please take a look at the CI errors? Thanks

Hey @4brunu as far I can tell these CI errors aren't related to my changes, not really sure what to do with them.

[4brunu]

You are correct, CI failures are not related to this PR. Can you please take a look at the open issues, related to detecting unknown in containers? Thanks

[jasperpatterson]

You are correct, CI failures are not related to this PR. Can you please take a look at the open issues, related to detecting unknown in containers? Thanks

@4brunu Thanks, fixed up that issue.