fix(kotlin): support oneOf with primitive types in kotlinx_serialization

[pvcresin]

Description

The kotlin-client generator with jvm-retrofit2 library and kotlinx_serialization failed to generate working code for oneOf schemas containing primitive types (e.g. string, integer) without a discriminator. The existing template only handled discriminator-based oneOf via sealed interfaces, producing an empty class for primitive oneOf.

company_id:
  oneOf:
    - type: string
    - type: integer
      format: int64

Changes:

• Add a non-discriminator fallback in oneof_class.mustache that generates a data class with actualInstance and a custom KSerializer, ported from the multiplatform template
• Add isLong (int64) handling to the serialize branch, with an isInteger guard to avoid generating a dead branch for int64 types
• Add test for oneOf primitive types with kotlinx_serialization

Before

@Serializable
class TestModelCompanyId () {
}

After

@Serializable(with = TestModelCompanyId.TestModelCompanyIdSerializer::class)
data class TestModelCompanyId(var actualInstance: Any? = null) {

    object TestModelCompanyIdSerializer : KSerializer<TestModelCompanyId> {
        override fun serialize(encoder: Encoder, value: TestModelCompanyId) {
            when (val instance = value.actualInstance) {
                is kotlin.String -> jsonEncoder.encodeString(instance)
                is kotlin.Long -> jsonEncoder.encodeLong(instance)
                null -> jsonEncoder.encodeJsonElement(JsonNull)
                else -> throw SerializationException(...)
            }
        }

        override fun deserialize(decoder: Decoder): TestModelCompanyId {
            // try each type in schema order
            try { decodeFromJsonElement<kotlin.String>(...) } catch { ... }
            try { decodeFromJsonElement<kotlin.Long>(...) } catch { ... }
        }
    }
}

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---

Summary by cubic

Fixes Kotlin client generation with jvm-retrofit2 + kotlinx_serialization for oneOf schemas that include primitive types without a discriminator. Now generates a data-class wrapper with a custom KSerializer that correctly handles primitives and null, and tries each type on decode.

• Bug Fixes
  • Added non-discriminator fallback in oneof_class.mustache: generate a data class with actualInstance and an in-file KSerializer (ported from multiplatform).
  • Serializer covers String, Boolean, Int, Long (int64 via isInteger guard), Float, Double, and null (JsonNull); delegates non-primitives to their serializers.
  • Added a test validating wrapper generation, try-each deserialization, and that the parent model references the wrapper type.

^{Written for commit cff7fbe. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (all 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/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:217">
P2: oneOf deserialization returns on the first successful decode without checking for multiple matches, so ambiguous primitives (e.g., Int/Long/Double) will silently select the first schema instead of enforcing exactly one match.</violation>
</file>

---

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[pvcresin]

@cubic-dev-ai rerun a review

[cubic-dev-ai]

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[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (all 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/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:198">
P2: `{{{dataType}}}.serializer()` will not compile for parameterized/collection oneOf types (e.g., `List<Foo>`/`Map<String,Bar>`), so the generated Kotlin for oneOf with array/map schemas will fail to compile. Use a collection serializer or the top‑level `serializer<T>()` instead.</violation>
</file>

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

[pvcresin]

@cubic-dev-ai rerun a review

[cubic-dev-ai]

@cubic-dev-ai rerun a review

@pvcresin I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (all 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/kotlin-client/oneof_class.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/kotlin-client/oneof_class.mustache:217">
P2: Non-discriminator oneOf deserialization returns on the first successful decode, so overlapping primitive schemas can match multiple types but the first one wins, violating oneOf’s “exactly one” semantics.</violation>
</file>

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

[pvcresin]

This is a PR regarding Kotlin. Could you please review it?

@karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l @dennisameling

[e5l]

Hey @pvcresin, thank you for the PR!

lgtm

[pvcresin]

This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
These must match the expectations made by your contribution.

Rebased on the latest master. The code has not been changed since the PR was created.