feat: add support for oneOf in sttp4 client

[plaflamme]

Address #19891 for sttp4 clients.

@Fish86

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---

Summary by cubic

Adds oneOf support to the Scala sttp4 client with sealed-trait parents and inline members, and aligns JSON and request/response handling with sttp4. Fixes discriminator mapping and makes circe constructor-name mapping total; adds a circe sample and updates to sttp 4.0.15 (fixes #19891).

• New Features

  • oneOf: sealed-trait parents; inline case classes when used by a single parent; supports discriminator + mapping; removes discriminator fields in children; circe/json4s codecs; tests incl. empty members; circe transformConstructorNames is total to avoid match errors.
  • JSON & API alignment: .body(asJson(...)); Either[ResponseException[String], T]; pass Option[String] directly to .header; container query/form values encode via mkString(","); default Seq params = Seq.empty; AnyType/object map to circe Json/JsonObject or json4s JValue/JObject based on jsonLibrary.
  • Enums & samples/deps: enums replaced with sealed-trait ADTs with per-type codecs; new scala-sttp4-circe sample and config; sttp client 4.0.15; add circe-generic and circe-generic-extras.

• Migration

  • Enums are now sealed traits; update pattern matches/usages and imports.
  • oneOf parents are sealed traits; some members are generated inline in the parent file; adjust imports if referenced directly.
  • Response type is Either[ResponseException[String], T] (was ResponseException[String, Exception]).
  • Container query/form values now encode via mkString(","); verify server expectations if relying on previous formatting.

^{Written for commit 7aba995. Summary will update on new commits.}

[cubic-dev-ai]

4 issues found across 8 files

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="modules/openapi-generator/src/main/resources/scala-sttp4/api.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/scala-sttp4/api.mustache:38">
P2: Request bodies are now always JSON‑serialized with `asJson(...)`, which breaks operations that consume non‑JSON media types by sending JSON instead of the raw body.</violation>
</file>

<file name="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache:52">
P2: json4s oneOf deserialization aborts on first mismatch because `Extraction.extract` throws, so later oneOf types are never tried.</violation>

<violation number="2" location="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache:74">
P2: Discriminator handling hardcodes the Scala class name as the wire value, ignoring OpenAPI discriminator mappings, so specs with mapped discriminator values will fail to deserialize/serialize correctly.</violation>

<violation number="3" location="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache:172">
P2: Inline oneOf member case classes are generated inside the parent model file, but the parent model’s import list isn’t augmented with the child models’ imports. If a oneOf member uses a type not referenced by the parent, the inline case class will compile without the necessary import.</violation>
</file>

---

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[cubic-dev-ai]

P2: Inline oneOf member case classes are generated inside the parent model file, but the parent model’s import list isn’t augmented with the child models’ imports. If a oneOf member uses a type not referenced by the parent, the inline case class will compile without the necessary import.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache, line 172:

<comment>Inline oneOf member case classes are generated inside the parent model file, but the parent model’s import list isn’t augmented with the child models’ imports. If a oneOf member uses a type not referenced by the parent, the inline case class will compile without the necessary import.</comment>

<file context>
@@ -15,46 +20,218 @@ import {{import}}
+{{/vendorExtensions.x-isEnum}}
+{{#vendorExtensions.x-isRegularModel}}
+{{^isEnum}}
+case class {{classname}}(
+  {{#vars}}
+  {{#description}}
</file context>

[wing328]

thanks for the PR

cc @clasnake (2017/07), @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04) @Fish86 (2023/06)

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/21774387501/job/62832048821?pr=22916

please follow step 3 to update the samples

[cubic-dev-ai]

1 issue found across 3 files (changes from recent commits).

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache:107">
P2: `transformConstructorNames` is defined as a partial match over only `x-oneOfMembers`. Because `x-oneOfMembers` excludes shared oneOf subtypes, the function is not total and will throw `MatchError` when encoding/decoding those subtypes. Add a default case (identity) so unmapped constructor names don’t crash serialization.</violation>
</file>

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

[plaflamme]

@wing328 I've updated the samples in e594023

[cubic-dev-ai]

1 issue found across 1 file (changes from recent commits).

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/scala-sttp4/model.mustache:110">
P2: Wildcard `case other` is emitted inside the oneOfMembers loop, so the first wildcard matches everything and makes subsequent member cases unreachable; only the first oneOf member can be encoded/decoded.</violation>
</file>

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