[feat][scala-sttp] Add oneOf and allOf discriminator support with sealed traits for circe generator

[nikhilsu]

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

Adds discriminator-based polymorphism support to the scala-sttp generator when using circe as the JSON library. This enables correct deserialization of polymorphic types.

Implementation

Three discriminator patterns are supported:

1. allOf + discriminator (no oneOf on parent) - e.g., Animal → Cat/Dog with mapped values
2. oneOf + discriminator (no explicit mapping) - dispatches using schema names
3. oneOf + discriminator + mapping - dispatches using explicit mapped values

Falls back to regular (non-sealed) traits for edge cases like nested oneOf or shared members.

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---

Summary by cubic

Adds discriminator-based polymorphism to the scala-sttp generator with circe, supporting both oneOf and allOf. Generates sealed traits and discriminator-aware codecs for correct deserialization; updates docs, tests, and samples.

• New Features

  • Advertises support for oneOf/allOf; synthesizes oneOf from allOf discriminator mappings for uniform handling.
  • Generates a sealed trait when all children can be inlined; otherwise a regular trait. Inlining only when a child is referenced once and not used as a field type or inside containers.
  • Circe codecs write discriminator fields on encode and dispatch on decode (using explicit mappings or schema names). Falls back to a union decoder when no discriminator.
  • Docs mark oneOf/allOf as supported; config now points to scala-sttp-circe/petstore.yaml. Samples show Animal (regular trait due to Seq[Dog]), Collar (mapped), and Treat (schema-name dispatch). Tests cover baseName field mapping, allOf/oneOf discriminator flows, and no-inlining when children are reused.

• Bug Fixes

  • Unknown discriminator values now return clear DecodingFailure errors; added missing imports.
  • Prevented incorrect inlining when a child model is referenced in containers or as a property.

^{Written for commit 821db67. Summary will update on new commits.}

[nikhilsu]

cc: @wing328

[cubic-dev-ai]

3 issues found across 21 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/scala-sttp/model.mustache">

<violation number="1" location="modules/openapi-generator/src/main/resources/scala-sttp/model.mustache:67">
P2: Regular-trait encoders are non-exhaustive; unknown implementations will throw MatchError during encoding.</violation>
</file>

<file name="samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/model/Animal.scala">

<violation number="1" location="samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/model/Animal.scala:45">
P2: Subtype codecs use unconstrained `className` strings while the polymorphic codec expects canonical discriminator constants, causing inconsistent serialization and potential wrong subtype decoding.</violation>
</file>

<file name="modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java">

<violation number="1" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java:363">
P2: Inlining decision only counts oneOf parents; a child referenced elsewhere can still be marked inlineable and removed, leading to missing model classes for non-oneOf references.</violation>
</file>

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

[cubic-dev-ai]

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

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/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java">

<violation number="1" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java:316">
P2: Model reference counting uses exact `dataType` matching, missing container-wrapped model references and causing incorrect inlining decisions.</violation>
</file>

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

[cubic-dev-ai]

2 issues found across 6 files (changes from recent commits).

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/test/java/org/openapitools/codegen/scala/ScalaSttpCirceCodegenTest.java">

<violation number="1" location="modules/openapi-generator/src/test/java/org/openapitools/codegen/scala/ScalaSttpCirceCodegenTest.java:158">
P2: Test assertion does not verify non-sealed trait and will pass for `sealed trait Animal` too.</violation>
</file>

<file name="modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java">

<violation number="1" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java:367">
P1: Shared oneOf child models can be rebound to the last processed parent because parent vendor extensions are overwritten unconditionally.</violation>
</file>

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

[cubic-dev-ai]

P1: Shared oneOf child models can be rebound to the last processed parent because parent vendor extensions are overwritten unconditionally.

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/java/org/openapitools/codegen/languages/ScalaSttpClientCodegen.java, line 367:

<comment>Shared oneOf child models can be rebound to the last processed parent because parent vendor extensions are overwritten unconditionally.</comment>

<file context>
@@ -345,16 +352,26 @@ private void markOneOfTraits(Map<String, ModelsMap> modelsMap,
+            if (child == null) continue;
+
+            // All children extend the parent trait
+            child.getVendorExtensions().put("x-oneOfParent", parent.classname);
+            if (parent.discriminator != null) {
+                child.getVendorExtensions().put("x-parentDiscriminatorName",
</file context>

[cubic-dev-ai]

1 issue found across 5 files (changes from recent commits).

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="samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/model/Animal.scala">

<violation number="1" location="samples/client/petstore/scala-sttp-circe/src/main/scala/org/openapitools/client/model/Animal.scala:18">
P2: `Animal` was made non-sealed, but `encoderAnimal` still matches only `Cat`/`Dog`, leaving a non-exhaustive runtime match for `Animal` values.</violation>
</file>

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

[GerretS]

Note that both this PR and #22916 are for solving this issue: #19891

I didn't look into the code myself but mentioning it here to prevent duplicate work.

[nikhilsu]

#22916 adds oneOf support for scala-sttp4-circe generator. This one is for sttp3