Fix allOf with $ref to discriminator-less oneOf flattening all variant properties

[jasperpatterson]

When a schema uses allOf to compose with a $ref to a oneOf schema that has no discriminator, the codegen incorrectly resolves the oneOf by merging properties from all variants into the model's vars. This produces an interface requiring every variant-specific field simultaneously — an impossible conjunction.

ParentC:
  allOf:
    - type: object
      properties:
        type: { type: string }
    - $ref: "#/components/schemas/Child"

Child:
  oneOf:       # no discriminator
    - $ref: "#/components/schemas/ChildX"
    - $ref: "#/components/schemas/ChildY"

Before (broken): All variant properties merged into one flat interface, instanceOf check becomes unsatisfiable.

After (fixed): The oneOf ref is recognized as a parent. Only the model's own properties appear in vars. Generators with inheritance support emit proper type extension/delegation to the parent.

Root cause

Two independent issues compound into the bug:

1. ModelUtils.getParentName() gates parent recognition on hasOrInheritsDiscriminator(). A oneOf without a discriminator is rejected as a parent candidate, so parentName comes back null.

2. DefaultCodegen.updateModelForComposedSchema() — when no parent is recognized, the composition path calls addProperties() on the oneOf schema. addProperties() recurses into every oneOf variant, collecting all their properties into a single map. This flattens the disjunction into a conjunction.

Fix

ModelUtils.getParentName() / getAllParentsName() — Added a hasOneOf(s) check so that a $ref to a oneOf schema is treated as a parent candidate even without a discriminator.

DefaultCodegen.updateModelForComposedSchema() — Added a ModelUtils.hasOneOf(refSchema) guard at the top of the property-resolution chain. When a $ref resolves to a oneOf schema, addProperties() is skipped entirely to prevent variant flattening. This fires before the inheritance/composition branches, protecting all generators regardless of supportsInheritance.

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---

Summary by cubic

Fixes the bug where allOf with a $ref to a discriminator-less oneOf merged all variant properties into the model. The oneOf ref is now treated as a parent and union semantics are preserved; wrapper-level properties are inherited correctly.

• Bug Fixes
  • In ModelUtils.getParentName()/getAllParentsName(), treat a $ref to a oneOf schema as a parent even without a discriminator.
  • In DefaultCodegen.updateModelForComposedSchema(), pre-seed visited with oneOf variants before addProperties() to suppress flattening variant fields while still inheriting the wrapper’s own properties and requireds; works with or without inheritance.
  • Added a 3.0 spec and unit tests for parent assignment, variant property isolation, and wrapper property flow-through (including when inheritance is disabled); updated C# to mark oneOf/anyOf wrappers as mutable and keep union constructors public in AbstractCSharpCodegen; regenerated samples.

^{Written for commit 398e391. Summary will update on new commits.}

[cubic-dev-ai]

6 issues found across 178 files

Note: This PR contains a large number of files. cubic only reviews up to 75 files per PR, so some files may not have been reviewed. cubic prioritises the most important files to review.

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/csharp/generichost/net10/NullReferenceTypes/src/Org.OpenAPITools/Model/ShapeOrNull.cs">

<violation number="1" location="samples/client/petstore/csharp/generichost/net10/NullReferenceTypes/src/Org.OpenAPITools/Model/ShapeOrNull.cs:38">
P2: Making the union constructors internal leaves the public ShapeOrNull type without any public constructor, preventing external consumers from creating instances.</violation>
</file>

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

<violation number="1" location="modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java:2805">
P2: Skipping addProperties for any refSchema with oneOf drops wrapper-level properties/required fields, not just variant properties, so shared required fields on a oneOf wrapper will never be added to allProperties/allRequired and disappear from generated models.</violation>
</file>

<file name="samples/client/petstore/csharp/generichost/net4.8/Petstore/src/Org.OpenAPITools/Model/ShapeOrNull.cs">

<violation number="1" location="samples/client/petstore/csharp/generichost/net4.8/Petstore/src/Org.OpenAPITools/Model/ShapeOrNull.cs:36">
P2: Changing the only constructors to internal makes the public ShapeOrNull type non-instantiable for consumers, preventing outbound creation of ShapeOrNull payloads.</violation>
</file>

<file name="samples/client/petstore/csharp/generichost/net8/Petstore/src/Org.OpenAPITools/Model/ShapeOrNull.cs">

<violation number="1" location="samples/client/petstore/csharp/generichost/net8/Petstore/src/Org.OpenAPITools/Model/ShapeOrNull.cs:36">
P2: ShapeOrNull is a public model but both constructors were changed to internal, leaving no public instantiation path for SDK consumers.</violation>
</file>

<file name="samples/client/petstore/csharp/generichost/latest/UseDateTimeOffset/src/Org.OpenAPITools/Model/ShapeOrNull.cs">

<violation number="1" location="samples/client/petstore/csharp/generichost/latest/UseDateTimeOffset/src/Org.OpenAPITools/Model/ShapeOrNull.cs:37">
P2: ShapeOrNull no longer has any public constructor or factory; changing the union constructors to internal makes the public model non-instantiable for SDK consumers.</violation>
</file>

<file name="samples/client/petstore/csharp/generichost/net8/FormModels/src/Org.OpenAPITools/Model/Shape.cs">

<violation number="1" location="samples/client/petstore/csharp/generichost/net8/FormModels/src/Org.OpenAPITools/Model/Shape.cs:36">
P2: Making the only constructors internal leaves public Shape with no public instantiation path for SDK consumers, creating a breaking API regression.</violation>
</file>

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