[typescript-fetch] Fix nested oneOf generating uncompilable interface extends on union type

[jasperpatterson]

When a oneOf discriminated union has a variant that references another oneOf via allOf (nested discriminated unions), the generator produces interface Child extends Parent where Parent is a TypeScript union type. This fails to compile with TS2312: "An interface can only extend an object type or intersection of object types with statically known members."

For example, given an outer oneOf discriminated by outerDiscriminator where one variant composes (via allOf) a fixed discriminator value with a $ref to an inner oneOf discriminated by innerDiscriminator:

OuterComposed:
  allOf:
    - type: object
      properties:
        outerDiscriminator: { type: string }
    - $ref: "#/components/schemas/Inner"   # Inner is a oneOf

Inner:
  oneOf:
    - $ref: "#/components/schemas/InnerA"
    - $ref: "#/components/schemas/InnerB"
  discriminator:
    propertyName: innerDiscriminator

Before:

// TS2312: An interface can only extend an object type or intersection of object types.
export interface OuterComposed extends Inner { ... }

After:

export type OuterComposed = Inner & { ... };

Changes

• TypeScriptFetchClientCodegen.java: In postProcessAllModels(), detect when a model's parent is a oneOf model and set a parentIsOneOf flag on the model.
• modelGenericInterfaces.mustache: When parentIsOneOf is true, emit export type X = Parent & { ... }; instead of export interface X extends Parent { ... }. Models with non-oneOf parents are unaffected.
• TypeScriptFetchClientCodegenTest.java: New test verifying the composed variant uses a type alias, the inner/outer discriminator dispatch is preserved, and plain models still generate as interfaces.
• nested-oneOf.yaml: Test spec exercising the nested discriminated union pattern.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  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.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

---

Summary by cubic

Fix TS2312 compile errors in the TypeScript Fetch generator when a model extends a oneOf union. Models with a oneOf parent now emit type X = Parent & { ... } instead of interface X extends Parent.

• Bug Fixes
  • Detect oneOf parents during post-processing and set parentIsOneOf, switching the template to emit an intersection type alias.
  • Added tests and a nested-oneOf.yaml spec to verify the alias is used, discriminators still dispatch correctly, and non-oneOf parents still generate interface.

^{Written for commit b4a4ecf. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 4 files