fix(core): preserve distinct object alternatives in oneOf/anyOf unions

[schani]

The bug

Given a JSON Schema oneOf (or anyOf) of two or more disjoint object
schemas, quicktype merged them into a single object type with all properties
made optional (plus an added index signature) instead of generating a
TypeScript union of the distinct object shapes.

Repro (from the issue):

{
  "oneOf": [
    { "type": "object", "required": ["id"], "properties": { "id": { "type": "integer" } } },
    { "type": "object", "required": ["externalId"], "properties": { "externalId": { "type": "string" } } }
  ]
}

quicktype --lang ts --src-lang schema --src AssetIdEither.json --just-types

Before:

export interface AssetIDEither {
    id?: number;
    externalId?: string;
    [property: string]: unknown;
}

After:

export type AssetIDEither = (PurpleAssetIDEither & { "externalId"?: never; }) | (FluffyAssetIDEither & { "id"?: never; });

export interface PurpleAssetIDEither {
    id: number;
    [property: string]: unknown;
}

export interface FluffyAssetIDEither {
    externalId: string;
    [property: string]: unknown;
}

Root cause

Schema parsing built the correct union, but flattenUnions/UnifyUnionBuilder
grouped all object-kind union members together and unconditionally merged
their properties into a single object (UnionType.canBeInlinedInTypeScript
disallowed more than one object-typed member, and UnifyUnionBuilder.makeObject
unioned all their properties as optional). There was no mechanism for marking
some object union members as distinct alternatives that must survive
unification.

The fix

• Added a new type attribute (packages/quicktype-core/src/attributes/UnionMembers.ts)
  to mark object types that originated as standalone oneOf/anyOf
  alternatives (not allOf/intersection members) as "distinct" union members,
  and to mark oneOf unions as mutually exclusive.
• JSONSchemaInput.ts now tags standalone oneOf/anyOf object cases with
  this attribute instead of letting them unify like ordinary compatible
  objects.
• UnionType.canBeInlinedInTypeScript, UnifyUnionBuilder, and
  UnionBuilder were updated to keep multiple distinct-object union members
  separate through flattening/unification instead of collapsing them.
• Added TargetLanguage.supportsUnionsWithMultipleObjectTypes (default
  false) and enabled it for TypeScript, Flow, JavaScript, and JSON Schema
  output, so other languages are unaffected.
• For TypeScript specifically, when a union is marked exclusive (from
  oneOf), sibling properties are excluded per-member with "prop"?: never
  guards so the generated union stays exclusive under TypeScript's excess
  property checks.

Test coverage

• test/inputs/schema/one-of-objects.schema (+ .1.json valid sample and
  .2.fail.one-of.json expected-failure sample exercising the new one-of
  fixture feature) — enabled for the typescript, javascript, and flow
  fixtures via test/languages.ts.
• test/unit/schema-object-unions.test.ts — a focused unit test reproducing
  the exact issue Union type not supported for objects #1266 schema for both oneOf and anyOf, asserting the
  output is a union of separate interfaces rather than one merged interface.

Coverage was added and confirmed failing against the unmodified code before
implementing the fix, per repo convention.

Verification

• npm run build — passes.
• npm run lint (Biome) — zero errors.
• npx vitest run test/unit — 172/172 tests pass.
• QUICKTEST=true FIXTURE=typescript,schema-typescript,javascript script/test
  — 208/208 tests pass (run twice to rule out flakiness; an initial run that
  showed a couple of unrelated transient failures under heavy concurrent
  machine load did not reproduce on repeat, or in isolation, or against a
  clean baseline worktree).
• Manually re-ran the exact repro command from the issue and confirmed the
  corrected union output shown above.
• Other language fixtures (beyond typescript/javascript/flow, which are the
  only ones opting into this behavior) are left to CI, since
  supportsUnionsWithMultipleObjectTypes defaults to false and this change
  should not affect their generated output.

Fixes #1266

🤖 Generated with Claude Code

[github-actions]

Generated-output differences

57 files differ — 22 modified, 35 new, 0 deleted
14619 changed lines — +10259 / −4360

Open the generated-output report →

[github-actions]

Generated-output differences

58 files differ — 22 modified, 36 new, 0 deleted
14695 changed lines — +10335 / −4360

Open the generated-output report →