fix(schema): keep oneOf/anyOf object alternatives as a distinct union

[schani]

Bug

A JSON Schema oneOf/anyOf of multiple object schemas was always collapsed into a single interface/class with every property made optional, discarding the mutual-exclusivity the schema expressed.

Repro:

{
  "type": "array",
  "items": {
     "oneOf": [
        {"type": "object", "additionalProperties": false, "properties": { "aa": { "type": "string" }, "bb": { "type": "string" }}},
        {"type": "object", "additionalProperties": false, "properties": { "cc": { "type": "string" }, "dd": { "type": "string" }}}
     ]
  }
}

quicktype --lang typescript --src-lang schema schema.json

Before:

export interface PurpleArray {
    aa?: string;
    bb?: string;
    cc?: string;
    dd?: string;
}

{"aa":"x","cc":"y"} — invalid per the schema (belongs to neither oneOf branch) — type-checked and passed runtime validation against this interface.

After:

export interface PurpleSchema {
    aa?: string;
    bb?: string;
}

export interface FluffySchema {
    cc?: string;
    dd?: string;
}

with the array typed as Array<(PurpleSchema & { "cc"?: never; "dd"?: never }) | (FluffySchema & { "aa"?: never; "bb"?: never })>, and the generated runtime cast/Convert code rejects objects mixing properties from both branches (each branch is a closed class with additionalProperties: false, so an object containing a key from the other branch fails every union alternative and throws).

Root cause

quicktype's union-unification machinery (UnionBuilder/UnifyClasses.ts, invoked via unifyTypes/flattenUnions) assumed at most one object-shaped member per union type, so all object alternatives from a oneOf/anyOf always got merged into one class/clique with optional properties, regardless of whether the schema intended them to be mutually exclusive.

Fix

• JSONSchemaInput.ts now tags each object alternative of a standalone oneOf/anyOf (i.e. not nested under allOf/combined with sibling properties at the same schema level, which represents an intersection — see implicit-one-of.schema) with a new unionMemberDistinct type attribute, and marks oneOf unions themselves as unionIsExclusive.
• UnionType.isCanonical (Type/Type.ts) and the union-building machinery (UnionBuilder.ts, UnifyClasses.ts) now allow a canonical union to contain multiple object members when they're all tagged distinct, instead of forcing exactly one.
• This is gated per target language via a new TargetLanguage#supportsUnionsWithMultipleObjectTypes getter (default false), currently enabled for TypeScript, Flow, JavaScript, and JSON Schema output — the languages whose renderers can represent a union of multiple object types. All other languages keep their previous (safe) merging behavior unchanged.
• The TypeScript/Flow renderer (TypeScriptRenderer.ts) adds "key"?: never guards for each alternative's sibling keys on unionIsExclusive unions, so the emitted static TypeScript union type also rejects object literals mixing properties from more than one branch, not just the runtime cast.

Test coverage

• Added test/inputs/schema/one-of-objects.schema, mirroring the issue's exact repro, with:
  • one-of-objects.1.json — a valid two-element array (one object per branch).
  • one-of-objects.2.fail.one-of.json — an object mixing properties from both oneOf branches, which must make the generated program exit nonzero.
• Enabled the new "one-of" fixture feature for typescript, javascript, and flow in test/languages.ts so the fail sample runs for those languages.

Verification

• npm run build — passes.
• npm run test:unit — 163 tests passed.
• npx biome check . — clean.
• QUICKTEST=true FIXTURE=schema-typescript script/test <all 65 non-broken schema fixtures> — all pass, including the new one-of-objects.schema and the adjacent union.schema, implicit-one-of.schema, nested-intersection-union.schema, union-list.schema, mutually-recursive.schema, postman-collection.schema (verifying no regression to schemas with related but semantically different combinator patterns).
  • vega-lite.schema, class-with-additional.schema, and class-map-union.schema were excluded from that sweep because they already fail identically (same tsc errors) on a clean master checkout — confirmed by stashing this change and re-running them, unrelated pre-existing failures.
• QUICKTEST=true FIXTURE=schema-javascript script/test — new fixture and adjacent ones pass.
• QUICKTEST=true FIXTURE=schema-python|schema-golang|schema-rust script/test — new fixture and adjacent ones pass, confirming non-opted-in languages are unaffected.
• schema-csharp could not be verified locally (dotnet is not installed in this environment); left to CI.

Fixes #2310

🤖 Generated with Claude Code

[github-actions]

Generated-output differences

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13930 changed lines — +9570 / −4360

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[github-actions]

Generated-output differences

53 files differ — 22 modified, 31 new, 0 deleted
14003 changed lines — +9643 / −4360

Open the generated-output report →