fix(schema-input): unify $ref to another top-level schema source

[schani]

Bug

When generating from multiple JSON Schema sources where one schema's property $refs another top-level source, quicktype emitted two separate, structurally-identical interfaces instead of reusing the top-level type.

Repro (--src=<dir> with two files, one $ref-ing the other):

node dist/index.js --src=res --src-lang=schema --just-types --lang=ts

res/WebsiteOrder.schema has a property info that is "$ref": "OrderCustomer.schema#", and res/OrderCustomer.schema is itself a top-level source. Before the fix, quicktype produced both OrderCustomer (from the top-level source) and a duplicate Info interface (synthesized for the $ref-ed property), with WebsiteOrder.info: Info. Expected: a single OrderCustomer interface reused for WebsiteOrder.info: OrderCustomer.

Root cause

In addTypesInSchema (packages/quicktype-core/src/input/JSONSchemaInput.ts), a property schema that was a pure $ref alias (only $ref, a matching type, or the draft-3 boolean required keyword present) was still routed through the same "needs a union/intersection" path as schemas with real narrowing keywords (properties, items, enum, etc.). That path always built a fresh type for the schema instead of reusing the identity of the resolved $ref target, so a $ref pointing at another top-level source got re-synthesized as a brand-new type rather than unified with the type already generated for that source.

Fix

$ref resolution is hoisted earlier so it's available when deciding whether a union/intersection is needed. A new isRefAlias check recognizes schemas that are pure $ref aliases (no narrowing keywords beyond a same-typed type or boolean required) and skips the union path for them, letting the existing $ref-target-reuse logic (already present further down) unify the type instead of synthesizing a duplicate.

Test coverage

• test/inputs/schema/multi-source-1543/ plus positive and negative JSON samples: the exact directory-as-source repro is registered for schema-typescript and runs end to end through the standard TypeScript compile/round-trip fixture.
• test/unit/schema-multiple-sources.test.ts: the round trip cannot detect structurally identical duplicate interfaces, so this targeted assertion verifies that info reuses OrderCustomer and no redundant Info interface is emitted.

Verification

• npm run build passes.
• The exact base/head repro changes from duplicate OrderCustomer and Info interfaces to one OrderCustomer, with TopLevel.info: OrderCustomer.
• npx vitest run test/unit/schema-multiple-sources.test.ts passes.
• QUICKTEST=true CPUs=2 FIXTURE=schema-typescript npm run test:fixtures passes all 81 cases, including the positive and expected-failure multi-source-1543 samples.
• A focused generated-output snapshot comparison reports exactly one added fixture output, schema-typescript/test/inputs/schema/multi-source-1543/default/TopLevel.ts, containing info: OrderCustomer and no Info interface.

Fixes #1543

🤖 Generated with Claude Code

[schani]

Heads up on a change to the test setup here. The multi-source-1543 directory fixture couldn't run through the round-trip harness: a directory source always produces multiple top-level types (one per file), and several backends emit per-type entry points for that case (java's Converter.TopLevelFromJsonString instead of fromJsonString, elm's per-type decoders, etc.) rather than the single top-level the shared fixture drivers assume. That made the sample fail to compile for java and elm, and it would fail the same way for other backends; the scenario can't be reduced to a single top-level because the bug specifically needs two top-level sources where one $refs the other.

The round-trip harness also can't observe the actual bug — the fix only removes a duplicate structurally-identical interface, so buggy and fixed output round-trip the sample identically. That regression is fully covered by test/unit/schema-multiple-sources.test.ts, which asserts the duplicate Info interface is not emitted and info reuses OrderCustomer.

I dropped the fixture registration and its round-trip sample JSON, kept the schema files under test/inputs/schema/multi-source-1543 as the unit test's input, and clarified the test comment. The core JSONSchemaInput fix is unchanged.

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219 changed lines — +219 / −0

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