Fix "Type creator didn't return its forwarding ref" crash on sibling maps

[schani]

Problem

Valid JSON input with sibling map-typed fields — where one map has entries and a sibling map is empty — crashes quicktype for every target language with:

Error: Internal error: Type creator didn't return its forwarding ref.

(The web app swallows the error, so it just silently produces nothing.) Minimal 126-byte repro:

{"mission_specs": {"1": {"objectives": {"2": {"rewards": {"3": {}, "5": {}}}}}, "4": {"objectives": {"6": {"rewards": {}}}}}}

The only workaround was --no-maps.

Root cause

In UnifyClasses.ts, UnifyUnionBuilder.makeObject has three branches for combining object types. The class branch (getUniqueClassType) and the object branch (getUniqueObjectType) both pass the pending forwardingRef through, but the map branch called this.typeBuilder.getMapType(typeAttributes, ...) without it. When the inferMaps rewrite unifies sibling classes into a map, the newly created map type gets a fresh ref instead of consuming the forwarding ref, tripping the assert in GraphRewriteBuilder.withForwardingRef (GraphRewriting.ts).

Fix

One line: pass forwardingRef to getMapType in the map branch, matching the other two branches (getMapType already accepts an optional forwardingRef parameter).

Tests

Test first: the minimized JSON was added as test/inputs/json/priority/bug2037.json before the fix, and FIXTURE=golang script/test test/inputs/json/priority/bug2037.json was run against the unpatched build — it failed with the exact crash (Internal error: Type creator didn't return its forwarding ref, thrown from GraphRewriteBuilder.withForwardingRef via UnifyUnionBuilder.makeObject). After the fix, the same run passes (Go generation, compile, and JSON round-trip).

Since priority/ inputs run for all languages, this fixture guards the fix everywhere.

Verification

• Post-fix, QUICKTEST=1 FIXTURE=golang script/test (all 41 priority/schema samples incl. the new one) passes locally — no regressions from the new priority input.
• The original 1.1 MB sync.txt attachment from the issue, plus all minimized variants from triage, now convert successfully (verified with --lang csharp).
• Generation-level spot check across typescript, rust, python, swift, java, kotlin, csharp, dart, php, and elm: all succeed on the new input.
• Not verified locally: compile/round-trip fixtures for toolchains not installed here (C#, Java, Kotlin, Dart, PHP, …) — CI covers those since the input is in priority/.

This also fixes the duplicates #1320 and #1509 (same forwarding-ref crash).

Fixes #2037

🤖 Generated with Claude Code