Kotlin/Klaxon: fix deserialization of maps with empty-object values

[schani]

Problem

The Kotlin/Klaxon renderer renders empty object types as typealias X = JsonObject. When such a type appears as a map value (val rewards: Map<String, Reward> with typealias Reward = JsonObject), the generated code fails to parse valid input:

KlaxonException: Couldn't find a suitable constructor for class JsonObject to initialize with {}

Minimal repro (from #2877's fixture, test/inputs/json/priority/bug2037.json):

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

Root cause

quicktype already registers a custom converter for JsonObject::class so empty objects deserialize at class-property positions. But Klaxon's DefaultConverter.fromJsonObject looks up converters for map values with

val typeValue = jt.actualTypeArguments[1]
val converter = klaxon.findConverterFromClass(typeValue.javaClass, null)

typeValue.javaClass is java.lang.Class — the class of the reflection Type object itself, not the value type — so custom converters are never consulted for map values (present in Klaxon 3.0.1 through current master). Klaxon then falls back to constructing JsonObject reflectively, and its only constructor (JsonObject(map: MutableMap<String, Any?>)) can't be satisfied from {}.

Fix

Klaxon does consult field-level converters (Klaxon.fieldConverter), which receive the whole field value before the broken map-value lookup is reached. The Klaxon renderer now:

• detects class properties whose type is a map (directly, or nested in further maps, incl. nullable ones) whose values are empty-object types;
• when any exist, declares a @Target(AnnotationTarget.FIELD) annotation KlaxonJsonObjectMap plus a converter object, registers it via .fieldConverter(KlaxonJsonObjectMap::class, jsonObjectMapConverter), and annotates those properties.

The converter's fromJson simply returns the parsed JsonValue.obj — a JsonObject whose values are already JsonObjects, i.e. exactly a runtime Map<String, JsonObject> — and its toJson delegates to klaxon.toJsonString, so round-tripping is unchanged. Empty objects everywhere else (direct properties, arrays, top levels) keep the existing JsonObject typealias + converter path untouched.

KotlinRenderer.renameAttribute now also receives the ClassProperty so the Klaxon renderer can inspect the property type when emitting attribute annotations (other Kotlin renderers are unaffected).

Tests (written first)

• New unit test test/unit/kotlin-klaxon-empty-object-map.test.ts:

  • the issue's repro must produce the field-converter declaration/registration and annotate rewards: Map<String, Reward>;
  • nested Map<String, Map<String, Empty>> properties are annotated too;
  • maps of non-empty classes get none of the workaround machinery.

  These were committed first (80a0aec) and fail without the fix (2 failed / 1 passed); with the fix all pass (npm run test:unit: 74/74 green).

• Fixture re-enabled: removed bug2037.json from the Kotlin/Klaxon skipJSON list that Fix "Type creator didn't return its forwarding ref" crash on sibling maps #2877 had to add, so the priority fixture now guards this end-to-end in CI.

Verification

• Runtime verified against the pinned klaxon-3.0.1.jar with a locally downloaded kotlinc: the exact issue scenario now parses and round-trips ({"mission_specs" : {"1": {...{"3":{},"5":{}}...}, "4": {...{}}}}), where the pre-fix code reproduced the exact KlaxonException from the issue. Nested (Map<String, Map<String, Empty>>) and nullable (Map<String, Empty>?) shapes verified the same way.
• QUICKTEST=true FIXTURE=kotlin script/test passes locally (all 44 tests, incl. the re-enabled bug2037.json with compile + run + round-trip diff + schema diff).
• npm run build, npm run test:unit, and npx biome check on all changed files are clean.

Fixes #2881

🤖 Generated with Claude Code