Also check collisions for JsonNamingStrategy-transformed names during serialization

Author: sandwwraith

formats/json-tests/commonTest/src/kotlinx/serialization/features/JsonNamingStrategyTest.kt

@@ -182,6 +182,11 @@ class JsonNamingStrategyTest : JsonTestBase() {
         val json = Json(jsonWithNaming) {
             ignoreUnknownKeys = true
         }
+        parametrizedTest { mode ->
+            assertFailsWithMessage<SerializationException>("The transformed name 'test_case' for property test_case already exists") {
+                json.encodeToString(CollisionCheckPrimary("a", "b"))
+            }
+        }
         parametrizedTest { mode ->
             assertFailsWithMessage<SerializationException>("The suggested name 'test_case' for property test_case is already one of the names for property testCase") {
                 json.decodeFromString<CollisionCheckPrimary>("""{"test_case":"a"}""", mode)

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonNamesMap.kt

@@ -55,9 +55,15 @@ internal fun Json.deserializationNamesMap(descriptor: SerialDescriptor): Map<Str
 
 internal fun SerialDescriptor.serializationNamesIndices(json: Json, strategy: JsonNamingStrategy): Array<String> =
     json.schemaCache.getOrPut(this, JsonSerializationNamesKey) {
+        val trackingSet = mutableSetOf<String>()
         Array(elementsCount) { i ->
             val baseName = getElementName(i)
-            strategy.serialNameForJson(this, i, baseName)
+            val name = strategy.serialNameForJson(this, i, baseName)
+            if (!trackingSet.add(name)) throw JsonEncodingException(
+                "The transformed name '$name' for property $baseName already exists " +
+                    "in ${this@serializationNamesIndices}"
+            )
+            name
         }
     }