Fix error triggered by 'consume leading class discriminator' polymorphic parsing optimization: (#2362)

In case of an empty object, no exception should be thrown, it should be treated as missing class discriminator.
peekString() correctly handles this case.

Also fixes misplaced calls in lenient vs non-lenient mode.

Fixes #2353

1 files changed, 38 insertions, 0 deletions

diff --git a/formats/json-tests/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt b/formats/json-tests/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt
index d05403b8..c4938871 100644
--- a/formats/json-tests/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt
+++ b/formats/json-tests/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt
@@ -143,4 +143,42 @@ class PolymorphismTest : JsonTestBase() {
         val s = json.encodeToString(Wrapper.serializer(), obj, jsonTestingMode)
         assertEquals("""{"polyBase1":{"type":"even","parity":"even"},"polyBase2":{"type":"odd","parity":"odd"}}""", s)
     }
+
+    @Serializable
+    sealed class Conf {
+        @Serializable
+        @SerialName("empty")
+        object Empty : Conf() // default
+
+        @Serializable
+        @SerialName("simple")
+        data class Simple(val value: String) : Conf()
+    }
+
+    private val jsonForConf = Json {
+        isLenient = false
+        ignoreUnknownKeys = true
+        serializersModule = SerializersModule {
+            polymorphicDefaultDeserializer(Conf::class) { Conf.Empty.serializer() }
+        }
+    }
+
+    @Test
+    fun defaultSerializerWithEmptyBodyTest() = parametrizedTest { mode ->
+        assertEquals(Conf.Simple("123"), jsonForConf.decodeFromString<Conf>("""{"type": "simple", "value": "123"}""", mode))
+        assertEquals(Conf.Empty, jsonForConf.decodeFromString<Conf>("""{"type": "default"}""", mode))
+        assertEquals(Conf.Empty, jsonForConf.decodeFromString<Conf>("""{"unknown": "Meow"}""", mode))
+        assertEquals(Conf.Empty, jsonForConf.decodeFromString<Conf>("""{}""", mode))
+    }
+
+    @Test
+    fun testTypeKeysInLenientMode() = parametrizedTest { mode ->
+        val json = Json(jsonForConf) { isLenient = true }
+
+        assertEquals(Conf.Simple("123"), json.decodeFromString<Conf>("""{type: simple, value: 123}""", mode))
+        assertEquals(Conf.Empty, json.decodeFromString<Conf>("""{type: default}""", mode))
+        assertEquals(Conf.Empty, json.decodeFromString<Conf>("""{unknown: Meow}""", mode))
+        assertEquals(Conf.Empty, json.decodeFromString<Conf>("""{}""", mode))
+
+    }
 }