Properly skip unknown keys for objects and structures with zero prope…

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

@@ -7,6 +7,7 @@ package kotlinx.serialization.json.internal
 import kotlinx.serialization.*
 import kotlinx.serialization.descriptors.*
 import kotlinx.serialization.encoding.*
+import kotlinx.serialization.encoding.CompositeDecoder.Companion.DECODE_DONE
 import kotlinx.serialization.encoding.CompositeDecoder.Companion.UNKNOWN_NAME
 import kotlinx.serialization.json.*
 import kotlinx.serialization.modules.*
@@ -56,9 +57,21 @@ internal open class StreamingJsonDecoder(
     }
 
     override fun endStructure(descriptor: SerialDescriptor) {
+        // If we're ignoring unknown keys, we have to skip all undecoded elements,
+        // e.g. for object serialization. It can be the case when the descriptor does
+        // not have any elements and decodeElementIndex is not invoked at all
+        if (json.configuration.ignoreUnknownKeys && descriptor.elementsCount == 0) {
+            skipLeftoverElements(descriptor)
+        }
         lexer.consumeNextToken(mode.end)
     }
 
+    private fun skipLeftoverElements(descriptor: SerialDescriptor) {
+        while (decodeElementIndex(descriptor) != DECODE_DONE) {
+            // Skip elements
+        }
+    }
+
     override fun decodeNotNullMark(): Boolean {
         return !(elementMarker?.isUnmarkedNull ?: false) && lexer.tryConsumeNotNull()
     }

formats/json/commonTest/src/kotlinx/serialization/json/JsonModesTest.kt

@@ -110,4 +110,21 @@ class JsonModesTest : JsonTestBase() {
 
     @Serializable
     data class Box(val double: Double, val float: Float)
+
+
+    @Serializable
+    object Object
+
+    @Serializable
+    data class Holder(val o: Object)
+
+    @Test
+    fun testIgnoreUnknownKeysObject() = parametrizedTest { jsonTestingMode ->
+        noLegacyJs {
+            assertEquals(Holder(Object), lenient.decodeFromString("""{"o":{}}""", jsonTestingMode))
+            assertEquals(Holder(Object), lenient.decodeFromString("""{"o":{"unknown":{"b":"c"}}}""", jsonTestingMode))
+            assertEquals(Object, lenient.decodeFromString("""{}""", jsonTestingMode))
+            assertEquals(Object, lenient.decodeFromString("""{"o":{"unknown":{"b":"c"}}}""", jsonTestingMode))
+        }
+    }
 }