Fix incorrect behavior while deserializing maps to sealed classes

formats/properties/commonMain/src/kotlinx/serialization/properties/Properties.kt

@@ -53,6 +53,18 @@ public sealed class Properties(
 
         protected abstract fun encode(value: Any): Value
 
+        @Suppress("UNCHECKED_CAST")
+        final override fun <T> encodeSerializableValue(serializer: SerializationStrategy<T>, value: T) {
+            if (serializer is AbstractPolymorphicSerializer<*>) {
+                val casted = serializer as AbstractPolymorphicSerializer<Any>
+                val actualSerializer = casted.findPolymorphicSerializer(this, value as Any)
+
+                return actualSerializer.serialize(this, value)
+            }
+
+            return serializer.serialize(this, value)
+        }
+
         override fun encodeTaggedValue(tag: String, value: Any) {
             map[tag] = encode(value)
         }
@@ -89,6 +101,19 @@ public sealed class Properties(
             return structure(descriptor).also { copyTagsTo(it) }
         }
 
+        final override fun <T> decodeSerializableValue(deserializer: DeserializationStrategy<T>): T {
+            val type = map["type"]?.toString()
+
+            if (deserializer is AbstractPolymorphicSerializer<*>) {
+                val actualSerializer: DeserializationStrategy<out Any> = deserializer.findPolymorphicSerializer(this, type)
+
+                @Suppress("UNCHECKED_CAST")
+                return actualSerializer.deserialize(this) as T
+            }
+
+            return deserializer.deserialize(this)
+        }
+
         final override fun decodeTaggedValue(tag: String): Value {
             return map.getValue(tag)
         }

formats/properties/commonTest/src/kotlinx/serialization/properties/SealedClassSerializationFromPropertiesTest.kt

@@ -0,0 +1,50 @@
+package kotlinx.serialization.properties
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertIs
+
+class SealedClassSerializationFromPropertiesTest {
+    @Serializable
+    sealed class BaseClass {
+        abstract val firstProperty: Long
+        abstract val secondProperty: String
+    }
+
+    @SerialName("FIRSTCHILD")
+    @Serializable
+    data class FirstChild(override val firstProperty: Long, override val secondProperty: String) : BaseClass()
+
+    @SerialName("SECONDCHILD")
+    @Serializable
+    data class SecondChild(override val firstProperty: Long, override val secondProperty: String) : BaseClass()
+
+    @Test
+    fun testPropertiesDeserialization() {
+        val props = mapOf(
+            "type" to "FIRSTCHILD",
+            "firstProperty" to 1L,
+            "secondProperty" to "one"
+        )
+
+        val instance: BaseClass = Properties.decodeFromMap(props)
+
+        assertIs<FirstChild>(instance)
+        assertEquals(instance.firstProperty, 1)
+        assertEquals(instance.secondProperty, "one")
+    }
+
+    @Test
+    fun testPropertiesSerialization() {
+        val instance: BaseClass = FirstChild(
+            firstProperty = 1L, secondProperty = "one"
+        )
+
+        val instanceProperties = Properties.encodeToMap(instance)
+
+        assertEquals(1L, instanceProperties["firstProperty"])
+        assertEquals("one", instanceProperties["secondProperty"])
+    }
+}