Support polymorphic serialization

yamlkt/src/commonMain/kotlin/net.mamoe.yamlkt/internal/YamlDecoder.kt

@@ -5,6 +5,7 @@ package net.mamoe.yamlkt.internal
 
 import kotlinx.serialization.DeserializationStrategy
 import kotlinx.serialization.SerializationException
+import kotlinx.serialization.descriptors.PolymorphicKind
 import kotlinx.serialization.descriptors.SerialDescriptor
 import kotlinx.serialization.descriptors.SerialKind
 import kotlinx.serialization.descriptors.StructureKind
@@ -747,6 +748,7 @@ internal class YamlDecoder(
                     else -> throw contextualDecodingException("expected list(sequence), but found $token")
                 }
             }
+            is PolymorphicKind,
             StructureKind.CLASS -> {
                 return when (val token = nextToken()) {
                     END_OF_FILE -> {

yamlkt/src/commonMain/kotlin/net.mamoe.yamlkt/internal/YamlEncoder.kt

@@ -8,6 +8,7 @@ package net.mamoe.yamlkt.internal
 import kotlinx.serialization.KSerializer
 import kotlinx.serialization.SerializationStrategy
 import kotlinx.serialization.builtins.serializer
+import kotlinx.serialization.descriptors.PolymorphicKind
 import kotlinx.serialization.descriptors.SerialDescriptor
 import kotlinx.serialization.descriptors.StructureKind
 import kotlinx.serialization.encoding.CompositeEncoder
@@ -55,7 +56,7 @@ internal class YamlEncoder(
     private fun beginStructureImpl(parent: AbstractEncoder?, descriptor: SerialDescriptor): CompositeEncoder {
         writer.levelIncrease()
         return when (descriptor.kind) {
-            StructureKind.CLASS
+            StructureKind.CLASS, is PolymorphicKind
             -> {
                 when (configuration.classSerialization) {
                     YamlBuilder.MapSerialization.BLOCK_MAP -> {

yamlkt/src/jvmTest/kotlin/net/mamoe/yamlkt/decoder/ContextualTest.kt

@@ -0,0 +1,41 @@
+package net.mamoe.yamlkt.decoder
+
+import kotlinx.serialization.Contextual
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.modules.SerializersModule
+import net.mamoe.yamlkt.Yaml
+import org.junit.Test
+import kotlin.test.assertEquals
+
+
+internal class ContextualTest {
+    interface Fruit
+
+    @Serializable
+    data class Banana(
+        val prop: String
+    ) : Fruit
+
+    @Serializable
+    data class Poly(
+        val fruit: @Contextual Fruit
+    )
+
+    val yaml = Yaml {
+        serializersModule = SerializersModule {
+            contextual(Banana::class, Banana.serializer())
+        }
+    }
+
+    @Test
+    fun `test simple dump`() {
+        val poly = Poly(Banana("prop"))
+        val result = yaml.encodeToString(poly)
+        assertEquals(
+            """
+            fruit: 
+              prop: prop
+        """.trimIndent(), result
+        )
+    }
+}

yamlkt/src/jvmTest/kotlin/net/mamoe/yamlkt/decoder/PolymorphismTest.kt

@@ -0,0 +1,40 @@
+package net.mamoe.yamlkt.decoder
+
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.modules.SerializersModule
+import kotlinx.serialization.modules.polymorphic
+import net.mamoe.yamlkt.Yaml
+import org.junit.Test
+import kotlin.test.assertEquals
+
+
+internal class PolymorphismTest {
+    interface Fruit
+
+    @Serializable
+    data class Banana(
+        val prop: String
+    ) : Fruit
+
+    @Serializable
+    data class Poly(
+        val fruit: Fruit
+    )
+
+    val yaml = Yaml {
+        serializersModule = SerializersModule {
+            polymorphic(Fruit::class) {
+                subclass(Banana::class, Banana.serializer())
+            }
+        }
+    }
+
+    @Test
+    fun `test simple dump`() {
+        val poly = Poly(Banana("prop"))
+        val result = yaml.encodeToString(poly)
+        println(result)
+        val decode = yaml.decodeFromString(Poly.serializer(), result)
+        assertEquals(poly, decode)
+    }
+}