Support nullable types at top-level for JsonElement decoding (#1117)

Fixes #1102

Author: qwwdfsad

core/api/kotlinx-serialization-core.api

@@ -1031,7 +1031,7 @@ public abstract class kotlinx/serialization/internal/TaggedEncoder : kotlinx/ser
 	public final fun encodeLong (J)V
 	public final fun encodeLongElement (Lkotlinx/serialization/descriptors/SerialDescriptor;IJ)V
 	public final fun encodeNotNullMark ()V
-	public final fun encodeNull ()V
+	public fun encodeNull ()V
 	public final fun encodeNullableSerializableElement (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/SerializationStrategy;Ljava/lang/Object;)V
 	public fun encodeNullableSerializableValue (Lkotlinx/serialization/SerializationStrategy;Ljava/lang/Object;)V
 	public final fun encodeSerializableElement (Lkotlinx/serialization/descriptors/SerialDescriptor;ILkotlinx/serialization/SerializationStrategy;Ljava/lang/Object;)V
@@ -1048,7 +1048,6 @@ public abstract class kotlinx/serialization/internal/TaggedEncoder : kotlinx/ser
 	protected fun encodeTaggedFloat (Ljava/lang/Object;F)V
 	protected fun encodeTaggedInt (Ljava/lang/Object;I)V
 	protected fun encodeTaggedLong (Ljava/lang/Object;J)V
-	protected fun encodeTaggedNotNullMark (Ljava/lang/Object;)V
 	protected fun encodeTaggedNull (Ljava/lang/Object;)V
 	protected fun encodeTaggedShort (Ljava/lang/Object;S)V
 	protected fun encodeTaggedString (Ljava/lang/Object;Ljava/lang/String;)V

core/commonMain/src/kotlinx/serialization/internal/Tagged.kt

@@ -30,7 +30,6 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
     protected open fun encodeTaggedValue(tag: Tag, value: Any): Unit =
         throw SerializationException("Non-serializable ${value::class} is not supported by ${this::class} encoder")
 
-    protected open fun encodeTaggedNotNullMark(tag: Tag) {}
     protected open fun encodeTaggedNull(tag: Tag): Unit = throw SerializationException("null is not supported")
     protected open fun encodeTaggedInt(tag: Tag, value: Int): Unit = encodeTaggedValue(tag, value)
     protected open fun encodeTaggedByte(tag: Tag, value: Byte): Unit = encodeTaggedValue(tag, value)
@@ -56,8 +55,8 @@ public abstract class TaggedEncoder<Tag : Any?> : Encoder, CompositeEncoder {
         return true
     }
 
-    final override fun encodeNotNullMark(): Unit = encodeTaggedNotNullMark(currentTag)
-    final override fun encodeNull(): Unit = encodeTaggedNull(popTag())
+    final override fun encodeNotNullMark() {} // Does nothing, open because is not really required
+    open override fun encodeNull(): Unit = encodeTaggedNull(popTag())
     final override fun encodeBoolean(value: Boolean): Unit = encodeTaggedBoolean(popTag(), value)
     final override fun encodeByte(value: Byte): Unit = encodeTaggedByte(popTag(), value)
     final override fun encodeShort(value: Short): Unit = encodeTaggedShort(popTag(), value)

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

@@ -24,7 +24,7 @@ internal fun <T> Json.writeJson(value: T, serializer: SerializationStrategy<T>):
 @ExperimentalSerializationApi
 private sealed class AbstractJsonTreeEncoder(
     final override val json: Json,
-    val nodeConsumer: (JsonElement) -> Unit
+    private val nodeConsumer: (JsonElement) -> Unit
 ) : NamedValueEncoder(), JsonEncoder {
 
     final override val serializersModule: SerializersModule
@@ -39,11 +39,19 @@ private sealed class AbstractJsonTreeEncoder(
         encodeSerializableValue(JsonElementSerializer, element)
     }
 
-    override fun shouldEncodeElementDefault(descriptor: SerialDescriptor, index: Int): Boolean = configuration.encodeDefaults
+    override fun shouldEncodeElementDefault(descriptor: SerialDescriptor, index: Int): Boolean =
+        configuration.encodeDefaults
+
     override fun composeName(parentName: String, childName: String): String = childName
     abstract fun putElement(key: String, element: JsonElement)
     abstract fun getCurrent(): JsonElement
 
+
+    override fun encodeNull() {
+        val tag = currentTagOrNull ?: return nodeConsumer(JsonNull)
+        encodeTaggedNull(tag)
+    }
+
     override fun encodeTaggedNull(tag: String) = putElement(tag, JsonNull)
 
     override fun encodeTaggedInt(tag: String, value: Int) = putElement(tag, JsonPrimitive(value))
@@ -120,8 +128,10 @@ private sealed class AbstractJsonTreeEncoder(
 
 internal const val PRIMITIVE_TAG = "primitive" // also used in JsonPrimitiveInput
 
-private class JsonPrimitiveEncoder(json: Json, nodeConsumer: (JsonElement) -> Unit) :
-    AbstractJsonTreeEncoder(json, nodeConsumer) {
+private class JsonPrimitiveEncoder(
+    json: Json,
+    nodeConsumer: (JsonElement) -> Unit
+) : AbstractJsonTreeEncoder(json, nodeConsumer) {
     private var content: JsonElement? = null
 
     init {

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

@@ -0,0 +1,43 @@
+package kotlinx.serialization.json
+
+import kotlinx.serialization.Serializable
+import kotlin.test.*
+
+class JsonElementDecodingTest : JsonTestBase() {
+
+    @Serializable
+    data class A(val a: Int = 42)
+
+    @Test
+    fun testTopLevelClass() = assertSerializedForm(A(), """{}""".trimMargin())
+
+    @Test
+    fun testTopLevelNullableClass() {
+        assertSerializedForm<A?>(A(), """{}""")
+        assertSerializedForm<A?>(null, "null")
+    }
+
+    @Test
+    fun testTopLevelPrimitive() = assertSerializedForm(42, """42""")
+
+    @Test
+    fun testTopLevelNullablePrimitive() {
+        assertSerializedForm<Int?>(42, """42""")
+        assertSerializedForm<Int?>(null, """null""")
+    }
+
+    @Test
+    fun testTopLevelList() = assertSerializedForm(listOf(42), """[42]""")
+
+    @Test
+    fun testTopLevelNullableList() {
+        assertSerializedForm<List<Int>?>(listOf(42), """[42]""")
+        assertSerializedForm<List<Int>?>(null, """null""")
+    }
+
+    private inline fun <reified T> assertSerializedForm(value: T, expectedString: String) {
+        val element = Json.encodeToJsonElement(value)
+        assertEquals(expectedString, element.toString())
+        assertEquals(value, Json.decodeFromJsonElement(element))
+    }
+}