Make type argument in JsonTransformingSerializer nullable (#2911)

This will allow for more flexible Json transformations, replacing invalid/unexpected input data with `null`s.

Fixes #1927

Author: sandwwraith

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonTransformingSerializerTest.kt

@@ -98,4 +98,29 @@ class JsonTransformingSerializerTest : JsonTestBase() {
         assertEquals(correctExample, json.decodeFromString(DocExample.serializer(), """{"data":["str1"]}""", streaming))
         assertEquals(correctExample, json.decodeFromString(DocExample.serializer(), """{"data":"str1"}""", streaming))
     }
+
+    // Wraps/unwraps {"data":null} to just `null`, because StringData.data is not nullable
+    object NullableStringDataSerializer : JsonTransformingSerializer<StringData?>(StringData.serializer().nullable) {
+        override fun transformDeserialize(element: JsonElement): JsonElement {
+            val data = element.jsonObject["data"]?.jsonPrimitive
+            if (data?.contentOrNull.isNullOrBlank()) return JsonNull
+            return element
+        }
+
+        override fun transformSerialize(element: JsonElement): JsonElement {
+            return if (element is JsonNull) JsonObject(mapOf("data" to JsonNull))
+            else element
+        }
+    }
+
+    @Serializable
+    data class NullableStringDataHolder(@Serializable(NullableStringDataSerializer::class) val stringData: StringData?)
+
+    @Test
+    fun testNullableTransformingSerializer() {
+        val normalInput = """{"stringData":{"data":"str1"}}"""
+        val nullInput = """{"stringData":{"data":null}}"""
+        assertJsonFormAndRestored(NullableStringDataHolder.serializer(), NullableStringDataHolder(StringData("str1")), normalInput)
+        assertJsonFormAndRestored(NullableStringDataHolder.serializer(), NullableStringDataHolder(null), nullInput)
+    }
 }

formats/json/api/kotlinx-serialization-json.klib.api

@@ -111,7 +111,7 @@ abstract class <#A: kotlin/Any> kotlinx.serialization.json/JsonContentPolymorphi
     final fun serialize(kotlinx.serialization.encoding/Encoder, #A) // kotlinx.serialization.json/JsonContentPolymorphicSerializer.serialize|serialize(kotlinx.serialization.encoding.Encoder;1:0){}[0]
 }
 
-abstract class <#A: kotlin/Any> kotlinx.serialization.json/JsonTransformingSerializer : kotlinx.serialization/KSerializer<#A> { // kotlinx.serialization.json/JsonTransformingSerializer|null[0]
+abstract class <#A: kotlin/Any?> kotlinx.serialization.json/JsonTransformingSerializer : kotlinx.serialization/KSerializer<#A> { // kotlinx.serialization.json/JsonTransformingSerializer|null[0]
     constructor <init>(kotlinx.serialization/KSerializer<#A>) // kotlinx.serialization.json/JsonTransformingSerializer.<init>|<init>(kotlinx.serialization.KSerializer<1:0>){}[0]
 
     open val descriptor // kotlinx.serialization.json/JsonTransformingSerializer.descriptor|{}descriptor[0]

formats/json/commonMain/src/kotlinx/serialization/json/JsonTransformingSerializer.kt

@@ -52,7 +52,7 @@ import kotlinx.serialization.json.internal.*
  *        Should be able to parse [JsonElement] from [transformDeserialize] function.
  *        Usually, default [serializer] is sufficient.
  */
-public abstract class JsonTransformingSerializer<T : Any>(
+public abstract class JsonTransformingSerializer<T : Any?>(
     private val tSerializer: KSerializer<T>
 ) : KSerializer<T> {