From 83d0faa2a26b7c7504d3ce94834ca76a2b33c76c Mon Sep 17 00:00:00 2001
From: Matthijs van den Bos <mvdbos@users.noreply.github.com>
Date: Tue, 13 Apr 2021 20:31:20 +0200
Subject: [PATCH] Fix serializer lookup by KType for third party generics
 (#1397)

This lookup used to fail, because `SerializersModule.serializerByKTypeImpl()`
never tried `getContextual()` after constructSerializerForGivenTypeArgs() fails
to find a serializer (and that *will* fail, because `ThirdPartyBox` is not
annotated as @Serializable, since it is a third party class).

This does work when doing the look up by Java type with
`serializerByJavaTypeImpl.serializerByJavaTypeImpe()`.

This commit adds the call to `reflectiveOrContextual()` to
`SerializersModule.serializerByKTypeImpl()`.

The accompanying test illustrates the scenario where this used to fail.
---
 .../src/kotlinx/serialization/Serializers.kt  |  7 +++
 .../features/ThirdPartyGenericsTest.kt        | 45 +++++++++++++++++++
 .../kotlinx/serialization/SerializersJvm.kt   |  5 ---
 .../features/JvmThirdPartyGenericsTest.kt     | 13 ++++++
 4 files changed, 65 insertions(+), 5 deletions(-)
 create mode 100644 core/commonTest/src/kotlinx/serialization/features/ThirdPartyGenericsTest.kt
 create mode 100644 core/jvmTest/src/kotlinx/serialization/features/JvmThirdPartyGenericsTest.kt

diff --git a/core/commonMain/src/kotlinx/serialization/Serializers.kt b/core/commonMain/src/kotlinx/serialization/Serializers.kt
index cc1b347aa9..76faf0f7cc 100644
--- a/core/commonMain/src/kotlinx/serialization/Serializers.kt
+++ b/core/commonMain/src/kotlinx/serialization/Serializers.kt
@@ -115,10 +115,17 @@ private fun SerializersModule.builtinSerializer(
                 return ArraySerializer<Any, Any?>(typeArguments[0].classifier as KClass<Any>, serializers[0]).cast()
             }
             rootClass.constructSerializerForGivenTypeArgs(*serializers.toTypedArray())
+                ?: reflectiveOrContextual(rootClass)
         }
     }
 }
 
+@OptIn(ExperimentalSerializationApi::class)
+internal fun <T : Any> SerializersModule.reflectiveOrContextual(kClass: KClass<T>): KSerializer<T>? {
+    return kClass.serializerOrNull() ?: getContextual(kClass)
+}
+
+
 /**
  * Retrieves a [KSerializer] for the given [KClass].
  * The given class must be annotated with [Serializable] or be one of the built-in types.
diff --git a/core/commonTest/src/kotlinx/serialization/features/ThirdPartyGenericsTest.kt b/core/commonTest/src/kotlinx/serialization/features/ThirdPartyGenericsTest.kt
new file mode 100644
index 0000000000..e8d3eed6d5
--- /dev/null
+++ b/core/commonTest/src/kotlinx/serialization/features/ThirdPartyGenericsTest.kt
@@ -0,0 +1,45 @@
+package kotlinx.serialization.features
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.modules.*
+import kotlin.test.*
+
+open class ThirdPartyGenericsTest {
+    // This is a 3rd party class that we can't annotate as @Serializable
+    data class ThirdPartyBox<T>(val contents: T)
+
+    // This is the item that we put in the ThirdPartyBox, we control it, so can annotate it
+    @Serializable
+    data class Item(val name: String)
+
+    // The serializer for the ThirdPartyBox<T>
+    class BoxSerializer<T>(dataSerializer: KSerializer<T>) : KSerializer<ThirdPartyBox<T>> {
+        @Serializable
+        data class BoxSurrogate<T>(val contents: T)
+
+        private val strategy = BoxSurrogate.serializer(dataSerializer)
+        override val descriptor: SerialDescriptor = strategy.descriptor
+
+        override fun deserialize(decoder: Decoder): ThirdPartyBox<T> {
+            return ThirdPartyBox(decoder.decodeSerializableValue(strategy).contents)
+        }
+
+        override fun serialize(encoder: Encoder, value: ThirdPartyBox<T>) {
+            encoder.encodeSerializableValue(strategy, BoxSurrogate(value.contents))
+        }
+    }
+
+    // Register contextual serializer for ThirdPartyBox<Item>
+    protected val boxWithItemSerializer = BoxSerializer(Item.serializer())
+    protected val serializersModule = SerializersModule {
+        contextual(boxWithItemSerializer)
+    }
+
+    @Test
+    fun testSurrogateSerializerFoundForGenericWithKotlinType() {
+        val serializer = serializersModule.serializer<ThirdPartyBox<Item>>()
+        assertEquals(boxWithItemSerializer.descriptor, serializer.descriptor)
+    }
+}
\ No newline at end of file
diff --git a/core/jvmMain/src/kotlinx/serialization/SerializersJvm.kt b/core/jvmMain/src/kotlinx/serialization/SerializersJvm.kt
index 34943229c1..a89b531c87 100644
--- a/core/jvmMain/src/kotlinx/serialization/SerializersJvm.kt
+++ b/core/jvmMain/src/kotlinx/serialization/SerializersJvm.kt
@@ -154,11 +154,6 @@ private fun SerializersModule.genericArraySerializer(
     return ArraySerializer(kclass, serializer) as KSerializer<Any>
 }
 
-@OptIn(ExperimentalSerializationApi::class)
-private fun <T : Any> SerializersModule.reflectiveOrContextual(kClass: KClass<T>): KSerializer<T>? {
-    return kClass.serializerOrNull() ?: getContextual(kClass)
-}
-
 private fun Type.kclass(): KClass<*> = when (val it = this) {
     is KClass<*> -> it
     is Class<*> -> it.kotlin
diff --git a/core/jvmTest/src/kotlinx/serialization/features/JvmThirdPartyGenericsTest.kt b/core/jvmTest/src/kotlinx/serialization/features/JvmThirdPartyGenericsTest.kt
new file mode 100644
index 0000000000..0a43047aee
--- /dev/null
+++ b/core/jvmTest/src/kotlinx/serialization/features/JvmThirdPartyGenericsTest.kt
@@ -0,0 +1,13 @@
+package kotlinx.serialization.features
+
+import kotlinx.serialization.*
+import kotlin.test.*
+
+class JvmThirdPartyGenericsTest : ThirdPartyGenericsTest() {
+    @Test
+    fun testSurrogateSerializerFoundForGenericWithJavaType() {
+        val filledBox = ThirdPartyBox(contents = Item("Foo"))
+        val serializer = serializersModule.serializer(filledBox::class.java)
+        assertEquals(boxWithItemSerializer.descriptor, serializer.descriptor)
+    }
+}
\ No newline at end of file