Support Serializer for Nothing on the JS target (#2330)

Workaround for KT-51333: serialization of `Nothing` on the JS target and related tests

See also #932 

Co-authored-by: Leonid Startsev <[email protected]>

Author: ShreckYe

core/commonTest/src/kotlinx/serialization/SerializersModuleTest.kt

@@ -37,6 +37,9 @@ class SerializersModuleTest {
     @Serializable
     class Parametrized<T : Any>(val a: T)
 
+    @Serializable
+    class ParametrizedOfNullable<T>(val a: T)
+
     class ContextualType(val i: Int)
 
     @Serializer(forClass = ContextualType::class)
@@ -81,6 +84,27 @@ class SerializersModuleTest {
         assertEquals(PrimitiveKind.INT, mapSerializer.descriptor.getElementDescriptor(1).kind)
     }
 
+    @Suppress("UNCHECKED_CAST")
+    @Test
+    fun testNothingAndParameterizedOfNothing() {
+        assertEquals(NothingSerializer, Nothing::class.serializer())
+        //assertEquals(NothingSerializer, serializer<Nothing>()) // prohibited by compiler
+        assertEquals(NothingSerializer, serializer(Nothing::class, emptyList(), false) as KSerializer<Nothing>)
+        //assertEquals(NullableSerializer(NothingSerializer), serializer<Nothing?>()) // prohibited by compiler
+        assertEquals(
+            NullableSerializer(NothingSerializer),
+            serializer(Nothing::class, emptyList(), true) as KSerializer<Nothing?>
+        )
+
+        val parameterizedNothingSerializer = serializer<Parametrized<Nothing>>()
+        val nothingDescriptor = parameterizedNothingSerializer.descriptor.getElementDescriptor(0)
+        assertEquals(NothingSerialDescriptor, nothingDescriptor)
+
+        val parameterizedNullableNothingSerializer = serializer<ParametrizedOfNullable<Nothing?>>()
+        val nullableNothingDescriptor = parameterizedNullableNothingSerializer.descriptor.getElementDescriptor(0)
+        assertEquals(SerialDescriptorForNullable(NothingSerialDescriptor), nullableNothingDescriptor)
+    }
+
     @Test
     fun testUnsupportedArray() {
         assertFails {

core/jsMain/src/kotlinx/serialization/internal/Platform.kt

@@ -16,9 +16,12 @@ internal actual fun BooleanArray.getChecked(index: Int): Boolean {
     if (index !in indices) throw IndexOutOfBoundsException("Index $index out of bounds $indices")
     return get(index)
 }
-@Suppress("UNCHECKED_CAST")
+
 internal actual fun <T : Any> KClass<T>.compiledSerializerImpl(): KSerializer<T>? =
-    this.constructSerializerForGivenTypeArgs() ?: this.js.asDynamic().Companion?.serializer() as? KSerializer<T>
+    this.constructSerializerForGivenTypeArgs() ?: (
+        if (this === Nothing::class) NothingSerializer // Workaround for KT-51333
+        else this.js.asDynamic().Companion?.serializer()
+        ) as? KSerializer<T>
 
 internal actual fun <T> createCache(factory: (KClass<*>) -> KSerializer<T>?): SerializerCache<T> {
     return object: SerializerCache<T> {

formats/protobuf/commonTest/src/kotlinx/serialization/protobuf/ProtobufNothingTest.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2017-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.protobuf
+
+import kotlinx.serialization.*
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class ProtobufNothingTest {
+    @Serializable
+    /*private*/ data class NullableNothingBox(val value: Nothing?) // `private` doesn't work on the JS legacy target
+
+    @Serializable
+    private data class ParameterizedBox<T : Any>(val value: T?)
+
+    private inline fun <reified T> testConversion(data: T, expectedHexString: String) {
+        val string = ProtoBuf.encodeToHexString(data).uppercase()
+        assertEquals(expectedHexString, string)
+        assertEquals(data, ProtoBuf.decodeFromHexString(string))
+    }
+
+    @Test
+    fun testNothing() {
+        testConversion(NullableNothingBox(null), "")
+        if (isJsLegacy()) return
+        testConversion(ParameterizedBox(null), "")
+    }
+}