ObjectSerializer: Respect sequential decoding (#2273)

This was broken in #1916

Since ObjectSerializer calls decodeElementIndex() now, it should also call decodeSequentially().

Co-authored-by: Pavel Vasin <git@vasin.nl>

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

@@ -41,6 +41,9 @@ internal class ObjectSerializer<T : Any>(serialName: String, private val objectI
 
     override fun deserialize(decoder: Decoder): T {
         decoder.decodeStructure(descriptor) {
+            if (decodeSequentially())
+                return@decodeStructure
+
             when (val index = decodeElementIndex(descriptor)) {
                 CompositeDecoder.DECODE_DONE -> {
                     return@decodeStructure

core/commonTest/src/kotlinx/serialization/internal/DummySequentialDecoder.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.internal
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import kotlinx.serialization.modules.*
+
+/**
+ * The purpose of this decoder is to check whether its methods were called currectly,
+ * rather than implement any concrete format.
+ */
+class DummySequentialDecoder(
+    override val serializersModule: SerializersModule = EmptySerializersModule()
+) : Decoder, CompositeDecoder {
+    private fun notImplemented(): Nothing = throw Error("Implement this method if needed")
+
+    override fun decodeSequentially(): Boolean = true
+    override fun decodeElementIndex(descriptor: SerialDescriptor): Int = throw Error("This method shouldn't be called in sequential mode")
+
+    override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder = this
+    override fun endStructure(descriptor: SerialDescriptor): Unit = Unit
+
+    override fun decodeInline(descriptor: SerialDescriptor): Decoder = notImplemented()
+
+    override fun decodeBoolean(): Boolean = notImplemented()
+    override fun decodeByte(): Byte = notImplemented()
+    override fun decodeShort(): Short = notImplemented()
+    override fun decodeInt(): Int = notImplemented()
+    override fun decodeLong(): Long = notImplemented()
+    override fun decodeFloat(): Float = notImplemented()
+    override fun decodeDouble(): Double = notImplemented()
+    override fun decodeChar(): Char = notImplemented()
+    override fun decodeString(): String = notImplemented()
+    override fun decodeEnum(enumDescriptor: SerialDescriptor): Int = notImplemented()
+
+    override fun decodeNotNullMark(): Boolean = notImplemented()
+    override fun decodeNull(): Nothing? = notImplemented()
+
+    override fun decodeBooleanElement(descriptor: SerialDescriptor, index: Int): Boolean = notImplemented()
+    override fun decodeByteElement(descriptor: SerialDescriptor, index: Int): Byte = notImplemented()
+    override fun decodeShortElement(descriptor: SerialDescriptor, index: Int): Short = notImplemented()
+    override fun decodeIntElement(descriptor: SerialDescriptor, index: Int): Int = notImplemented()
+    override fun decodeLongElement(descriptor: SerialDescriptor, index: Int): Long = notImplemented()
+    override fun decodeFloatElement(descriptor: SerialDescriptor, index: Int): Float = notImplemented()
+    override fun decodeDoubleElement(descriptor: SerialDescriptor, index: Int): Double = notImplemented()
+    override fun decodeCharElement(descriptor: SerialDescriptor, index: Int): Char = notImplemented()
+    override fun decodeStringElement(descriptor: SerialDescriptor, index: Int): String = notImplemented()
+
+    override fun decodeInlineElement(descriptor: SerialDescriptor, index: Int): Decoder = notImplemented()
+    override fun <T : Any?> decodeSerializableElement(descriptor: SerialDescriptor, index: Int, deserializer: DeserializationStrategy<T>, previousValue: T?): T = notImplemented()
+    override fun <T : Any> decodeNullableSerializableElement(descriptor: SerialDescriptor, index: Int, deserializer: DeserializationStrategy<T?>, previousValue: T?): T? = notImplemented()
+}

core/commonTest/src/kotlinx/serialization/internal/ObjectSerializerTest.kt

@@ -0,0 +1,18 @@
+/*
+ * Copyright 2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.internal
+
+import kotlin.test.*
+import kotlinx.serialization.*
+
+class ObjectSerializerTest {
+    @Test
+    fun testSequentialDecoding() {
+        SimpleObject.serializer().deserialize(DummySequentialDecoder())
+    }
+
+    @Serializable
+    object SimpleObject
+}