Fix beginStructure in JsonTreeDecoder when inner structure descriptor…

… is same as outer (#2346)

Instead of returning the same instance of decoder (with invalid position) a new instance is created, and polyDiscriminator and polyDescriptor are passed to the new instance.

This way check for unknown keys in endStructure can properly filter out polymorphic discriminator (by default "type) from potential unknown keys.

Fixes #2343

formats/json-tests/commonTest/src/kotlinx/serialization/json/polymorphic/JsonTreeDecoderPolymorphicTest.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2017-2023 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json.polymorphic
+
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.*
+import kotlin.test.*
+
+class JsonTreeDecoderPolymorphicTest : JsonTestBase() {
+
+    @Serializable
+    sealed class Sealed
+
+    @Serializable
+    data class ClassContainingItself(
+        val a: String,
+        val b: String,
+        val c: ClassContainingItself? = null,
+        val d: String?
+    ) : Sealed()
+
+    val inner = ClassContainingItself(
+        "InnerA",
+        "InnerB",
+        null,
+        "InnerC"
+    )
+    val outer = ClassContainingItself(
+        "OuterA",
+        "OuterB",
+        inner,
+        "OuterC"
+    )
+
+    @Test
+    fun testDecodingWhenClassContainsItself() = parametrizedTest { jsonTestingMode ->
+        val encoded = default.encodeToString(outer as Sealed, jsonTestingMode)
+        val decoded: Sealed = Json.decodeFromString(encoded, jsonTestingMode)
+        assertEquals(outer, decoded)
+    }
+}

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

@@ -44,7 +44,7 @@ private sealed class AbstractJsonTreeDecoder(
     @JvmField
     protected val configuration = json.configuration
 
-    private fun currentObject() = currentTagOrNull?.let { currentElement(it) } ?: value
+    protected fun currentObject() = currentTagOrNull?.let { currentElement(it) } ?: value
 
     override fun decodeJsonElement(): JsonElement = currentObject()
 
@@ -256,11 +256,14 @@ private open class JsonTreeDecoder(
     override fun currentElement(tag: String): JsonElement = value.getValue(tag)
 
     override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder {
-        /*
-         * For polymorphic serialization we'd like to avoid excessive decoder creating in
-         * beginStructure to properly preserve 'polyDiscriminator' field and filter it out.
-         */
-        if (descriptor === polyDescriptor) return this
+        // polyDiscriminator needs to be preserved so the check for unknown keys
+        // in endStructure can filter polyDiscriminator out.
+        if (descriptor === polyDescriptor) {
+            return JsonTreeDecoder(
+                json, cast(currentObject(), polyDescriptor), polyDiscriminator, polyDescriptor
+            )
+        }
+
         return super.beginStructure(descriptor)
     }