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 

1 files changed, 43 insertions, 0 deletions

diff --git a/formats/json-tests/commonTest/src/kotlinx/serialization/json/polymorphic/JsonTreeDecoderPolymorphicTest.kt b/formats/json-tests/commonTest/src/kotlinx/serialization/json/polymorphic/JsonTreeDecoderPolymorphicTest.kt
new file mode 100644
index 00000000..9d8e861d
--- /dev/null
+++ b/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)
+    }
+}