test: add test for error when invalid type used in serializing

Associated issue: [issue-2938](Kotlin#2938)

Author: Stream29

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

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2017-2025 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.json.Json
+import kotlin.jvm.JvmInline
+import kotlin.test.Test
+import kotlin.test.assertFailsWith
+
+class InvalidPolymorphicTypeTest {
+    sealed interface InvalidType
+    enum class InvalidEnum : InvalidType {
+        A, B
+    }
+    @JvmInline
+    value class InvalidInline(val value: Int) : InvalidType
+
+    @Test
+    fun invalidEnum() {
+        assertFailsWith<IllegalArgumentException> {
+            Json.encodeToString<InvalidType>(InvalidEnum.A)
+        }
+    }
+
+    @Test
+    fun invalidInline() {
+        assertFailsWith<IllegalArgumentException> {
+            Json.encodeToString<InvalidType>(InvalidInline(1))
+        }
+    }
+}