Fix issue #2938. Add check for sealed value class when serializing.

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))
+        }
+    }
+}

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

@@ -54,6 +54,11 @@ private fun validateIfSealed(
     classDiscriminator: String
 ) {
     if (serializer !is SealedClassSerializer<*>) return
+
+    if (actualSerializer.descriptor.run { isInline && getElementDescriptor(0).kind is PrimitiveKind }) {
+        error("Value class that contains primitive kind cannot be serialized polymorphically with 'type' parameter. You can use 'JsonBuilder.useArrayPolymorphism' instead")
+    }
+
     @Suppress("DEPRECATION_ERROR")
     if (classDiscriminator in actualSerializer.descriptor.jsonCachedSerialNames()) {
         val baseName = serializer.descriptor.serialName