Prohibit Array<T> in @Serializable classes

because it's impossible to correctly create array reflectively without
knowing correct KClass.

Fixes https://github.com/Kotlin/kotlinx.serialization/issues/1243

plugins/kotlinx-serialization/kotlinx-serialization.k1/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationErrors.java

@@ -32,6 +32,8 @@ public interface SerializationErrors {
     DiagnosticFactory1<PsiElement, KotlinType> LOCAL_SERIALIZER_USAGE = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory0<PsiElement> TRANSIENT_MISSING_INITIALIZER = DiagnosticFactory0.create(ERROR);
 
+    DiagnosticFactory0<PsiElement> GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED = DiagnosticFactory0.create(ERROR);
+
     DiagnosticFactory0<PsiElement> TRANSIENT_IS_REDUNDANT = DiagnosticFactory0.create(WARNING);
     DiagnosticFactory0<PsiElement> JSON_FORMAT_REDUNDANT_DEFAULT = DiagnosticFactory0.create(WARNING);
     DiagnosticFactory0<PsiElement> JSON_FORMAT_REDUNDANT = DiagnosticFactory0.create(WARNING);

plugins/kotlinx-serialization/kotlinx-serialization.k1/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginDeclarationChecker.kt

@@ -69,7 +69,8 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
             val entry = classDescriptor.findAnnotationDeclaration(SerializationAnnotations.serializerAnnotationFqName)
             val inSameModule =
                 trace.bindingContext[BindingContext.FQNAME_TO_CLASS_DESCRIPTOR, serializableDescriptor.fqNameUnsafe] != null
-            val diagnostic = if (inSameModule) SerializationErrors.EXTERNAL_CLASS_NOT_SERIALIZABLE else SerializationErrors.EXTERNAL_CLASS_IN_ANOTHER_MODULE
+            val diagnostic =
+                if (inSameModule) SerializationErrors.EXTERNAL_CLASS_NOT_SERIALIZABLE else SerializationErrors.EXTERNAL_CLASS_IN_ANOTHER_MODULE
 
             trace.report(diagnostic.on(entry ?: declaration, classDescriptor.defaultType, serializableKType))
         }
@@ -263,6 +264,7 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
         }
 
     }
+
     private fun ClassDescriptor.isSerializableEnumWithMissingSerializer(): Boolean {
         if (kind != ClassKind.ENUM_CLASS) return false
         if (hasSerializableOrMetaAnnotation) return false
@@ -344,10 +346,23 @@ open class SerializationPluginDeclarationChecker : DeclarationChecker {
                 generatorContextForAnalysis.checkTypeArguments(it.module, it.type, element, trace, propertyPsi)
             } else {
                 generatorContextForAnalysis.checkType(it.module, it.type, ktType, trace, propertyPsi)
+                checkGenericArrayType(it.type, ktType, trace, propertyPsi)
             }
         }
     }
 
+    private fun checkGenericArrayType(
+        type: KotlinType,
+        ktType: KtTypeReference?,
+        trace: BindingTrace,
+        fallbackElement: PsiElement
+    ) {
+        if (KotlinBuiltIns.isArray(type) && type.arguments.first().type.genericIndex != null) {
+            // Array<T> is unsupported, since we can't get T::class from KSerializer<T>
+            trace.report(SerializationErrors.GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED.on(ktType ?: fallbackElement))
+        }
+    }
+
     private fun AbstractSerialGenerator.checkTypeArguments(
         module: ModuleDescriptor,
         type: KotlinType,

plugins/kotlinx-serialization/kotlinx-serialization.k1/src/org/jetbrains/kotlinx/serialization/compiler/diagnostic/SerializationPluginErrorsRendering.kt

@@ -95,6 +95,10 @@ object SerializationPluginErrorsRendering : DefaultErrorMessages.Extension {
             SerializationErrors.TRANSIENT_IS_REDUNDANT,
             "Property does not have backing field which makes it non-serializable and therefore @Transient is redundant"
         )
+        MAP.put(
+            SerializationErrors.GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED,
+            "Serialization of Arrays with generic type arguments is impossible because of unknown compile-time type."
+        )
         MAP.put(
             SerializationErrors.JSON_FORMAT_REDUNDANT_DEFAULT,
             "Redundant creation of Json default format. Creating instances for each usage can be slow."

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationErrors.kt

@@ -29,10 +29,10 @@ object FirSerializationErrors {
     val SERIALIZER_NULLABILITY_INCOMPATIBLE by error2<PsiElement, ConeKotlinType, ConeKotlinType>()
     val SERIALIZER_TYPE_INCOMPATIBLE by warning3<PsiElement, ConeKotlinType, ConeKotlinType, ConeKotlinType>()
     val LOCAL_SERIALIZER_USAGE by error1<PsiElement, ConeKotlinType>()
+    val GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED by error0<PsiElement>()
     val TRANSIENT_MISSING_INITIALIZER by error0<PsiElement>()
 
     val TRANSIENT_IS_REDUNDANT by warning0<PsiElement>()
-
     val INCORRECT_TRANSIENT by warning0<PsiElement>()
 
     val REQUIRED_KOTLIN_TOO_HIGH by error3<KtAnnotationEntry, String, String, String>()

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt

@@ -204,9 +204,9 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
             )
             return false
         }
-        
+
         if (!classSymbol.hasSerializableOrMetaAnnotation) return false
-        
+
         if (classSymbol.isAnonymousObjectOrInsideIt) {
             reporter.reportOn(classSymbol.serializableOrMetaAnnotationSource, FirSerializationErrors.ANONYMOUS_OBJECTS_NOT_SUPPORTED)
             return false
@@ -370,10 +370,21 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
                 checkSerializerNullability(propertyType, serializerType, source, reporter)
             } else {
                 checkType(propertyType, source, reporter)
+                checkGenericArrayType(propertyType, source, reporter)
             }
         }
     }
 
+    context(CheckerContext)
+    private fun checkGenericArrayType(propertyType: ConeKotlinType, source: KtSourceElement?, reporter: DiagnosticReporter) {
+        if (propertyType.isNonPrimitiveArray && propertyType.typeArguments.first().type?.isTypeParameter == true) {
+            reporter.reportOn(
+                source,
+                FirSerializationErrors.GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED,
+            )
+        }
+    }
+
     context(CheckerContext)
     @Suppress("IncorrectFormatting") // KTIJ-22227
     private fun checkTypeArguments(type: ConeKotlinType, source: KtSourceElement?, reporter: DiagnosticReporter) {

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/KtDefaultErrorMessagesSerialization.kt

@@ -102,6 +102,10 @@ object KtDefaultErrorMessagesSerialization {
             FirSerializationErrors.INCORRECT_TRANSIENT,
             "@kotlin.jvm.Transient does not affect @Serializable classes. Please use @kotlinx.serialization.Transient instead."
         )
+        put(
+            FirSerializationErrors.GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED,
+            "Serialization of Arrays with generic type arguments is impossible because of unknown compile-time type."
+        )
         put(
             FirSerializationErrors.REQUIRED_KOTLIN_TOO_HIGH,
             "Your current Kotlin version is {0}, while kotlinx.serialization core runtime {1} requires at least Kotlin {2}. " +

plugins/kotlinx-serialization/testData/diagnostics/GenericArrays.kt

@@ -0,0 +1,13 @@
+// FIR_IDENTICAL
+// WITH_STDLIB
+// SKIP_TXT
+import kotlinx.serialization.*
+
+@Serializable
+class C(val values: IntArray) // OK
+
+@Serializable
+class B(val values: Array<String>) // OK
+
+@Serializable
+class A<T>(val values: <!GENERIC_ARRAY_ELEMENT_NOT_SUPPORTED!>Array<T><!>)

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirDiagnosticTestGenerated.java

@@ -44,6 +44,12 @@ public class SerializationFirDiagnosticTestGenerated extends AbstractSerializati
             runTest("plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt");
         }
 
+        @Test
+        @TestMetadata("GenericArrays.kt")
+        public void testGenericArrays() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/diagnostics/GenericArrays.kt");
+        }
+
         @Test
         @TestMetadata("IncorrectTransient.kt")
         public void testIncorrectTransient() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationPluginDiagnosticTestGenerated.java

@@ -42,6 +42,12 @@ public class SerializationPluginDiagnosticTestGenerated extends AbstractSerializ
         runTest("plugins/kotlinx-serialization/testData/diagnostics/ExternalSerializers.kt");
     }
 
+    @Test
+    @TestMetadata("GenericArrays.kt")
+    public void testGenericArrays() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/diagnostics/GenericArrays.kt");
+    }
+
     @Test
     @TestMetadata("IncorrectTransient.kt")
     public void testIncorrectTransient() throws Exception {