Do not enable PolymorphicSerializer without special annotation

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/backend/common/TypeUtil.kt

@@ -91,6 +91,22 @@ fun AbstractSerialGenerator.getSerialTypeInfo(property: SerializableProperty): S
     }
 }
 
+/**
+ * Returns class descriptor for ContextSerializer or PolymorphicSerializer
+ * if [annotations] contains @Contextual or @Polymorphic annotation
+ */
+fun analyzeSpecialSerializers(
+    moduleDescriptor: ModuleDescriptor,
+    annotations: Annotations
+): ClassDescriptor? = when {
+    annotations.hasAnnotation(SerializationAnnotations.contextualFqName) ->
+        moduleDescriptor.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
+    // can be annotation on type usage, e.g. List<@Polymorphic Any>
+    annotations.hasAnnotation(SerializationAnnotations.polymorphicFqName) ->
+        moduleDescriptor.getClassFromSerializationPackage(SpecialBuiltins.polymorphicSerializer)
+    else -> null
+}
+
 fun AbstractSerialGenerator.findTypeSerializerOrContext(
     module: ModuleDescriptor,
     kType: KotlinType,
@@ -100,12 +116,8 @@ fun AbstractSerialGenerator.findTypeSerializerOrContext(
     if (kType.isTypeParameter()) return null
     if (kType.isMarkedNullable) return findTypeSerializerOrContext(module, kType.makeNotNullable(), annotations, sourceElement)
     additionalSerializersInScopeOfCurrentFile[kType]?.let { return it }
-    fun getContextualSerializer() =
-        if (annotations.hasAnnotation(SerializationAnnotations.contextualFqName) || kType in contextualKClassListInCurrentFile)
-            module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
-        else
-            null
-    return getContextualSerializer() ?: findTypeSerializer(module, kType) ?: throw CompilationException(
+    if (kType in contextualKClassListInCurrentFile) return module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer)
+    return analyzeSpecialSerializers(module, annotations) ?: findTypeSerializer(module, kType) ?: throw CompilationException(
         "Serializer for element of type $kType has not been found.\n" +
                 "To use context serializer as fallback, explicitly annotate element with @ContextualSerialization",
         null,
@@ -116,7 +128,6 @@ fun AbstractSerialGenerator.findTypeSerializerOrContext(
 fun findTypeSerializer(module: ModuleDescriptor, kType: KotlinType): ClassDescriptor? {
     val userOverride = kType.overridenSerializer
     if (userOverride != null) return userOverride.toClassDescriptor
-    if (kType.requiresPolymorphism()) return findPolymorphicSerializer(module)
     if (kType.isTypeParameter()) return null
     if (KotlinBuiltIns.isArray(kType)) return module.getClassFromInternalSerializationPackage(SpecialBuiltins.referenceArraySerializer)
     return kType.typeSerializer.toClassDescriptor // check for serializer defined on the type
@@ -170,16 +181,6 @@ fun findEnumTypeSerializer(module: ModuleDescriptor, kType: KotlinType): ClassDe
     return if (classDescriptor.kind == ClassKind.ENUM_CLASS) module.findClassAcrossModuleDependencies(enumSerializerId) else null
 }
 
-fun KotlinType.requiresPolymorphism(): Boolean {
-    return this.toClassDescriptor?.getSuperClassNotAny()?.isInternalSerializable == true
-            || (this.toClassDescriptor?.modality != Modality.FINAL && this.toClassDescriptor?.unsubstitutedPrimaryConstructor != null) // open not java class
-            || this.containsTypeProjectionsInTopLevelArguments() // List<*>
-}
-
-fun findPolymorphicSerializer(module: ModuleDescriptor): ClassDescriptor {
-    return requireNotNull(module.findClassAcrossModuleDependencies(polymorphicSerializerId)) { "Can't locate polymorphic serializer definition" }
-}
-
 fun KtPureClassOrObject.bodyPropertiesDescriptorsMap(bindingContext: BindingContext): Map<PropertyDescriptor, KtProperty> = declarations
     .asSequence()
     .filterIsInstance<KtProperty>()

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/NamingConventions.kt

@@ -22,6 +22,7 @@ object SerializationAnnotations {
     internal val serialInfoFqName = FqName("kotlinx.serialization.SerialInfo")
 
     internal val contextualFqName = FqName("kotlinx.serialization.ContextualSerialization")
+    internal val polymorphicFqName = FqName("kotlinx.serialization.Polymorphic")
     internal val additionalSerializersFqName = FqName("kotlinx.serialization.UseSerializers")
 }
 

plugins/kotlin-serialization/kotlin-serialization-compiler/src/org/jetbrains/kotlinx/serialization/compiler/resolve/SerializableProperty.kt

@@ -19,13 +19,12 @@ package org.jetbrains.kotlinx.serialization.compiler.resolve
 import org.jetbrains.kotlin.descriptors.ClassDescriptor
 import org.jetbrains.kotlin.descriptors.PropertyDescriptor
 import org.jetbrains.kotlin.descriptors.ValueParameterDescriptor
-import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.psi.KtDeclarationWithInitializer
 import org.jetbrains.kotlin.psi.KtParameter
 import org.jetbrains.kotlin.psi.ValueArgument
 import org.jetbrains.kotlin.resolve.descriptorUtil.module
 import org.jetbrains.kotlin.resolve.source.getPsi
-import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlinx.serialization.compiler.backend.common.analyzeSpecialSerializers
 
 class SerializableProperty(
     val descriptor: PropertyDescriptor,
@@ -36,18 +35,11 @@ class SerializableProperty(
     val type = descriptor.type
     val genericIndex = type.genericIndex
     val module = descriptor.module
-    val serializableWith = extractSerializableWith(descriptor.annotations)
+    val serializableWith = descriptor.serializableWith ?: analyzeSpecialSerializers(module, descriptor.annotations)?.defaultType
     val optional = !descriptor.annotations.serialRequired && descriptor.declaresDefaultValue
     val transient = descriptor.annotations.serialTransient || !hasBackingField
     val annotationsWithArguments: List<Triple<ClassDescriptor, List<ValueArgument>, List<ValueParameterDescriptor>>> =
         descriptor.annotationsWithArguments()
-
-    private fun extractSerializableWith(annotations: Annotations): KotlinType? {
-        descriptor.serializableWith?.let { return it }
-        return if (annotations.hasAnnotation(SerializationAnnotations.contextualFqName))
-            module.getClassFromSerializationPackage(SpecialBuiltins.contextSerializer).defaultType
-        else null
-    }
 }
 
 val PropertyDescriptor.declaresDefaultValue: Boolean