Introduce ClassDescriptor.getSealedSubclasses

#KT-12795 In Progress

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/NotFoundClasses.kt

@@ -110,6 +110,7 @@ class NotFoundClasses(private val storageManager: StorageManager, private val mo
         override fun getConstructors(): Collection<ClassConstructorDescriptor> = emptySet()
         override fun getUnsubstitutedPrimaryConstructor(): ClassConstructorDescriptor? = null
         override fun getCompanionObjectDescriptor(): ClassDescriptor? = null
+        override fun getSealedSubclasses(): Collection<ClassDescriptor> = emptyList()
 
         override fun toString() = "class $name (not found)"
     }

core/deserialization/src/org/jetbrains/kotlin/serialization/deserialization/descriptors/DeserializedClassDescriptor.kt

@@ -28,6 +28,7 @@ import org.jetbrains.kotlin.resolve.DescriptorFactory
 import org.jetbrains.kotlin.resolve.NonReportingOverrideStrategy
 import org.jetbrains.kotlin.resolve.OverridingUtil
 import org.jetbrains.kotlin.resolve.descriptorUtil.classId
+import org.jetbrains.kotlin.resolve.descriptorUtil.computeSealedSubclasses
 import org.jetbrains.kotlin.resolve.scopes.DescriptorKindFilter
 import org.jetbrains.kotlin.resolve.scopes.MemberScope
 import org.jetbrains.kotlin.resolve.scopes.StaticScopeForKotlinEnum
@@ -46,7 +47,7 @@ class DeserializedClassDescriptor(
         val classProto: ProtoBuf.Class,
         nameResolver: NameResolver,
         private val sourceElement: SourceElement
-) : ClassDescriptor, AbstractClassDescriptor(
+) : AbstractClassDescriptor(
         outerContext.storageManager,
         nameResolver.getClassId(classProto.fqName).shortClassName
 ) {
@@ -67,6 +68,7 @@ class DeserializedClassDescriptor(
     private val primaryConstructor = c.storageManager.createNullableLazyValue { computePrimaryConstructor() }
     private val constructors = c.storageManager.createLazyValue { computeConstructors() }
     private val companionObjectDescriptor = c.storageManager.createNullableLazyValue { computeCompanionObjectDescriptor() }
+    private val sealedSubclasses = c.storageManager.createLazyValue { computeSubclassesForSealedClass() }
 
     internal val thisAsProtoContainer: ProtoContainer.Class = ProtoContainer.Class(
             classProto, c.nameResolver, c.typeTable, sourceElement,
@@ -147,6 +149,13 @@ class DeserializedClassDescriptor(
     internal fun hasNestedClass(name: Name): Boolean =
             name in memberScope.classNames
 
+    private fun computeSubclassesForSealedClass(): Collection<ClassDescriptor> {
+        // TODO: store the list of subclasses to metadata
+        return computeSealedSubclasses(this)
+    }
+
+    override fun getSealedSubclasses() = sealedSubclasses()
+
     override fun toString() = "deserialized class $name" // not using descriptor render to preserve laziness
 
     override fun getSource() = sourceElement