Add KClass.sealedSubclasses to get direct subclasses of sealed class

#KT-14657 Fixed

core/builtins/src/kotlin/reflect/KClass.kt

@@ -76,6 +76,12 @@ public interface KClass<T : Any> : KDeclarationContainer, KAnnotatedElement, KCl
     @SinceKotlin("1.1")
     public val supertypes: List<KType>
 
+    /**
+     * The list of the immediate subclasses if this class is a sealed class, or an empty list otherwise.
+     */
+    @SinceKotlin("1.3")
+    public val sealedSubclasses: List<KClass<out T>>
+
     /**
      * Visibility of this class, or `null` if its visibility cannot be represented in Kotlin.
      */

core/reflection.jvm/src/kotlin/reflect/jvm/internal/KClassImpl.kt

@@ -147,6 +147,14 @@ internal class KClassImpl<T : Any>(override val jClass: Class<T>) : KDeclaration
             result.compact()
         }
 
+        val sealedSubclasses: List<KClass<out T>> by ReflectProperties.lazySoft {
+            descriptor.sealedSubclasses.mapNotNull { subclass ->
+                @Suppress("UNCHECKED_CAST")
+                val jClass = (subclass as ClassDescriptor).toJavaClass() as Class<out T>?
+                jClass?.let { KClassImpl(it) }
+            }
+        }
+
         val declaredNonStaticMembers: Collection<KCallableImpl<*>>
                 by ReflectProperties.lazySoft { getMembers(memberScope, DECLARED) }
         private val declaredStaticMembers: Collection<KCallableImpl<*>>
@@ -239,6 +247,11 @@ internal class KClassImpl<T : Any>(override val jClass: Class<T>) : KDeclaration
 
     override val supertypes: List<KType> get() = data().supertypes
 
+    /**
+     * The list of the immediate subclasses if this class is a sealed class, or an empty list otherwise.
+     */
+    override val sealedSubclasses: List<KClass<out T>> get() = data().sealedSubclasses
+
     override val visibility: KVisibility?
         get() = descriptor.visibility.toKVisibility()