[Concurrency] Base priority should be present on UnsafeTask too (swiftlang#63563)

Author: ktoso

stdlib/public/Concurrency/Task.swift

@@ -287,10 +287,10 @@ extension Task where Success == Never, Failure == Never {
   /// If the system can't provide a priority,
   /// this property's value is `Priority.default`.
   public static var currentPriority: TaskPriority {
-    withUnsafeCurrentTask { task in
+    withUnsafeCurrentTask { unsafeTask in
       // If we are running on behalf of a task, use that task's priority.
-      if let unsafeTask = task {
-         return TaskPriority(rawValue: _taskCurrentPriority(unsafeTask._task))
+      if let unsafeTask {
+         return unsafeTask.priority
       }
 
       // Otherwise, query the system.
@@ -311,6 +311,7 @@ extension Task where Success == Never, Failure == Never {
       return nil
     }
   }
+
 }
 
 @available(SwiftStdlib 5.1, *)
@@ -820,6 +821,15 @@ public struct UnsafeCurrentTask {
     TaskPriority(rawValue: _taskCurrentPriority(_task))
   }
 
+  /// The current task's base priority.
+  ///
+  /// - SeeAlso: `TaskPriority`
+  /// - SeeAlso: `Task.basePriority`
+  @available(SwiftStdlib 5.9, *)
+  public var basePriority: TaskPriority {
+    TaskPriority(rawValue: _taskBasePriority(_task))
+  }
+
   /// Cancel the current task.
   public func cancel() {
     _taskCancel(_task)

test/Concurrency/async_task_priority.swift

@@ -39,6 +39,13 @@ func expectedBasePri(priority: TaskPriority) -> TaskPriority {
   let basePri = Task.basePriority!
   print("Testing basePri matching expected pri - \(basePri) == \(priority)")
   expectEqual(basePri, priority)
+  Task.withUnsafeCurrentTask { unsafeTask in
+    guard let unsafeTask else {
+      fatalError("Expected to be able to get current task, but could not!")
+    }
+    // The UnsafeCurrentTask must return the same value
+    expectEqual(basePri, unsafeTask.basePriority)
+  }
 
   return basePri
 }