Merge pull request #65272 from etcwilde/ewilde/5.9/fix-startOnMainActor

[🍒 5.9] Fix main executor check in startOnMainActor

Author: DougGregor

stdlib/public/Concurrency/Task.cpp

@@ -1598,7 +1598,7 @@ SWIFT_CC(swift)
 static void swift_task_startOnMainActorImpl(AsyncTask* task) {
   AsyncTask * originalTask = _swift_task_clearCurrent();
   ExecutorRef mainExecutor = swift_task_getMainExecutor();
-  if (swift_task_getCurrentExecutor() != swift_task_getMainExecutor())
+  if (!swift_task_isCurrentExecutor(mainExecutor))
     swift_Concurrency_fatalError(0, "Not on the main executor");
   swift_retain(task);
   swift_job_run(task, mainExecutor);

stdlib/public/Concurrency/Task.swift

@@ -192,6 +192,7 @@ extension Task where Failure == Error {
     @_spi(MainActorUtilities)
     @MainActor
     @available(SwiftStdlib 5.9, *)
+    @discardableResult
     public static func startOnMainActor(
         priority: TaskPriority? = nil,
         @_inheritActorContext @_implicitSelfCapture _ work: __owned @Sendable @escaping @MainActor() async throws -> Success
@@ -213,6 +214,7 @@ extension Task where Failure == Never {
     @_spi(MainActorUtilities)
     @MainActor
     @available(SwiftStdlib 5.9, *)
+    @discardableResult
     public static func startOnMainActor(
         priority: TaskPriority? = nil,
         @_inheritActorContext @_implicitSelfCapture _ work: __owned @Sendable @escaping @MainActor() async -> Success