Merge pull request #2651 from Frizlab/swift-5.1-branch

[5.1] Backport of #2543 for Swift 5.1 branch

Author: weissi

Foundation/Operation.swift

@@ -347,7 +347,7 @@ open class Operation : NSObject {
             _state = .executing
             Operation.observeValue(forKeyPath: _NSOperationIsExecuting, ofObject: self)
 
-            _queue?._execute(self)
+            _queue?._execute(self) ?? main()
         }
 
         if __NSOperationState.executing == _state {

TestFoundation/TestOperationQueue.swift

@@ -15,6 +15,7 @@ class TestOperationQueue : XCTestCase {
             ("test_OperationPriorities", test_OperationPriorities),
             ("test_OperationCount", test_OperationCount),
             ("test_AsyncOperation", test_AsyncOperation),
+            ("test_SyncOperationWithoutAQueue", test_SyncOperationWithoutAQueue),
             ("test_isExecutingWorks", test_isExecutingWorks),
             ("test_MainQueueGetter", test_MainQueueGetter),
             ("test_CancelOneOperation", test_CancelOneOperation),
@@ -104,6 +105,18 @@ class TestOperationQueue : XCTestCase {
         XCTAssertTrue(operation.isFinished)
     }
 
+    func test_SyncOperationWithoutAQueue() {
+        let operation = SyncOperation()
+        XCTAssertFalse(operation.isExecuting)
+        XCTAssertFalse(operation.isFinished)
+
+        operation.start()
+
+        XCTAssertFalse(operation.isExecuting)
+        XCTAssertTrue(operation.isFinished)
+        XCTAssertTrue(operation.hasRun)
+    }
+    
     func test_MainQueueGetter() {
         XCTAssertTrue(OperationQueue.main === OperationQueue.main)
 
@@ -330,3 +343,14 @@ class AsyncOperation: Operation {
     }
 
 }
+
+class SyncOperation: Operation {
+
+    var hasRun = false
+
+    override func main() {
+        Thread.sleep(forTimeInterval: 1)
+        hasRun = true
+    }
+
+}