style(TaskOperation): add separate error for retrieving result with…

…out starting

Sources/AsyncObjects/TaskOperation.swift

@@ -93,7 +93,7 @@ public final class TaskOperation<R: Sendable>: Operation, AsyncObject,
     /// Will be success if provided operation completed successfully,
     /// or failure returned with error.
     public var result: Result<R, Error> {
-        get async { (await execTask?.result) ?? .failure(CancellationError()) }
+        get async { (await execTask?.result) ?? .failure(EarlyInvokeError()) }
     }
 
     /// Creates a new operation that executes the provided asynchronous task.
@@ -268,3 +268,11 @@ public final class TaskOperation<R: Sendable>: Operation, AsyncObject,
         try? await _withPromisedContinuation()
     }
 }
+
+/// An error that indicates that operation result
+/// requested without starting operation.
+///
+/// Error is thrown by ``TaskOperation/result``
+/// if the operation hasn't been started yet with either
+/// ``TaskOperation/start()`` or ``TaskOperation/signal()``.
+public struct EarlyInvokeError: Error, Sendable {}

Tests/AsyncObjectsTests/TaskOperationTests.swift

@@ -208,4 +208,18 @@ class TaskOperationTests: XCTestCase {
             await operation.wait()
         }
     }
+
+    func testNotStartedError() async throws {
+        let operation = TaskOperation { try await Self.sleep(seconds: 1) }
+        let result = await operation.result
+        switch result {
+        case .success: XCTFail("Unexpected operation result")
+        case .failure(let error):
+            XCTAssertTrue(type(of: error) == EarlyInvokeError.self)
+            print(
+                "[\(#function)] [\(type(of: error))] \(error.localizedDescription)"
+            )
+            XCTAssertFalse(error.localizedDescription.isEmpty)
+        }
+    }
 }