# expect(_:sourceLocation:performing:throws:) Check that an expression always throws an error matching some condition. ``` @discardableResult @freestanding(expression) macro expect(_ comment: @autoclosure () -> Comment? = nil, sourceLocation: SourceLocation = #_sourceLocation, performing expression: () async throws -> R, throws errorMatcher: (any Error) async throws -> Bool) -> (any Error)? ``` ## Parameters `comment` A comment describing the expectation. `sourceLocation` The source location to which recorded expectations and issues should be attributed. `expression` The expression to be evaluated. `errorMatcher` A closure to invoke when `expression` throws an error that indicates if it matched or not. ## Return Value If the expectation passes, the error that was thrown by `expression`. If the expectation fails, the result is `nil`. ## Overview Use this overload of `#expect()` when the expression `expression` *should* throw an error, but the logic to determine if the error matches is complex: ```swift #expect { FoodTruck.shared.engine.batteryLevel = 0 try FoodTruck.shared.engine.start() } throws: { error in return error == EngineFailureError.batteryDied || error == EngineFailureError.stillCharging } ``` If `expression` does not throw an error, if it throws an error that is not matched by `errorMatcher`, or if `errorMatcher` throws an error (including the error passed to it), an [`Issue`](/documentation/Testing/Issue) is recorded for the test that is running in the current task. Any value returned by `expression` is discarded. If the thrown error need only be an instance of a particular type, use [`expect(throws:_:sourceLocation:performing:)`](/documentation/Testing/expect(throws:_:sourceLocation:performing:)-1hfms) instead. If the thrown error need only equal another instance of [`Error`](https://developer.apple.com/documentation/swift/error), use [`expect(throws:_:sourceLocation:performing:)`](/documentation/Testing/expect(throws:_:sourceLocation:performing:)-7du1h) instead. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)