Swift 6.4: Continuation Makes Double-Resume a Compile Error

Nov 02, 2026 |

swift concurrency swift6 noncopyable

Until now, bridging callbacks to async meant picking your poison. UnsafeContinuation costs nothing, but resuming twice is undefined behavior and forgetting to resume hangs the task forever, silently. CheckedContinuation catches both mistakes, but only at runtime, and it pays for that with an allocation and atomic operations. SE-0528 adds a third option, a ~Copyable Continuation:

let data = try await withContinuation(of: Data.self, throwing: NetworkError.self) { continuation in
    client.fetch { result in
        continuation.resume(with: result)
    }
}

Every resume is consuming, so once you’ve called it the binding is gone:

func complete(with value: String) {
    if let continuation {
        continuation.resume(returning: value)
        continuation.resume(returning: value) // ❌ 'continuation' used after consuming use
    }
}

Dropping it without resuming hits a deinit that calls fatalError. A hang you’d never notice becomes a crash with a clear message:

func cancel() {
    self.continuation = nil // 💥 "The continuation was dropped without resuming."
}

	Unsafe	Checked	Continuation
Double resume	undefined behavior	runtime trap	compile error
Missing resume	silent hang	runtime warning	runtime trap
Overhead	none	allocation + atomics	none

Two smaller wins:

• of: and throwing: put the types at the call site, so you no longer need a (continuation: CheckedContinuation<Data, Never>) in annotation.
• Thanks to typed throws, one function covers all three cases. Never needs no try, NetworkError.self gives you a typed error, and (any Error).self works like the old withCheckedThrowingContinuation.

The catch: a noncopyable value can’t be captured by an escaping closure, and it can’t be consumed in two places. An API with separate onSuccess and onFailure callbacks won’t compile. For that case, wrap it with let checked = CheckedContinuation(continuation). That’s exactly why Checked and Unsafe aren’t being deprecated.

Takeaway: make withContinuation(of:) your default for new bridging code, and fall back to CheckedContinuation(c) only when the callback API makes you capture it in more than one place.

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