AsyncStream+CombineLatest.swift

func combineLatest<A, B, R>(_ aSequence: A, _ bSequence: B, transform: @escaping (A.Element, B.Element) -> R) -> AsyncStream<R> where A : AsyncSequence, B : AsyncSequence {
    return AsyncStream { (continuation) async in
        let aCurrentValue = Ref<A.Element?>(initialValue: nil)
        let aTask = Ref<Task<Void, Error>?>(initialValue: nil)
        
        let bCurrentValue = Ref<B.Element?>(initialValue: nil)
        let bTask = Ref<Task<Void, Error>?>(initialValue: nil)
        
        aTask.value = Task {
            defer { bTask.value?.cancel() }
            
            for try await aValue in aSequence {
                aCurrentValue.value = aValue
                
                if let bValue = bCurrentValue.value {
                    continuation.yield(transform(aValue, bValue))
                }
            }
        }
        
        bTask.value = Task {
            defer { aTask.value?.cancel() }
            
            for try await bValue in bSequence {
                bCurrentValue.value = bValue
                
                if let aValue = aCurrentValue.value {
                    continuation.yield(transform(aValue, bValue))
                }
            }
        }
        
        do {
            try await aTask.value?.value
            try await bTask.value?.value
        } catch {
            // Nothing
        }
    }
}