`scan`-based approach to `Publisher.pairwiseWithDuplicatedStart`

scan_based_pairwise_with_dublicated_start.swift

// `optionalZip` because `zip` caused ambiguity with `Publisher.zip` in the implementation below.
private func optionalZip<Left, Right>(_ pair: (Left?, Right?)) -> (Left, Right)? {
	pair.0.flatMap { left in pair.1.map { right in (left, right) } }
}

extension Publisher {
	func pairwiseWithDuplicatedStart() -> AnyPublisher<(Output, Output), Failure> {
		scan((nil, nil)) { previousPair, next in
			optionalZip(previousPair)
				.map { ($0.1, next) } ?? (next, next) // (1)
		}
		.compactMap(optionalZip)
		.eraseToAnyPublisher()
	}
}
let cancellable = (1...2)
	.publisher
	.pairwiseWithDuplicatedStart()
	.sink(receiveValue: { print($0) })

// Outputs (without the annotations):
// ```
// (1, 1)
// (1, 2)
// ```