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Initial, composed implementation of `Publisher.withLatestFrom`’s handling of error events.
// **Scenario one**: `withLatestFrom`’s argument erroring out first.
struct SomeError: Error {}
let first = PassthroughSubject<Int, SomeError>()
let second = PassthroughSubject<String, SomeError>()
var cancellables = Set<AnyCancellable>()
first
.withLatestFrom(second) // Our composed operator implementation of `withLatestFrom`.
.sink(receiveCompletion: { print($0) }, receiveValue: { print($0) })
.store(in: &cancellables)
first.send(1)
second.send("one")
first.send(2)
second.send("two")
second.send(completion: .failure(.init()))
// Outputs:
//
// ```
// one
// failure(SomeError())
// ```
// **Scenario two**: Upstream erroring out first.
//
first.send(1)
second.send("one")
first.send(2)
second.send("two")
first.send(completion: .failure(.init())) // `first` is subbed in for `second` here.
// Outputs:
//
// ```
// one
// failure(SomeError())
// ```
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