CheekySwift.swift

import Combine

func stringIfOver42(_ value: Int) -> String? {
    value > 42 ? "\(value)" : nil
}

let voidSubject = PassthroughSubject<Void, Never>()
let intSubject = PassthroughSubject<Int, Never>()

let publisher = voidSubject
    // Changing "String?" to "String" below fixes the bug.
    .map { () -> AnyPublisher<String?, Never> in
        intSubject
            .compactMap { stringIfOver42($0) }
            .eraseToAnyPublisher()
    }
    .switchToLatest()

let cancellable = publisher.sink { print($0) }
voidSubject.send(())

// This should not print anything, but does
intSubject.send(10)
// This should print "50"
intSubject.send(50)