https://twitter.com/harlanhaskins/status/1318814859404800002

DeinitTimingTests.swift

import XCTest
import Combine

class DeinitTimingTests: XCTestCase {

    func testExample() throws {
        let publisher = PassthroughSubject<Void, Never>()
        var cancellables = Set<AnyCancellable>()
        var isDone = false

        publisher
            .sink {
                isDone = true
            }
            .store(in: &cancellables)

        // NOTE: `cancellables` will be deallocated here for release (-O) build.

        publisher.send(())

        XCTAssertTrue(isDone)

        // NOTE: `cancellables` will be deallocated here for debug build.
    }

}