Sketch of a possible AnyCancellable.store(in:) thread-safety issue.

anycancellable_store_in.swift

import Combine
import Foundation
import SwiftUI

final class SomeObservableObject: ObservableObject {
	@Published var count: Int?
	private var cancellables = Set<AnyCancellable>()

	func fetchCount() {
		countPublisher // (Assume it’s defined elsewhere.)
			.receive(on: DispatchQueue.main)
			.sink(receiveValue: { [weak self] in self?.count = $0 })
			.store(in: &cancellables) // Very likely triggers a `Set.insert(_:)` call
			// under the hood.
			// https://github.com/OpenCombine/OpenCombine/blob/28993ae57de5a4ea7e164787636cafad442d568c/Sources/OpenCombine/AnyCancellable.swift#L48-L65
	}
}