class A {
    let backgroundColor: Int = 0
}

class C {
        // Note: changing this to something other than tableView (since that's the prefix in all the methods) does produce the correct diagnostic.
    private let tableView = A()
}

protocol P {
    func numberOfSections(in tableView: A)
    func tableView(_ tableView: A, numberOfRowsInSection section: Int)
    func tableView(_ tableView: A, cellForRowAt indexPath: Int)
}

extension C: P {
    func numberOfSections(in tableView: A) { }

    func tableView(_ tableView: A, numberOfRowsInSection section: Int) {}

    func tableView(_ tableView: A, cellForRowAt indexPath: Int) {
        print(self.tableView.backgroundColor)
    }
}