Discarded Notification Center Observer Violation false trigger

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New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

Discarded Notification Center Observer Violation is triggered when I use the below code pattern where the notification observer result is appended to an array instead of assigning it to a variable.

Complete output when running SwiftLint, including the stack trace and command used

$ swiftlint lint App/NotificationController.swift 
Loading configuration from '.swiftlint.yml'
Linting Swift files at paths App/NotificationController.swift
Linting 'NotificationController.swift' (1/1)
/Users/jsloop/dev/App/App/NotificationController.swift:34:27: warning: Discarded Notification Center Observer Violation: When registering for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
/Users/jsloop/dev/App/App/NotificationController.swift:36:27: warning: Discarded Notification Center Observer Violation: When registering for a notification using a block, the opaque observer that is returned should be stored so it can be removed later. (discarded_notification_center_observer)
Done linting! Found 2 violations, 0 serious in 1 file.

Environment

• SwiftLint version: 0.31.0
• Installation method used: Hombrew
• Paste your configuration file:

excluded:
- Carthage
- Pods
- SwiftLint/Common/3rdPartyLib

disabled_rules:
- trailing_whitespace
- identifier_name
- type_body_length
- large_tuple
- cyclomatic_complexity

opt_in_rules:
- unneeded_parentheses_in_closure_argument

force_cast: warning

line_length:
  warning: 160

file_length:
  warning: 1000
  
function_body_length:
  warning: 100
  error: 200

reporter: "xcode"

• Which Xcode version are you using: 10.1
• Do you have a sample that shows the issue?

// This triggers a violation:
class NotificationController {
    private var notifs: [Any?] = []

    init() {
        notifs.append(NotificationCenter.default.addObserver(forName: Notification.Name("n1"),
                                                             object: nil, queue: nil, using: {_ in}))
        notifs.append(NotificationCenter.default.addObserver(forName: Notification.Name("n2"), 
                                                            object: nil, queue: nil, using: {_ in}))
    }
}

If I assign the return to a variable, the violation is not triggered.

// This *does not* trigger a violation:
class NotificationController {
    private var notifs: [Any?] = []

    init() {
        let n1 = NotificationCenter.default.addObserver(forName: Notification.Name("n1"),
                                                        object: nil, queue: nil, using: {_ in})
        let n2 = NotificationCenter.default.addObserver(forName: Notification.Name("n2"), 
                                                        object: nil, queue: nil, using: {_ in})
        _ = [n1, n2].map { notifs.append($0!) }
    }
}