NestedFunctionRetainCycle.swift

import UIKit

class ViewController: UIViewController {
    
    func doSomething() {}
    
    func howDoesThisWork() {
        
        // Nested function that references `self` implicitly.
        func performDoSomething() {
            doSomething()
        }
        
        dispatch_async(dispatch_get_main_queue()) { [weak self] in
            // When we call `doSomething()` compiler asks us to explicitly add `self` to the call.
            self?.doSomething()
            
            // However, when we call the nested function it doesn't.
            // And the behaviour is ambiguous (probably retains `self`).
            performDoSomething()
        }
    }
}

https://twitter.com/jckarter/status/697100492288057345

Joe Groff
Yeah, that's a known bug. Capturing the local function will capture self strongly, even from a [weak self] closure.