Function references are strong references in Swift

StrongFunctionReference.swift

class BlockHolder {
    let block: () -> Void
    init(block: @escaping () -> Void) {
        self.block = block
    }
}

class Doer {
    func doIt() {
        print("done")
    }
}

weak var doer: Doer?
var blockHolder: BlockHolder?

do {
    doer = Doer()
    // Uncomment the line below and watch as the doer reference survives this scope!
//    blockHolder = BlockHolder(block: doer!.doIt)
}

// Without blockHolder, doer reference is dropped once control leaves the `do` scope, so this is nil
// With blockHolder, doer reference is strongly retained by blockHolder, so this is non-nil
print(doer as Any)