Does `Dictionary.Keys.contains(_:)` have `O(1)` time complexity?

[lorentey (Karoy Lorentey)]

@tera

(¹ NSSet is kind of broken anyway as you can put NSMutableAnything in it and then mutate it invalidating set's invariants).

Beware, this is also true for Set and Dictionary, when they contain values with reference semantics. Cocoa handles this with NSCopying; Swift prefers encouraging the use of value types instead.