[SR-11905] Objective-C interop allows creation of undefined behavior

Previous ID	SR-11905
Radar	rdar://problem/57711780
Original Reporter	@DevAndArtist
Type	Bug

Apple Swift version 5.1.2 (swiftlang-1100.0.278 clang-1100.0.33.9)

Votes	1
Component/s	Compiler
Labels	Bug, StarterBug
Assignee	faical (JIRA)
Priority	Medium

md5: df1a976b8eb12c28ba3696248c7605f6

Issue Description:

In pure Swift this wouldn't be possible because we shouldn't be able to override a property of a class we import and don't own.

extension UITabBarController {
  open override var shouldAutorotate: Bool {
    return true
  }
}

However due to Objective-C interop the swift compiler does not complain about this extension and let us create an undefined behavior at runtime.

If the name of a method declared in a category is the same as a method in the original class, or a method in another category on the same class (or even a superclass), the behavior is undefined as to which method implementation is used at runtime. This is less likely to be an issue if you’re using categories with your own classes, but can cause problems when using categories to add methods to standard Cocoa or Cocoa Touch classes.

Source: https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html