Can you override between extensions in Swift or not? (Compiler seems confused!)

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It seems that overriding methods and properties in an extension works with the
current Swift (Swift 1.1/Xcode 6.1) only for Objective-C compatible
methods and properties.

If a class is derived from NSObject then all its members are automatically available
in Objective-C (if possible, see below). So with

class A : NSObject { }

your example code compiles and works as expected. Your Code Data extension overrides
work because NSManagedObject is a subclass of NSObject.

Alternatively, you can use the @objc attribute for a method or property:

class A { }

class B : A { }

extension A
{
    @objc var y : String { get { return "YinA" } }
}

extension B
{
   @objc override var y : String { get { return "YinB" } }
}

Methods which are not representable in Objective-C cannot be marked with @objc
and cannot be overridden in a subclass extension. That applies for example to
methods having inout parameters or parameters of an enum type.

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