Attachment: Download

duplicates:

• SR-103 Protocol Extension: function's implementation cannot be overridden by a subclass

Issue Description:

If a protocol has an extension with default values for the protocol's properties, and a class implements that protocol without defining those property values, and a subclass of that class does define property values, passing an instance of that subclass to a function that takes a protocol argument doesn't

Steps to Reproduce:

1. Declare a protocol with a read-only properties.

    protocol Protocol {
        var string: String { get }
    }

2. Declare a protocol extension that implements the properties by returning fixed values.

    extension Protocol {
         var string: String { return "Protocol extension's string" }
    }

3. Declare an NSObject subclass that implements the protocol, but do NOT implement the protocol's properties.

    class EmptyClass: NSObject, Protocol { }

4. Subclass the empty class from step 3, and implement the protocol's properties to return the subclass's own values.

    class SubclassOfEmptyClass: EmptyClass {
         var string: String { return "SubclassOfEmptyClass's string" }
    }

5. Pass an instance of the subclass to a function that takes a parameter of the protocol's type.

    print("SubclassOfEmptyClass")
    printProperties(SubclassOfEmptyClass())

6. Inside the function, access the subclass instance's properties. Instead of seeing the subclass's own values, the protocol extension's values are returned.

    func printProperties(_ aProtocol: Protocol) {
        print(aProtocol.string)
     }

Expected Results:

Passing an instance to the function should print the SubclassOfEmptyClass's string property ("SubclassOfEmptyClass's string").

Actual Results:

It prints the protocol extension's string property ("Protocol extension's string"). This happens even if the original protocol is declared as a class-only protocol (i.e. protocol Protocol: class).

The obvious workaround is to implement the properties in the EmptyClass, which will force the SubclassOfEmptyClass's properties to specify that they override the superclass's values, but that requires any class that implements the protocol to lose access to the protocol extension's default values.