Swift protocol extension static-methods dispatch with superclass and subclass

Question

I'm having a similar but slightly different problem described in: Swift protocol extension method dispatch with superclass and subclass.

The problem is related to static methods on protocols.

I have the following code:

protocol Protocol: class {
    static var reuseID: String { get }
}

extension Protocol {
    static var reuseID: String { return String(Self) }
}

class MyClass {
    func registerClass<T where T: Protocol>(cell: T.Type) {
        print(cell)          // <-- Prints "SubClass"
        print(cell.self)     // <-- Prints "SubClass"
        print(cell.reuseID)  // <-- Prints "SuperClass", expected "SubClass"
    }
}

class SuperClass: Protocol {}
class SubClass: SuperClass {}

print(SubClass.self)    // <-- Prints "SubClass"
print(SubClass.reuseID) // <-- Prints "SubClass"
MyClass().registerClass(SubClass.self)

The behaviour is "fixed" if I remove the reuseID declaration from the protocol Protocol. Isn't this backward? According to https://nomothetis.svbtle.com/the-ghost-of-swift-bugs-future the rules for dispatch for protocol extensions are as follows:

1. IF the inferred type of a variable is the protocol:
2. AND the method is defined in the original protocol THEN the runtime type’s implementation is called, irrespective of whether there is a default implementation in the extension.
3. AND the method is not defined in the original protocol, THEN the default implementation is called.
4. ELSE IF the inferred type of the variable is the type THEN the type’s implementation is called.

The behaviour I'm observing is exactly the opposite. Any idea what I am doing wrong?

Answer 1

How about change to this:

extension Protocol {
    static var reuseID: String { return String(self) }
}