Calling protocol default implementation from regular method

I don't know if you are still looking for an answer to this, but the way to do it is to remove the function from the protocol definition, cast your object to Foo and then call the method on it:

protocol Foo { 
    // func testPrint() <- comment this out or remove it
}

extension Foo {
    func testPrint() {
        print("Protocol extension call")
    }
}

struct Bar: Foo {
    func testPrint() {
        print("Call from struct")
        (self as Foo).testPrint() // <- cast to Foo and you'll get the  default
                                  //    function defined in the extension
    }
}

Bar().testPrint()

// Output:    "Call from struct"
//            "Protocol extension call"

For some reason it only works if the function isn't declared as part of the protocol, but is defined in an extension to the protocol. Go figure. But it does work.

Well, you could create a nested type conforming to the protocol, instantiate it, and call the method on that one (it does not matter that you cannot access your type's data as the implementation inside the protocol extension cannot reference it anyway). But it's not a solution I'd call elegant.

struct Bar: Foo {
    func testPrint() {
        // Calling default implementation
        struct Dummy : Foo {}
        let dummy = Dummy()
        dummy.testPrint()
        print("Call from struct")
    }
}