Protocol extensions magic in Swift

gistfile1.txt

protocol SampleProtocol {
    func foo()
}

extension SampleProtocol {
    func foo() {
        print("Protocol foo")
    }
    func bar() {
        print("Protocol bar")
    }
}

final class Sampleclass: SampleProtocol {
    func foo() {
        print("class foo")
    }
    func bar() {
        print("class bar")
    }
}

        let a: SampleProtocol = Sampleclass()
        a.foo()
        a.bar()