Why does swift not compile when referring to protocol method directly?

Given the following struct:

struct TestStruct {
    func test() {
        print("Something")
    }
}

Referencing the test function like this works:

let testFunc = TestStruct.test // (TestStruct) -> () -> Void

In order to execute the function you would have to do the following:

testFunc(TestStruct())() // Prints "Something"

Say you make TestStruct conform to a protocol:

protocol Test {
    func test()
}

extension TestStruct: Test {}

Now trying to rather reference the test function through the protocol like this does not work:

let testFunc = Test.func // Does not compile, no error message

Shouldn't the type of testFunc be (Test) -> () -> Void? And why is it now compiling?