#if vs @available vs if #available

[wowbagger (冀卓疌)]

@lukasa

However, unlike with #available, the method must type check and compile. The code will always be emitted into your binary

I have 2 follow-on questions:

---

Say if I have this declaration:

protocol ExampleProtocol {}

@available(macOS 10.12)
extension DateInterval: ExampleProtocol {}

and try to run it on macOS 10.11. The type DateInterval isn't available on macOS 10.11. How does the compiler type check and compile it when it can't find the symbols?

---

By "#available", do you mean #if or if #available? I assume it's #if, because it seems to me that if the block of if #available needs to be executable, then it needs to be type-checked and compiled.