Open Radar

 
Given that XCTFail immediately throws an error, it should return `Never` to let the compiler know that this is going to happen.

Often when writing tests, we need to write helper functions like the following:

```
struct Type {
    let foo: String
}

func makeType(foo: String) -> Type {
    guard !foo.isEmpty else {
        XCTFail("An empty string was passed in for foo")
    }

    return Type(foo: foo)
}
```

Because XCTFail _doesn't_ return `Never`, we get a compiler error that the guard will fall through without a return statement. However, it's not actually appropriate to return anything here. We can modify the code to return an Optional, but in some cases that simply isn't appropriate.

Having XCTFail return `Never` will give our code clearer intention and allow us to communicate that intention to the compiler.