duplicates:

• SR-75 Referencing a protocol function crashes the compiler

Issue Description:

```
protocol Test {
func f()
}
let x = Test.f
```

This crashes the compiler in all versions of swift from 4 back to at least 3.0. I would expect this to capture a function type `(Test) -> (Void) -> Void`, which I could then call with an instance of a concrete type that conforms to `Test`.