```swift
func foo(f: Void) -> ()) {}
```
This compiler currently suggests we change this to:
```swift
func foo(f: (Void) -> ()) {}
```
That's `(()) -> ()`, almost certainly not what someone wants in Swift
4. We should suggest changing that to:
```swift
func foo(f: () -> ()) {}
```
Additionally,
```swift
func foo(f: (Void) -> ()) {}
```
Should trigger a warning that the `(Void)` input type is `(())`, and you
can't supply `()` to it in Swift 4, and suggest a fix-it change it to:
```swift
func foo(f: () -> ()) {}
```
rdar://problem/32143617
