E0269 (not all control paths return a value) misfiring on an () associated type.

Try on [playpen](https://play.rust-lang.org/?gist=a8c01b70bab1386ccb73&version=stable):

``` rust
trait Make {
    type Out;

    fn make() -> Self::Out;
}

impl Make for () {
    type Out = ();

    fn make() -> Self::Out {}
}
```

This looks like missing normalization when checking for the return type.
`fn make() -> Self::Out { () }` and `fn make() -> Self::Out { return; }` work just fine.
