Wrong type inferred for closure when returned from function

```
fn make_adder(x: int) -> @fn(int) -> int { |y| x + y }
```

The above function gives a compile error:
_mismatched types: expected `@fn(int) -> int` but found `&fn(int) -> int` (expected @ closure, found & closure)_

If I change the expression syntax into an explicit return:

```
fn make_adder(x: int) -> @fn(int) -> int { return |y| x + y; }
```

the code compiler (and runs) without error.
