ICE: trying to take the sizing type of A, an unsized type

Getting this ICE:
error: internal compiler error: trying to take the sizing type of A, an unsized type

when compiling this:

```
enum A {
    B(char),
    C([Box<A>]),
}

fn c(c:char) -> A {
    return B(c)
}

pub fn main() {
}
```

Heard on IRC that pinging @nick29581 may be good on this issue
