Compiler hangs when compiling mutually-defined enums

```rust
#[derive(Clone)]
enum A<T> {
    A1(B<T>),
    A2(B<Option<T>>),
}

#[derive(Clone)]
enum B<T> {
    B1(T),
    B2(Box<A<B<T>>>),
}

fn main() {}
```
The following fails to halt or emit any warnings or errors.