rustc overflows its stack when type checking ~~a simple program~~ polymorphmic recursion

The following program triggers a stack overflow in rustc:

```rust
struct S<T> {
    t : T,
    s : Box<S<fn(u : T)>>
}

fn f(x : S<u32>) { }

fn main () { }
```

My version of rustc:

```
$ rustc --version
rustc 1.15.0-nightly (71c06a56a 2016-12-18)
```

The problem also appears when replacing `fn(u : T)` with `fn() -> T`.