Testing Sync diverges on recursive type

[RalfJung]

Typechecking the following program diverges:

#![feature(never_type)]

struct Foo<'a, T: 'a> {
    ph: std::marker::PhantomData<T>,
    foo: &'a Foo<'a, (T, T)>,
}

fn wub(f: Foo<!>) {
    sync(f)
}

fn sync<T: Sync>(x: T) {}

fn main() {}

Cc @eddyb @nikomatsakis

[eddyb]

Replacing (T, T) with T makes this compile, so I think it's definitely something about the polymorphic recursion / coinduction aspect of the definition that's causing this problem.

[nikomatsakis]

Fun.