`impl Trait` ICE returning a recursive type

[durka]

We were trying to make a function that returns itself.

#![feature(conservative_impl_trait)]
fn foo() -> impl Copy {
    foo
}

rustc overflows its stack.

cc @aturon

[aturon]

cc @eddyb

[eddyb]

Well, you've sort of succeeded: what trans sees is fn foo() -> typeof foo (an infinitely recursive type).

However, I would've expected traits::project to catch this via the recursion limit, as the backtrace repeats traits::project::AssociatedTypeNormalizer::fold_ty. cc @nikomatsakis

[topecongiro]

rustc does not overflow its stack on rustc 1.24.0-nightly (cddc4a62d 2017-12-26).