ICE with `#![feature(existential_type)]` + closure

[Centril]

Encountered this when working on test cases for an RFC:

#![feature(existential_type)]

existential type A: Iterator;
fn def_a() -> A { 0..1 }
pub fn use_a() {
    def_a().map(|x| x);
}

cc @oli-obk

This seems related to other issues.

[oli-obk]

I'm confused. Either this was fixed since yesterday's nightly (it still ICEs on nightly but works locally on master), or something really odd is going on

[Centril]

@oli-obk Is that the upstream master? Otherwise maybe someone merged a PR or something..

[Centril]

This still ICEs on nightly.

[Aaron1011]

This now compiles successfully on the latest nightly.