error promoting type-tests from closures

[aliemjay]

The following should pass borrowck:

trait Trait {
    type Item<'a>: 'a;
}

fn assert_static<T: 'static>(_: T) {}

fn test_args<I: Trait>() {
    let closure = |a, _b| assert_static(a);
    //~^ ERROR the associated type may not live long enough

    closure(None::<I::Item::<'_>>, &None::<I::Item::<'_>>);
}

fn test_upvars<I: Trait>() {
    let upvars = (None::<I::Item::<'_>>, &None::<I::Item::<'_>>);
    let _closure = || {
        let (a, _b) = upvars;
        assert_static(a);
        //~^ ERROR the associated type may not live long enough
    };
}

When promoting the type-test I::Item::<'_>: 'static from the closure to the parent function, we fail to do so because we can't express '_ in terms of universal regions because the corresponding free region doesn't have an external_name?:

rust/compiler/rustc_borrowck/src/region_infer/mod.rs

Line 1147 in 0b4ba4c

self.definitions[upper_bound].external_name.unwrap_or(r)

@rustbot label C-bug T-types A-NLL NLL-complete A-borrow-checker