member constraints involving external regions are scuffed

struct Inv<'a>(Option<*mut &'a ()>);
fn foo() -> impl Sized {
    let _: fn() -> Inv<'static> = || {
        foo()
    };
    loop {}
}

When borrow checking the closure with signature fn() -> Inv<'external> we've got a defining use of the opaque with the hidden type Inv<'external>.

The member constraint 'external member ['static] does not choose 'static here as it fails the upper bound check. This should work, properly supporting it is hard :>

Start by compiling the provided Rust reproducer and tracing the borrow-checking handling of the closure signature fn() -> Inv<'external> and its member constraint 'external member ['static]. Done means the reproducer compiles with the opaque hidden type Inv<'external> and the member constraint can select 'static without failing its upper-bound check.