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However, for regular functions, the latter only includes late-bound lifetimes that appear in the function's inputs or output, missing unused lifetime parameters.

That doesn't seem to be quite true 🤔 from what I can tell all the ICE happen in cases where the fn-sig is malformed. The following compiles:

struct S<'a, T: ?Sized>(&'a T);

fn b<'a>() -> S<'static, u32> {
    S::<'a>(&0)
}

Hmm, so. This seems incompatible with the closure_lifetime_binder feature?

I think we should get @lcnr's thoughts on this, on why this switch was made (and if it's appropriate to go back to calling for_each_late_bound_region_in_item on everything).

I would also expect the same to be true for closures. The Binder should reference all late-bound regions, even if they're unused.

#![feature(closure_lifetime_binder)]
struct S<'a, T: ?Sized>(&'a T);

fn b() {
    for<'a> || -> S<'static, u32> {
        S::<'a>(&0)
    };
}

I personally believe the bug here to be that we don't include late bound regions in the binder if there's an error. We could even add an assert in query fn_sig that all late-bound regions are present in the binder to detect such issues more eagerly

on why this switch was made

Iirc the main reason behind #135479 was that previously we lazily converted bound regions which feels questionable to me. I switched us to first add nll vars for every late-bound region, and then convert as normal, without having to optionally create new bound regions

I would also expect the same to be true for closures. The Binder should reference all late-bound regions, even if they're unused.

Okay, yes, that's actually what I would expect too! But wasn't quite sure.

Okay, so, I think both @lcnr and I are on the same page that this isn't quite the right solution. Going to mark this as waiting on author until an alternate solution is proposed.

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