rust-analyzer shows {unknown} types when it's not clear why it wouldn't be able to

[glandium]

rust-analyzer version: 0.3.1607-standalone

rustc version: 1.71.0

In the following code:

pub fn foo<T, I: IntoIterator<Item=T>,J: IntoIterator<Item=T>>(i: I, j: J) -> impl Iterator {
    let mut a = i.into_iter();
    let mut b = j.into_iter();

    std::iter::from_fn(move || {
         match (a.next(), b.next()) {
            (Some(a), Some(b)) => Some((a, b)),
            _ => None,
         }
    })
}

Hovering at the end of a.next() shows Option<{unknown}> instead of Option<T>, likewise for b.next(). Hovering a in Some(a) shows {unknown} instead of T, etc.

I thought it could be related to from_fn, but in the following:

pub fn bar<T, I: IntoIterator<Item=T>>(i: I) -> impl Iterator {
    let mut a = i.into_iter();

    std::iter::from_fn(move || {
         match a.next() {
            Some(a) => Some(a),
            _ => None,
         }
    })
}

it works fine.

[flodiebold]

Duplicate of #10653 (and probably a bunch of others)