Compiler too restrictive for unused variables interacting with closures

[tbu-]

I'd like to write the following code with map:

fn foobar(map: Option<&BTreeMap<u32,u32>>) {
    let temp;
    let map = match map {
        Some(a) => a,
        None => { temp = BTreeMap::new(); &temp }
    }
    // Do something.
}

But the following thing doesn't work:

let temp;
map.unwrap_or_else(|| { temp = BTreeMap::new(); &temp })

[tbu-]

Looks similar to #8338.

[steveklabnik]

Triage: no change. reproducible: https://is.gd/ktvqme

[Mark-Simulacrum]

Closing as per #8338 (comment).