Weird error explanation, when returning closures from match

[timcryt]

I tried this code:

use std::collections::HashMap;
use std::fmt::Debug;

fn main() {
    let a: HashMap<String, Box<dyn Debug>> = HashMap::new();
    let b = "hello";
    println!(
        "{:?}",
        (match &b[0..=0] {
            "h" => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>)
                as Box<dyn FnMut(&str) -> &Box<dyn Debug>>,
            "H" => Box::new(|x| a.get(x).unwrap()),
            _ => unreachable!(),
        })("world")
    );
}

I expected to see this happen: Compilation error with logical explanation.

Instead, this happened: Сompilation error with weird explanation.

error[E0308]: mismatched types
  --> src/main.rs:10:20
   |
10 |             "h" => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>)
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected reference `&Box<dyn Debug>`
              found reference `&Box<dyn Debug>`

error[E0308]: mismatched types
  --> src/main.rs:10:20
   |
10 |             "h" => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>)
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `FnOnce<(&str,)>`
              found type `FnOnce<(&str,)>`

error[E0308]: mismatched types
  --> src/main.rs:9:9
   |
9  | /         (match &b[0..=0] {
10 | |             "h" => Box::new(|x| a.get_mut(x).unwrap() as &Box<dyn Debug>)
11 | |                 as Box<dyn FnMut(&str) -> &Box<dyn Debug>>,
12 | |             "H" => Box::new(|x| a.get(x).unwrap()),
13 | |             _ => unreachable!(),
14 | |         })("world")
   | |__________^ one type is more general than the other
   |
   = note: expected type `FnOnce<(&str,)>`
              found type `FnOnce<(&str,)>`

error: aborting due to 3 previous errors

Meta

The same error message on both stable (1.48.0), beta (1.49.0-beta.1) and nightly (2020-11-20)

[jyn514]

Duplicate of #41078