Why can the ? operator convert a &str into a Box<dyn Error>?

Please consider the following two functions:

use std::error::Error;

fn foo() -> Result<i32, Box<dyn Error>> {
    let x: Result<Result<i32, Box<dyn Error>>, &str> = Err("error");
    x?
}

fn bar() -> Result<i32, Box<dyn Error>> {
    Err("error")
}

foo() will compile, but bar() won’t.

I guess I understand why bar() doesn’t work: We expect an object implementing the Error trait in a Box but we pass a &str which neither implements Error nor is it in a Box.

I wonder why foo() compiles though. Won’t the ? operator also try to convert the &str into a Box<dyn Error> in this case? Why does it succeed?