error: concrete type differs from previous defining existential type use
  --> $DIR/issue-52843.rs:7:5
   |
LL |     fn _unused() -> Existential { String::new() }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, got `std::string::String`
   |
note: previous use here
  --> $DIR/issue-52843.rs:5:1
   |
LL | / fn main() {
LL | |     existential type Existential: Debug;
LL | |     fn _unused() -> Existential { String::new() }
LL | |
LL | |     let null = || -> Existential { 0 };
LL | |     println!("{:?}", null());
LL | | }
   | |_^

error: aborting due to previous error