Confusing error message with conservative impl trait

[iopq]

#![feature(conservative_impl_trait)]

fn main() {
    let mut x = &mut 1i32;
    let closure = return_closure(x);
    closure();
}


fn return_closure(x: &mut i32) -> impl Fn() {
    move || println!("{}", x)
}

this gives the error message

error[E0564]: only named lifetimes are allowed in `impl Trait`, but `` was found in the type `[closure@<anon>:11:5: 11:30 x:&mut i32]`
  --> <anon>:10:35
   |
10 | fn return_closure(x: &mut i32) -> impl Fn() {
   |                                   ^^^^^^^^^

but that's wrong, if we amend the code to

#![feature(conservative_impl_trait)]

fn main() {
    let mut x = &mut 1i32;
    let closure = return_closure(x);
    closure();
}


fn return_closure<'a>(x: &'a mut i32) -> impl Fn() {
    move || println!("{}", x)
}

it compiles and runs just fine, so the previous error message is incorrect

[gnzlbg]

Duplicate of #38615

[cramertj]

This is fixed.