Return impl Trait on unimplemented funtion fails to compile!

[andreytkachenko]

This snipped of rust code

fn fail() -> impl Iterator<Item = u32> {
    unimplemented!()
}

fn main() {}

fails to compile with error:

 --> src/main.rs:2:14
  |
2 | fn fail() -> impl Iterator<Item = u32> {
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not an iterator; maybe try calling `.iter()` or a similar method
  |
  = help: the trait `std::iter::Iterator` is not implemented for `()`
  = note: the return type of a function must have a statically known size

[hanna-kruppe]

Duplicate of #36375

[andreytkachenko]

As duplicate of #36375