Issue with impl Trait and panic!()

[futile]

The following code does not compile on nightly (and 1.29.1 stable) (playground):

fn foo() -> impl Iterator<Item = i32> {
    panic!()
}

I expected this code to compile, but instead I get the following error:

error[E0277]: the trait bound `(): std::iter::Iterator` is not satisfied
 --> src/lib.rs:1:13
  |
1 | fn foo() -> impl Iterator<Item = i32> {
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^ `()` 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

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: Could not compile `playground`.

To learn more, run the command again with --verbose.

This seems like it should work to me.

[killercup]

FYI: This also means that

fn foo() -> impl Iterator<Item = i32> {
    unimplemented!()
}

doesn't compile!

[futile]

Yeah, I always use panic!() instead of unimplemented!() because it's easier to type, but that is exactly how I found it.

[memoryruins]

Clicking through compiler explorer, the original example has never compiled on any stable release (since stabilizing impl trait on 1.26). The error also occurs on traits without associated types

trait Bar {}

fn foo() -> impl Bar {
    unimplemented!()
}

3 | fn foo() -> impl Bar {
  |             ^^^^^^^^ the trait `Bar` is not implemented for `()`

(It will compile when returning something like impl Eq due to () implementing Eq)

[varkor]

This is a known design question concerning ! in general: #35121.