unimplemented!() does not typecheck functions that return impl Trait

[BookOwl]

I tried this code:

trait Spam {
    fn eggs(&self) -> u8;
}

fn foo(bar: u8) -> impl Spam {
    unimplemented!();
}

From my reading of the docs for unimplemented!(), this code should typecheck. However, when I try to compile it I get this type error:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `(): Spam` is not satisfied
 --> src/lib.rs:5:20
  |
5 | fn foo(bar: u8) -> impl Spam {
  |                    ^^^^^^^^^ the trait `Spam` is not implemented for `()`
6 |     unimplemented!();
  |     -------- consider removing this semicolon
  |
  = note: the return type of a function must have a statically known size

error: aborting due to previous error

If I remove the semicolon I get this error:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `(): Spam` is not satisfied
 --> src/lib.rs:5:20
  |
5 | fn foo(bar: u8) -> impl Spam {
  |                    ^^^^^^^^^ the trait `Spam` is not implemented for `()`
6 |     unimplemented!()
  |     ---------------- this returned value is of type `!`
  |
  = note: the return type of a function must have a statically known size

error: aborting due to previous error

The same issue happens with the todo!() macro.

Meta

When tested in the playground I get this error in stable 1.43.1, beta, and nightly.
Playground link to example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c62d0cbe9dd4e72fe0aaff17a18b53d0

[jonas-schievink]

Duplicate of #69882