Cannot compile `todo!()` in functions returning `impl Trait` with associated types

[nicklimmm]

I tried this code:

trait Trait {
    type Type;
}

fn foo() -> impl Trait<Type = ()> {
    todo!()
}

Expected:

• Compile without errors, similar to when returning other types such as i32, etc.

Instead, this happened:

error[E0277]: the trait bound `(): Trait` is not satisfied
 --> src/main.rs:5:13
  |
5 | fn foo() -> impl Trait<Type = ()> {
  |             ^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `()`
  |

Question:

• Could this be caused by the behavior of having the ! type in the function body?
  • todo!() macro uses core::panicking::panic(), which returns !

Meta

rustc --version --verbose:

rustc 1.75.0-nightly (aa1a71e9e 2023-10-26)
binary: rustc
commit-hash: aa1a71e9e90f6eb3aed8cf79fc80bea304c17ecb
commit-date: 2023-10-26
host: aarch64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.3

Backtrace

[fmease]

Closing as duplicate of #113875.