`impl Trait` doesn't understand that empty functions return ()

[Stebalien]

The following should compile because (): Default:

#![feature(conservative_impl_trait)]
fn test() -> impl Default {}

However, it fails with:

error[E0282]: unable to infer enough type information about `_`
 --> tmp.rs:2:14
  |
2 | fn test() -> impl Default {}
  |              ^^^^^^^^^^^^ cannot infer type for `_`
  |
  = note: type annotations or generic parameter binding required

(Obviously, nobody will ever run into this in practice...)

[Mark-Simulacrum]

This works today; E-needstest.

[venkatagiri]

@Mark-Simulacrum would a run-pass be fine for this?

[Mark-Simulacrum]

Let's go with a compile-fail test that look something like this one; meaningful part being these two lines. Let me know if you have any more questions!