Failure to recognize and/or unify ! type with impl Trait in return

[TyPR124]

I couldn't find an issue exactly like this one, but also probably don't know enough to recognize if any existing issue covered this specific case or not.

I tried this code:

fn foo() -> impl Drop {
    todo!()
}

I expected to see this happen: code to compile, just as this would

fn foo() -> u32 {
    todo!()
}

Instead, this happened: code fails to compile, explaining that type () does not implement Drop

Meta

rustc --version --verbose:

rustc 1.46.0-nightly (346aec9b0 2020-07-11)

Also same behavior on the playground

[jonas-schievink]

duplicate of #69882