strange errors with multiple impls of FnOnce

[durka]

If one maliciously attempts to hack function overloading into rust

#![feature(unboxed_closures, fn_traits)]

use std::ops::FnOnce;

struct Nope;

impl FnOnce<()> for Nope {
    type Output = ();

    extern "rust-call" fn call_once(self, _: ()) {}
}

impl FnOnce<((),())> for Nope {
    type Output = ();

    extern "rust-call" fn call_once(self, _: ((),())) {}
}

fn main() {
    Nope();
}

then one receives appropriately inscrutable errors:

<anon>:20:5: 20:11 error: the type of this value must be known in this context
<anon>:20     Nope();
              ^~~~~~
<anon>:20:5: 20:11 error: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit [E0059]
<anon>:20     Nope();
              ^~~~~~
<anon>:20:5: 20:11 help: see the detailed explanation for E0059
error: aborting due to 2 previous errors
playpen: application terminated with error code 101

I don't understand which value rustc wants a type for, and the second error is clearly untrue. I assume this use case is not going to be supported, even if these features are stabilized, but the diagnostic is not good.

[qnighy]

Looks like a duplicate of #18952.

[Mark-Simulacrum]

Closing in favor, as duplicate.