Negative blanket OIBIT impl treated as positive

[bluss]

impl !Pod for .. {} looks like it produces a positive impl.

(playpen)

#![feature(optin_builtin_traits)]

unsafe trait Pod {}

unsafe impl !Pod for .. {}

struct Foo<X, Y>(X, Y);

fn is_pod<T: Pod>(_: &T) { }

fn main() {
    is_pod(&Foo(0, 0));
}

cc @nikomatsakis

OIBIT tracker issue: #13231

[arielb1]

Negative impls are only supposed to work if they are nominal (impl<U,V,W> !Foo for Bar<U,V,W>). We should however decide what this means.

[nikomatsakis]

impl !Pod for .. is certainly not legal, in any case.