🛠️ specialization permits empty impls when parent has no default items

It is potentially somewhat surprising that the following impls are accepted:

#![feature(specialization)]

trait Foo {
    fn bar();
}

impl<T> Foo for T {
    fn bar() { } // no default
}

impl Foo for i32 {
    // OK so long as we define no items:
    // fn bar() { }
}


fn main() {}

The original example was unearthed by @eddyb and @varkor as part of a PR.

To be clear, I don't see this as a soundness problem per se. Just a question of what we think it ought to do.

cc @aturon