Exhaustiveness should ignore non-constructable patterns

[brendanzab]

Currently this does not compile:

enum Void {}

fn main() {
    let x = None::<Void>;
    match x {
        None => println!("hi"),
    }
}

<anon>:5:5: 7:6 error: non-exhaustive patterns: `Some(_)` not covered [E0004]
<anon>:5     match x {
<anon>:6         None => println!("hi"),
<anon>:7     }

playpen

I contend that because Some::<Void> is impossible to construct (Void is an uninhabited enum), exhaustiveness should not require it to be matched against.

[huonw]

This is possibly a dupe of #12609.

@huonw It definitely is. I think we could close it.

[alexcrichton]

Closing as a dupe.