overlapping instances in Rust

overlapping.rs

trait Foo {}

trait Bar {}

trait Demo<T> {
    fn t(_: T) -> T;
}

struct Test;

impl <T: Foo> Demo<T> for Test {
    fn t(_: T) -> T { panic!() }
}

impl <T: Bar> Demo<T> for Test {
    fn t(_: T) -> T { panic!() }
}

fn main() {
    println!("Hello, world!");
}