#![feature(specialization)]

use std::ops::Deref;
pub struct UserId(i64);

impl<S> From<S> for UserId
    where
        S: Deref,
        S::Target: Into<UserId>,
{
    default fn from(s: S) -> Self {
        s.deref().into()
    }
}

impl From<i64> for UserId {
    fn from(id: i64) -> Self {
        UserId(id)
    }
}

fn main() {}