#![feature(arbitrary_self_types)]
trait Trait {
    fn f(self: *const Self)
    where
        Self: Send;
}

impl Trait for *const () {
    fn f(self: *const Self) {
        unreachable!()
    }
}

fn main() {
    let unsend: *const () = &();
    let unsend: *const dyn Trait = &unsend;
    let send_bad: *const (dyn Trait + Send) = unsend as _;
    send_bad.f(); // this crashes, since vtable for `*const ()` does not have an entry for `f`
    //~^ warning: adding an auto trait `Send` to a trait object in a pointer cast may cause UB later on
}

trait Cat {}
impl Cat for *const () {}

fn main() {
    let unsend: *const () = &();
    let unsend: *const dyn Cat = &unsend;
    let _send: *const (dyn Cat + Send) = unsafe {
        // Safety:
        // - Both types are pointers, to the same trait object (and thus have the same vtable)
        // - `Cat` does not have methods with `Send` bounds
        std::mem::transmute::<*const dyn Cat, *const (dyn Cat + Send)>(unsend)
    };
    // meow
}