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//@ run-pass
// Test that we evaluate projection predicates to winnow out
// candidates during trait selection and method resolution (#20296).
// If we don't properly winnow out candidates based on the output type
// `Target=[A]`, then the impl marked with `(*)` is seen to conflict
// with all the others.
use std::marker::PhantomData;
use std::ops::Deref;
pub trait MyEq<U: ?Sized=Self> {
fn eq(&self, u: &U) -> bool;
}
impl<A, B> MyEq<[B]> for [A]
where A : MyEq<B>
{
fn eq(&self, other: &[B]) -> bool {
self.len() == other.len() &&
self.iter().zip(other).all(|(a, b)| MyEq::eq(a, b))
}
}
// (*) This impl conflicts with everything unless the `Target=[A]`
// constraint is considered.
impl<'a, A, B, Lhs> MyEq<[B; 0]> for Lhs
where A: MyEq<B>, Lhs: Deref<Target=[A]>
{
fn eq(&self, other: &[B; 0]) -> bool {
MyEq::eq(&**self, other)
}
}
struct DerefWithHelper<H, T> {
pub helper: H,
pub marker: PhantomData<T>,
}
trait Helper<T> {
fn helper_borrow(&self) -> &T;
}
impl<T> Helper<T> for Option<T> {
fn helper_borrow(&self) -> &T {
self.as_ref().unwrap()
}
}
impl<T, H: Helper<T>> Deref for DerefWithHelper<H, T> {
type Target = T;
fn deref(&self) -> &T {
self.helper.helper_borrow()
}
}
pub fn check<T: MyEq>(x: T, y: T) -> bool {
let d: DerefWithHelper<Option<T>, T> = DerefWithHelper { helper: Some(x),
marker: PhantomData };
d.eq(&y)
}
pub fn main() {
}
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