Confusing/inconsistent lifetime error with `==` on types with different covariant lifetimes

```rust
#[derive(Eq, PartialEq)]
struct S<'a>(&'a ());

fn f<'a, 'b>(a: S<'a>, b: S<'b>) {
    a.eq(&b); // This is OK
    a == b; // This errors that 'a cannot outlive 'b so that the expression is assignable
}
```