Derived trait shadows a blanket default impl, specialization unshadows but the generic impl ends up being used

[mzabaluev]

This example does not compile with 1.31.0-nightly (2018-10-20 155510e):

#![feature(specialization)]

use std::borrow::Borrow;

#[derive(PartialEq)]
struct MyString(String);

impl Borrow<str> for MyString {
    fn borrow(&self) -> &str {
        &self.0
    }
}

impl<Rhs> PartialEq<Rhs> for MyString
where
    Rhs: ?Sized + Borrow<str>,
{
    default fn eq(&self, rhs: &Rhs) -> bool {
        println!("default impl used");
        self.0 == rhs.borrow()
    }
}

#[cfg(never)]
impl PartialEq<str> for MyString {
    fn eq(&self, other: &str) -> bool {
        println!("specialized impl used");
        self.0 == other
    }
}

fn main() {
    let s = MyString(String::from("Hello, world!"));
    println!("{}", s == "Здравствуй, мир!");
}

The error output:

error[E0308]: mismatched types
  --> src/main.rs:34:25
   |
34 |     println!("{}", s == "Здравствуй, мир!");
   |                         ^^^^^^^^^^^^^^^^^^ expected struct `MyString`, found reference
   |
   = note: expected type `MyString`
              found type `&'static str`

Enabling the specialized impl PartialEq<str> makes the example work, but it prints "default impl used".

Removing the derive attribute also fixes the example, but only if the manually specialized impl is not present.