require reimplementations of all items when a defaulted associated type is overriden

This is a
[breaking-change]
but it follows the RFC (not sure whether it will be accepted).

src/test/compile-fail/associated-types-overridden-default.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(associated_consts)]
+
+pub trait Tr {
+    type Assoc = u8;
+    type Assoc2 = Self::Assoc;
+    const C: u8 = 11;
+    fn foo(&self) {}
+}
+
+impl Tr for () {
+    type Assoc = ();
+    //~^ ERROR need to be reimplemented as `Assoc` was overridden: `Assoc2`, `C`, `foo`
+}
+
+fn main() {}