Auto merge of #29259 - arielb1:supertrait-self-2, r=eddyb

…being it

This is a [breaking-change]:lang, but the broken code does not make
much sense.

Fixes #26056

r? @eddyb

src/test/compile-fail/issue-26056.rs

@@ -0,0 +1,33 @@
+// 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.
+
+trait MapLookup<Q> {
+    type MapValue;
+}
+
+impl<K> MapLookup<K> for K {
+    type MapValue = K;
+}
+
+trait Map: MapLookup<<Self as Map>::Key> {
+    type Key;
+}
+
+impl<K> Map for K {
+    type Key = K;
+}
+
+
+fn main() {
+    let _ = &()
+        as &Map<Key=u32,MapValue=u32>;
+    //~^ ERROR the trait `Map` cannot be made into an object
+    //~| NOTE the trait cannot use `Self` as a type parameter
+}