Add tests related to #20220. Fixes #20220.

Author: nikomatsakis

src/test/compile-fail/traits-assoc-type-in-supertrait-bad.rs

@@ -0,0 +1,26 @@
+// 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.
+
+// Test case where an associated type is referenced from within the
+// supertrait definition, and the impl makes the wrong
+// associations. Issue #20220.
+
+use std::vec::IntoIter;
+
+pub trait Foo: Iterator<Item=<Self as Foo>::Key> {
+    type Key;
+}
+
+impl Foo for IntoIter<i32> { //~ ERROR type mismatch
+    type Key = u32;
+}
+
+fn main() {
+}

src/test/run-pass/traits-assoc-type-in-supertrait.rs

@@ -0,0 +1,31 @@
+// 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.
+
+// Test case where an associated type is referenced from within the
+// supertrait definition. Issue #20220.
+
+use std::vec::IntoIter;
+
+pub trait Foo: Iterator<Item=<Self as Foo>::Key> {
+    type Key;
+}
+
+impl Foo for IntoIter<i32> {
+    type Key = i32;
+}
+
+fn sum_foo<F:Foo<Key=i32>>(f: F) -> i32 {
+    f.fold(0, |a,b| a + b)
+}
+
+fn main() {
+    let x = sum_foo(vec![11, 10, 1].into_iter());
+    assert_eq!(x, 22);
+}