Fix a bug with yielding subtypes of the yield type.

src/test/run-pass/generator/yield-subtype.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 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(generators)]
+
+fn bar<'a>() {
+    let a: &'static str = "hi";
+    let b: &'a str = a;
+
+	|| {
+		yield a;
+		yield b;
+	};
+}
+
+fn main() {}