Add regression test for issue #10766

This test verifies that casting from the same lifetime on a value
to the same lifetime on a trait succeeds. Closes #10766.

src/test/run-pass/borrowck-trait-lifetime.rs

@@ -0,0 +1,21 @@
+// Copyright 2014 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.
+
+// This test verifies that casting from the same lifetime on a value
+// to the same lifetime on a trait succeeds. See issue #10766.
+
+#![allow(dead_code)]
+fn main() {
+    trait T {}
+
+    fn f<'a, V: T>(v: &'a V) -> &'a T {
+        v as &'a T
+    }
+}