Add testcase for issue-18088.

ref : https://github.com/rust-lang/rust/issues/18088.

src/test/run-pass/issue-18088.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 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.
+
+
+pub trait Indexable<T>: std::ops::Index<usize, Output = T> {
+    fn index2(&self, i: usize) -> &T {
+        &self[i]
+    }
+}
+fn main() {}