Add new test for #20232.

Author: nikomatsakis

src/test/compile-fail/borrowck-overloaded-index-and-overloaded-deref.rs

@@ -0,0 +1,47 @@
+// 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.
+
+// Check that we properly record borrows when we are doing an
+// overloaded, autoderef of a value obtained via an overloaded index
+// operator. The accounting of the all the implicit things going on
+// here is rather subtle. Issue #20232.
+
+use std::ops::{Deref, Index};
+
+struct MyVec<T> { x: T }
+
+impl<T> Index<usize> for MyVec<T> {
+    type Output = T;
+    fn index(&self, _: &usize) -> &T {
+        &self.x
+    }
+}
+
+struct MyPtr<T> { x: T }
+
+impl<T> Deref for MyPtr<T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &self.x
+    }
+}
+
+struct Foo { f: usize }
+
+fn main() {
+    let mut v = MyVec { x: MyPtr { x: Foo { f: 22 } } };
+    let i = &v[0].f;
+    v = MyVec { x: MyPtr { x: Foo { f: 23 } } };
+    //~^ ERROR cannot assign to `v`
+    read(*i);
+}
+
+fn read(_: usize) { }
+