Fix multiple mutable autoderefs with Box

Closes #26205.

src/test/run-pass/issue-26205.rs

@@ -0,0 +1,39 @@
+// 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.
+
+use std::ops::{Deref, DerefMut};
+
+struct Foo;
+
+impl Foo {
+    fn foo_mut(&mut self) {}
+}
+
+struct Bar(Foo);
+
+impl Deref for Bar {
+    type Target = Foo;
+
+    fn deref(&self) -> &Foo {
+        &self.0
+    }
+}
+
+impl DerefMut for Bar {
+    fn deref_mut(&mut self) -> &mut Foo {
+        &mut self.0
+    }
+}
+
+fn test(mut bar: Box<Bar>) {
+    bar.foo_mut();
+}
+
+fn main() {}