add test for #20605

closes #20605
closes #20989 (duplicate)
closes #21379 (duplicate)

src/test/compile-fail/issue-20605.rs

+// 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.
+
+fn changer<'a>(mut things: Box<Iterator<Item=&'a mut u8>>) {
+    for item in *things { *item = 0 }
+//~^ ERROR the trait `core::marker::Sized` is not implemented for the type `core::iter::Iterator`
+//~^^ ERROR
+//~^^^ ERROR
+// FIXME(#21528) error should be reported once, not thrice
+}
+
+fn main() {}