Add regression test for #20186

Closes #20186.

src/test/run-pass/issue-20186.rs

@@ -0,0 +1,23 @@
+// 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.
+
+struct Foo;
+
+impl Foo {
+    fn putc(&self, b: u8) { }
+
+    fn puts(&self, s: &str) {
+        for byte in s.bytes() {
+            self.putc(byte)
+        }
+    }
+}
+
+fn main() {}