Add regression test for rust-lang#17959

Closes rust-lang#17959.

src/test/compile-fail/issue-17959.rs

@@ -0,0 +1,31 @@
+// 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.
+
+extern crate core;
+
+use core::ops::Drop;
+
+trait Bar {}
+
+struct G<T: ?Sized> {
+    _ptr: *const T
+}
+
+impl<T> Drop for G<T> {
+//~^ ERROR: The requirement `T : core::marker::Sized` is added only by the Drop impl. [E0367]
+    fn drop(&mut self) {
+        if !self._ptr.is_null() {
+        }
+    }
+}
+
+fn main() {
+    let x:G<Bar>;
+}