Add a regression test for #23305.

Closes #23305.

src/test/compile-fail/issue-23305.rs

@@ -0,0 +1,17 @@
+// 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.
+
+pub trait ToNbt<T> {
+    fn new(val: T) -> Self;
+}
+
+impl ToNbt<Self> {} //~ ERROR use of `Self` outside of an impl or trait
+
+fn main() {}