Add a regression test for #18119.

Closes #18119

src/test/compile-fail/issue-18119.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.
+
+const X: u8 = 1;
+static Y: u8 = 1;
+fn foo() {}
+
+impl X {}
+//~^ ERROR use of undeclared type name `X`
+impl Y {}
+//~^ ERROR use of undeclared type name `Y`
+impl foo {}
+//~^ ERROR use of undeclared type name `foo`
+
+fn main() {}