Add regression test.

src/test/run-pass/issue-37020.rs

@@ -0,0 +1,25 @@
+// Copyright 2016 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.
+
+#![allow(private_in_public)]
+
+mod foo {
+    pub mod bar {
+        extern crate core;
+    }
+}
+
+mod baz {
+    pub use foo::bar::core;
+}
+
+fn main() {
+    baz::core::cell::Cell::new(0u32);
+}