add regression test for #37154

Fixes #37154

Author: nikomatsakis

src/test/run-pass/project-cache-issue-37154.rs

@@ -0,0 +1,28 @@
+// 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.
+
+// Regression test for #37154: the problem here was that the cache
+// results in a false error because it was caching skolemized results
+// even after those skolemized regions had been popped.
+
+trait Foo {
+    fn method(&self) {}
+}
+
+struct Wrapper<T>(T);
+
+impl<T> Foo for Wrapper<T> where for<'a> &'a T: IntoIterator<Item=&'a ()> {}
+
+fn f(x: Wrapper<Vec<()>>) {
+    x.method(); // This works.
+    x.method(); // error: no method named `method`
+}
+
+fn main() { }