add regression test for #48071

Fixes #48071

Author: nikomatsakis

src/test/ui/nll/relate_tys/issue-48071.rs

@@ -0,0 +1,38 @@
+// Copyright 2018 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 #48071. This test used to ICE because -- in
+// the leak-check -- it would pass since we knew that the return type
+// was `'static`, and hence `'static: 'a` was legal even for a
+// placeholder region, but in NLL land it would fail because we had
+// rewritten `'static` to a region variable.
+//
+// compile-pass
+
+#![allow(warnings)]
+#![feature(dyn_trait)]
+#![feature(nll)]
+
+trait Foo {
+    fn foo(&self) { }
+}
+
+impl Foo for () {
+}
+
+type MakeFooFn = for<'a> fn(&'a u8) -> Box<dyn Foo + 'a>;
+
+fn make_foo(x: &u8) -> Box<dyn Foo + 'static> {
+    Box::new(())
+}
+
+fn main() {
+    let x: MakeFooFn = make_foo as MakeFooFn;
+}