Handle ReEmpty for impl Trait

Closes #35668

src/librustc_typeck/check/writeback.rs

@@ -319,7 +319,8 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
             let outside_ty = gcx.fold_regions(&inside_ty, &mut false, |r, _| {
                 match *r {
                     // 'static is valid everywhere.
-                    ty::ReStatic => gcx.mk_region(ty::ReStatic),
+                    ty::ReStatic |
+                    ty::ReEmpty => gcx.mk_region(*r),
 
                     // Free regions that come from early-bound regions are valid.
                     ty::ReFree(ty::FreeRegion {
@@ -341,7 +342,6 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                     }
 
                     ty::ReVar(_) |
-                    ty::ReEmpty |
                     ty::ReErased => {
                         let span = reason.span(self.tcx());
                         span_bug!(span, "invalid region in impl Trait: {:?}", r);

src/test/compile-fail/issue-35668.rs

@@ -0,0 +1,24 @@
+// 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.
+
+#![feature(conservative_impl_trait)]
+
+fn func<'a, T>(a: &'a [T]) -> impl Iterator<Item=&'a T> {
+    a.iter().map(|a| a*a)
+    //~^ ERROR binary operation `*` cannot be applied to type `&T`
+}
+
+fn main() {
+    let a = (0..30).collect::<Vec<_>>();
+
+    for k in func(&a) {
+        println!("{}", k);
+    }
+}