return erased regions from fulfill_obligation

Fixes #26802.

src/librustc_trans/trans/common.rs

@@ -905,11 +905,13 @@ pub fn fulfill_obligation<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
     let vtable = selection.map(|predicate| {
         fulfill_cx.register_predicate_obligation(&infcx, predicate);
     });
-    let vtable = drain_fulfillment_cx_or_panic(span, &infcx, &mut fulfill_cx, &vtable);
+    let vtable = erase_regions(tcx,
+        &drain_fulfillment_cx_or_panic(span, &infcx, &mut fulfill_cx, &vtable)
+    );
 
-    info!("Cache miss: {:?}", trait_ref);
-    ccx.trait_cache().borrow_mut().insert(trait_ref,
-                                          vtable.clone());
+    info!("Cache miss: {:?} => {:?}", trait_ref, vtable);
+
+    ccx.trait_cache().borrow_mut().insert(trait_ref, vtable.clone());
 
     vtable
 }

src/test/run-pass/issue-26802.rs

@@ -0,0 +1,23 @@
+// 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.
+
+trait Foo<'a> {
+    fn bar<'b>(&self, x: &'b u8) -> u8 where 'a: 'b { *x+7 }
+}
+
+pub struct FooBar;
+impl Foo<'static> for FooBar {}
+fn test(foobar: FooBar) -> Box<Foo<'static>> {
+    Box::new(foobar)
+}
+
+fn main() {
+    assert_eq!(test(FooBar).bar(&4), 11);
+}