---

yaml
---
r: 208132
b: refs/heads/snap-stage3
c: df42976
h: refs/heads/master
v: v3

Author: arielb1

[refs]

@@ -1,7 +1,7 @@
 ---
 refs/heads/master: 38a97becdf3e6a6157f6f7ec2d98ade8d8edc193
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
-refs/heads/snap-stage3: 4d8d68f80d2fb474188c1c2ee26d6266b4eaa003
+refs/heads/snap-stage3: df42976e4db6565e8693460c34387e515d10ecfd
 refs/heads/try: 7b4ef47b7805a402d756fb8157101f64880a522f
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/dist-snap: ba4081a5a8573875fed17545846f6f6902c8ba8d

branches/snap-stage3/src/librustc/middle/traits/error_reporting.rs

@@ -57,8 +57,10 @@ pub fn report_projection_error<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
     let predicate =
         infcx.resolve_type_vars_if_possible(&obligation.predicate);
     // The ty_err created by normalize_to_error can end up being unified
-    // into all obligations - we still have to report an error in that
-    // case.
+    // into all obligations: for example, if our obligation is something
+    // like `$X = <() as Foo<$X>>::Out` and () does not implement Foo<_>,
+    // then $X will be unified with ty_err, but the error still needs to be
+    // reported.
     if !infcx.tcx.sess.has_errors() || !predicate.references_error() {
         span_err!(infcx.tcx.sess, obligation.cause.span, E0271,
                 "type mismatch resolving `{}`: {}",