From 7b2aeca10a91e2c2cb3f2efcc383e8addabebe58 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Thu, 13 Sep 2018 13:40:15 -0400
Subject: [PATCH] give a more informative failure in this case

---
 src/librustc/traits/query/evaluate_obligation.rs | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/librustc/traits/query/evaluate_obligation.rs b/src/librustc/traits/query/evaluate_obligation.rs
index 6bd926783625..1a906b5da6f3 100644
--- a/src/librustc/traits/query/evaluate_obligation.rs
+++ b/src/librustc/traits/query/evaluate_obligation.rs
@@ -51,7 +51,14 @@ impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
                 let mut selcx =
                     SelectionContext::with_query_mode(&self, TraitQueryMode::Standard);
                 selcx.evaluate_obligation_recursively(obligation)
-                     .expect("Overflow should be caught earlier in standard query mode")
+                    .unwrap_or_else(|r| {
+                        span_bug!(
+                            obligation.cause.span,
+                            "Overflow should be caught earlier in standard query mode: {:?}, {:?}",
+                            obligation,
+                            r,
+                        )
+                    })
             }
         }
     }