Prefer other implementations over default ones

src/librustc/middle/traits/select.rs

@@ -1273,6 +1273,10 @@ fn candidate_should_be_dropped_in_favor_of<'o>(&mut self,
                 // #18453.
                 true
             }
+            (&DefaultTraitCandidate(_), _) => {
+                // Prefer other candidates over default implementations.
+                true
+            }
             (&ProjectionCandidate, &ParamCandidate(_)) => {
                 // FIXME(#20297) -- this gives where clauses precedent
                 // over projections. Really these are just two means