This test now works

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -366,6 +366,7 @@ fn negative_impl_exists<'cx, 'tcx>(
         return true;
     }
 
+    // FIXME we should implement an `assemble_neg_candidates` fn
     for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
         if resolve_negative_obligation(infcx, param_env, region_context, &o) {
             return true;