Don't require intercrate mode for negative coherence

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -397,6 +397,8 @@ fn impl_intersection_has_negative_obligation(
 ) -> bool {
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
 
+    // N.B. We need to unify impl headers *with* intercrate mode, even if proving negative predicates
+    // do not need intercrate mode enabled.
     let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
     let root_universe = infcx.universe();
     assert_eq!(root_universe, ty::UniverseIndex::ROOT);
@@ -554,7 +556,11 @@ fn try_prove_negated_where_clause<'tcx>(
         return false;
     };
 
-    let ref infcx = root_infcx.fork();
+    // N.B. We don't need to use intercrate mode here because we're trying to prove
+    // the *existence* of a negative goal, not the non-existence of a positive goal.
+    // Without this, we over-eagerly register coherence ambiguity candidates when
+    // impl candidates do exist.
+    let ref infcx = root_infcx.fork_with_intercrate(false);
     let ocx = ObligationCtxt::new(infcx);
 
     ocx.register_obligation(Obligation::new(