Take the polarity into account in compute_applicable_impls

Author: Urgau

compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs

@@ -19,6 +19,8 @@ pub fn recompute_applicable_impls<'tcx>(
     let tcx = infcx.tcx;
     let param_env = obligation.param_env;
 
+    let predicate_polarity = obligation.predicate.skip_binder().polarity;
+
     let impl_may_apply = |impl_def_id| {
         let ocx = ObligationCtxt::new(infcx);
         infcx.enter_forall(obligation.predicate, |placeholder_obligation| {
@@ -40,6 +42,21 @@ pub fn recompute_applicable_impls<'tcx>(
                 return false;
             }
 
+            let impl_trait_header = tcx.impl_trait_header(impl_def_id).unwrap();
+
+            let impl_polarity = impl_trait_header.polarity;
+            match impl_polarity {
+                ty::ImplPolarity::Positive | ty::ImplPolarity::Negative => {
+                    match impl_polarity == predicate_polarity {
+                        true => {}
+                        false => return false,
+                    }
+                }
+                ty::ImplPolarity::Reservation => {
+                    return false;
+                }
+            }
+
             let impl_predicates = tcx.predicates_of(impl_def_id).instantiate(tcx, impl_args);
             ocx.register_obligations(impl_predicates.predicates.iter().map(|&predicate| {
                 Obligation::new(tcx, ObligationCause::dummy(), param_env, predicate)