Handle multiple trait-def projection candidates

compiler/rustc_trait_selection/src/traits/project.rs

@@ -977,6 +977,13 @@ fn assemble_candidates_from_predicates<'cx, 'tcx>(
 
             if is_match {
                 candidate_set.push_candidate(ctor(data));
+
+                if potentially_unnormalized_candidates && !obligation.predicate.needs_infer() {
+                    // HACK: Pick the first trait def candidate for a fully
+                    // inferred predicate. This is to allow duplicates that
+                    // differ only in normalization.
+                    return;
+                }
             }
         }
     }

src/test/ui/associated-types/associated-types-projection-bound-ambiguity.rs

@@ -0,0 +1,16 @@
+// Check that if we have multiple applicable projection bounds we pick one (for
+// backwards compatibility reasons).
+
+// check-pass
+use std::ops::Mul;
+
+trait A {
+    type V;
+    type U: Mul<Self::V, Output = ()> + Mul<(), Output = ()>;
+}
+
+fn g<T: A<V = ()>>() {
+    let y: <T::U as Mul<()>>::Output = ();
+}
+
+fn main() {}