Normalize super trait bounds when confirming object candidates

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -440,15 +440,26 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let upcast_trait_ref = upcast_trait_ref.unwrap();
 
         // Check supertraits hold
-        nested.extend(
-            tcx.super_predicates_of(trait_predicate.def_id())
-                .instantiate(tcx, trait_predicate.trait_ref.substs)
-                .predicates
-                .into_iter()
-                .map(|super_trait| {
-                    Obligation::new(obligation.cause.clone(), obligation.param_env, super_trait)
-                }),
-        );
+        for super_trait in tcx
+            .super_predicates_of(trait_predicate.def_id())
+            .instantiate(tcx, trait_predicate.trait_ref.substs)
+            .predicates
+            .into_iter()
+        {
+            let normalized_super_trait = normalize_with_depth_to(
+                self,
+                obligation.param_env,
+                obligation.cause.clone(),
+                obligation.recursion_depth + 1,
+                &super_trait,
+                &mut nested,
+            );
+            nested.push(Obligation::new(
+                obligation.cause.clone(),
+                obligation.param_env.clone(),
+                normalized_super_trait,
+            ));
+        }
 
         let assoc_types: Vec<_> = tcx
             .associated_items(trait_predicate.def_id())

src/test/ui/associated-types/object-normalization.rs

@@ -0,0 +1,26 @@
+// ignore-tidy-linelength
+
+// Check that we normalize super predicates for object candidates.
+
+// check-pass
+
+use std::ops::Index;
+
+fn next<'a, T>(s: &'a mut dyn SVec<Item = T, Output = T>) {
+    // To prove
+    // `dyn SVec<Item = T, Output = T>: SVec`
+    // we need to show
+    // `dyn SVec<Item = T, Output = T> as Index>::Output == <dyn SVec<Item = T, Output = T> as SVec>::Item`
+    // which, with the current normalization strategy, has to be eagerly
+    // normalized to:
+    // `dyn SVec<Item = T, Output = T> as Index>::Output == T`.
+    let _ = s.len();
+}
+
+trait SVec: Index<usize, Output = <Self as SVec>::Item> {
+    type Item;
+
+    fn len(&self) -> usize;
+}
+
+fn main() {}