From e674cf0200347c5e58f8a3867657187b12e7b496 Mon Sep 17 00:00:00 2001
From: Matthew Jasper <mjjasper1@gmail.com>
Date: Fri, 14 Aug 2020 21:51:28 +0100
Subject: [PATCH] Normalize super trait bounds when confirming object
 candidates

---
 .../src/traits/select/confirmation.rs         | 29 +++++++++++++------
 .../associated-types/object-normalization.rs  | 26 +++++++++++++++++
 2 files changed, 46 insertions(+), 9 deletions(-)
 create mode 100644 src/test/ui/associated-types/object-normalization.rs

diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 21293615bc9..8c58d4191c5 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/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())
diff --git a/src/test/ui/associated-types/object-normalization.rs b/src/test/ui/associated-types/object-normalization.rs
new file mode 100644
index 00000000000..1f93248e10e
--- /dev/null
+++ b/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() {}