Fix upcasting with normalization in old solver, add a test

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

@@ -887,9 +887,19 @@ fn confirm_trait_upcasting_unsize_candidate(
         let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else { bug!() };
 
         let source_principal = a_data.principal().unwrap().with_self_ty(tcx, a_ty);
-        let upcast_principal = util::supertraits(tcx, source_principal).nth(idx).unwrap();
+        let unnormalized_upcast_principal =
+            util::supertraits(tcx, source_principal).nth(idx).unwrap();
 
         let mut nested = vec![];
+        let upcast_principal = normalize_with_depth_to(
+            self,
+            obligation.param_env,
+            obligation.cause.clone(),
+            obligation.recursion_depth + 1,
+            unnormalized_upcast_principal,
+            &mut nested,
+        );
+
         for bound in b_data {
             match bound.skip_binder() {
                 // Check that a's supertrait (upcast_principal) is compatible
@@ -973,7 +983,7 @@ fn confirm_trait_upcasting_unsize_candidate(
                     }
                     VtblSegment::TraitOwnEntries { trait_ref, emit_vptr } => {
                         vptr_offset += count_own_vtable_entries(tcx, trait_ref);
-                        if trait_ref == upcast_principal {
+                        if trait_ref == unnormalized_upcast_principal {
                             if emit_vptr {
                                 return ControlFlow::Break(Some(vptr_offset));
                             } else {

tests/ui/traits/trait-upcasting/normalization.rs

@@ -0,0 +1,20 @@
+// check-pass
+// issue: 114113
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
+#![feature(trait_upcasting)]
+
+trait Mirror {
+    type Assoc;
+}
+impl<T> Mirror for T {
+    type Assoc = T;
+}
+
+trait Bar<T> {}
+trait Foo<T>: Bar<<T as Mirror>::Assoc> {}
+
+fn upcast<T>(x: &dyn Foo<T>) -> &dyn Bar<T> { x }
+
+fn main() {}