Deeply normalize obligations in refining_impl_trait

compiler/rustc_hir_analysis/src/check/compare_impl_item/refine.rs

@@ -136,11 +136,15 @@ pub(super) fn check_refining_return_position_impl_trait_in_trait<'tcx>(
     // 1. Project the RPITIT projections from the trait to the opaques on the impl,
     //    which means that they don't need to be mapped manually.
     //
-    // 2. Project any other projections that show up in the bound. That makes sure that
-    //    we don't consider `tests/ui/async-await/in-trait/async-associated-types.rs`
-    //    to be refining.
-    let (trait_bounds, impl_bounds) =
-        ocx.normalize(&ObligationCause::dummy(), param_env, (trait_bounds, impl_bounds));
+    // 2. Deeply normalize any other projections that show up in the bound. That makes sure
+    //    that we don't consider `tests/ui/async-await/in-trait/async-associated-types.rs`
+    //    or `tests/ui/impl-trait/in-trait/refine-normalize.rs` to be refining.
+    let Ok((trait_bounds, impl_bounds)) =
+        ocx.deeply_normalize(&ObligationCause::dummy(), param_env, (trait_bounds, impl_bounds))
+    else {
+        tcx.dcx().delayed_bug("encountered errors when checking RPITIT refinement (selection)");
+        return;
+    };
 
     // Since we've normalized things, we need to resolve regions, since we'll
     // possibly have introduced region vars during projection. We don't expect

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -107,6 +107,15 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
         self.register_infer_ok_obligations(infer_ok)
     }
 
+    pub fn deeply_normalize<T: TypeFoldable<TyCtxt<'tcx>>>(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        value: T,
+    ) -> Result<T, Vec<FulfillmentError<'tcx>>> {
+        self.infcx.at(cause, param_env).deeply_normalize(value, &mut **self.engine.borrow_mut())
+    }
+
     /// Makes `expected <: actual`.
     pub fn eq_exp<T>(
         &self,

tests/ui/impl-trait/in-trait/refine-normalize.rs

@@ -0,0 +1,20 @@
+//@ check-pass
+//@ edition: 2021
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+
+#![deny(refining_impl_trait)]
+
+pub trait Foo {
+    type Item;
+
+    fn hello() -> impl Iterator<Item = Self::Item>;
+}
+
+impl Foo for () {
+    type Item = ();
+
+    fn hello() -> impl Iterator<Item = ()> { [()].into_iter() }
+}
+
+fn main() {}