diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 878763dca14..97b87b985d0 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -987,8 +987,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 GenericArgs::AngleBracketed(data) => {
                     self.lower_angle_bracketed_parameter_data(data, ParamMode::Explicit, itctx).0
                 }
+                GenericArgs::Parenthesized(data)
+                    if self.tcx.features().return_type_notation =>
+                {
+                    // TODO: Check the parens + no return type
+                    GenericArgsCtor {
+                        args: Default::default(),
+                        bindings: &[],
+                        parenthesized: true,
+                        span: data.span,
+                    }
+                }
                 GenericArgs::Parenthesized(data) => {
                     self.emit_bad_parenthesized_trait_in_assoc_ty(data);
+                    // TODO: Add a RTN feature error if the parens are shaped correctly
                     self.lower_angle_bracketed_parameter_data(
                         &data.as_angle_bracketed_args(),
                         ParamMode::Explicit,
diff --git a/compiler/rustc_hir_analysis/src/astconv/mod.rs b/compiler/rustc_hir_analysis/src/astconv/mod.rs
index 4ab6bb5908b..0565bc6dffe 100644
--- a/compiler/rustc_hir_analysis/src/astconv/mod.rs
+++ b/compiler/rustc_hir_analysis/src/astconv/mod.rs
@@ -854,16 +854,15 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         )
     }
 
-    fn trait_defines_associated_type_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
+    fn trait_defines_associated_item_named(
+        &self,
+        trait_def_id: DefId,
+        assoc_kind: ty::AssocKind,
+        assoc_name: Ident,
+    ) -> bool {
         self.tcx()
             .associated_items(trait_def_id)
-            .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, trait_def_id)
-            .is_some()
-    }
-    fn trait_defines_associated_const_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
-        self.tcx()
-            .associated_items(trait_def_id)
-            .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Const, trait_def_id)
+            .find_by_name_and_kind(self.tcx(), assoc_name, assoc_kind, trait_def_id)
             .is_some()
     }
 
@@ -1087,24 +1086,42 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         let tcx = self.tcx();
 
-        let candidate =
-            if self.trait_defines_associated_type_named(trait_ref.def_id(), binding.item_name) {
-                // Simple case: X is defined in the current trait.
+        // TODO: rtn comment goes here
+        let associated_return_type_bound =
+            binding.gen_args.parenthesized && self.tcx().features().associated_return_type_bounds;
+
+        let candidate = if return_type_notation {
+            if self.trait_defines_associated_item_named(
+                trait_ref.def_id(),
+                ty::AssocKind::Fn,
+                binding.item_name,
+            ) {
                 trait_ref
             } else {
-                // Otherwise, we have to walk through the supertraits to find
-                // those that do.
-                self.one_bound_for_assoc_type(
-                    || traits::supertraits(tcx, trait_ref),
-                    trait_ref.print_only_trait_path(),
-                    binding.item_name,
-                    path_span,
-                    match binding.kind {
-                        ConvertedBindingKind::Equality(term) => Some(term),
-                        _ => None,
-                    },
-                )?
-            };
+                // TODO: error
+                todo!()
+            }
+        } else if self.trait_defines_associated_item_named(
+            trait_ref.def_id(),
+            ty::AssocKind::Type,
+            binding.item_name,
+        ) {
+            // Simple case: X is defined in the current trait.
+            trait_ref
+        } else {
+            // Otherwise, we have to walk through the supertraits to find
+            // those that do.
+            self.one_bound_for_assoc_type(
+                || traits::supertraits(tcx, trait_ref),
+                trait_ref.print_only_trait_path(),
+                binding.item_name,
+                path_span,
+                match binding.kind {
+                    ConvertedBindingKind::Equality(term) => Some(term),
+                    _ => None,
+                },
+            )?
+        };
 
         let (assoc_ident, def_scope) =
             tcx.adjust_ident_and_get_scope(binding.item_name, candidate.def_id(), hir_ref_id);
@@ -1116,9 +1133,13 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 .filter_by_name_unhygienic(assoc_ident.name)
                 .find(|i| i.kind == kind && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident)
         };
-        let assoc_item = find_item_of_kind(ty::AssocKind::Type)
-            .or_else(|| find_item_of_kind(ty::AssocKind::Const))
-            .expect("missing associated type");
+        let assoc_item = if return_type_notation {
+            find_item_of_kind(ty::AssocKind::Fn)
+        } else {
+            find_item_of_kind(ty::AssocKind::Type)
+                .or_else(|| find_item_of_kind(ty::AssocKind::Const))
+        }
+        .expect("missing associated type");
 
         if !assoc_item.visibility(tcx).is_accessible_from(def_scope, tcx) {
             tcx.sess
@@ -1145,28 +1166,54 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 .or_insert(binding.span);
         }
 
-        // Include substitutions for generic parameters of associated types
-        let projection_ty = candidate.map_bound(|trait_ref| {
-            let ident = Ident::new(assoc_item.name, binding.item_name.span);
-            let item_segment = hir::PathSegment {
-                ident,
-                hir_id: binding.hir_id,
-                res: Res::Err,
-                args: Some(binding.gen_args),
-                infer_args: false,
+        let projection_ty = if associated_return_type_bound {
+            let generics = self.tcx().generics_of(assoc_item.def_id);
+            if !generics.params.is_empty() {
+                todo!();
+            }
+            let output = self.tcx().fn_sig(assoc_item.def_id).skip_binder().output();
+            let fn_bound_vars = output.bound_vars();
+
+            let output = if let ty::Alias(ty::Projection, alias_ty) = *output.skip_binder().kind()
+                && tcx.def_kind(alias_ty.def_id) == DefKind::ImplTraitPlaceholder
+            {
+                alias_ty
+            } else {
+                todo!("found return type of {output:?}");
             };
 
-            let substs_trait_ref_and_assoc_item = self.create_substs_for_associated_item(
-                path_span,
-                assoc_item.def_id,
-                &item_segment,
-                trait_ref.substs,
-            );
+            let trait_bound_vars = candidate.bound_vars();
+            let shifted_output = tcx.shift_bound_var_indices(trait_bound_vars.len(), output);
+            let subst_output =
+                ty::EarlyBinder(shifted_output).subst(tcx, candidate.skip_binder().substs);
+            let bound_vars =
+                tcx.mk_bound_variable_kinds_from_iter(trait_bound_vars.iter().chain(fn_bound_vars));
 
-            debug!(?substs_trait_ref_and_assoc_item);
+            ty::Binder::bind_with_vars(subst_output, bound_vars)
+        } else {
+            // Include substitutions for generic parameters of associated types
+            candidate.map_bound(|trait_ref| {
+                let ident = Ident::new(assoc_item.name, binding.item_name.span);
+                let item_segment = hir::PathSegment {
+                    ident,
+                    hir_id: binding.hir_id,
+                    res: Res::Err,
+                    args: Some(binding.gen_args),
+                    infer_args: false,
+                };
 
-            self.tcx().mk_alias_ty(assoc_item.def_id, substs_trait_ref_and_assoc_item)
-        });
+                let substs_trait_ref_and_assoc_item = self.create_substs_for_associated_item(
+                    path_span,
+                    assoc_item.def_id,
+                    &item_segment,
+                    trait_ref.substs,
+                );
+
+                debug!(?substs_trait_ref_and_assoc_item);
+
+                self.tcx().mk_alias_ty(assoc_item.def_id, substs_trait_ref_and_assoc_item)
+            })
+        };
 
         if !speculative {
             // Find any late-bound regions declared in `ty` that are not
@@ -1206,6 +1253,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         }
 
         match binding.kind {
+            ConvertedBindingKind::Equality(..) if associated_return_type_bound => {
+                // TODO: error
+                todo!()
+            }
             ConvertedBindingKind::Equality(mut term) => {
                 // "Desugar" a constraint like `T: Iterator<Item = u32>` this to
                 // the "projection predicate" for:
@@ -1267,7 +1318,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
                 // parameter to have a skipped binder.
                 let param_ty = tcx.mk_alias(ty::Projection, projection_ty.skip_binder());
-                self.add_bounds(param_ty, ast_bounds.iter(), bounds, candidate.bound_vars());
+                self.add_bounds(param_ty, ast_bounds.iter(), bounds, projection_ty.bound_vars());
             }
         }
         Ok(())
@@ -1808,10 +1859,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     where
         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
     {
-        let mut matching_candidates = all_candidates()
-            .filter(|r| self.trait_defines_associated_type_named(r.def_id(), assoc_name));
-        let mut const_candidates = all_candidates()
-            .filter(|r| self.trait_defines_associated_const_named(r.def_id(), assoc_name));
+        let mut matching_candidates = all_candidates().filter(|r| {
+            self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Type, assoc_name)
+        });
+        let mut const_candidates = all_candidates().filter(|r| {
+            self.trait_defines_associated_item_named(r.def_id(), ty::AssocKind::Const, assoc_name)
+        });
 
         let (bound, next_cand) = match (matching_candidates.next(), const_candidates.next()) {
             (Some(bound), _) => (bound, matching_candidates.next()),
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index b50d2984a4e..72f230e628d 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -550,7 +550,11 @@ impl<'a> Parser<'a> {
 
                     // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
                     if let AssocConstraintKind::Bound { .. } = kind {
-                        self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+                        if gen_args.as_ref().map_or(false, |args| args.is_parenthesized()) {
+                            self.sess.gated_spans.gate(sym::return_type_notation, span);
+                        } else {
+                            self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+                        }
                     }
                     let constraint =
                         AssocConstraint { id: ast::DUMMY_NODE_ID, ident, gen_args, kind, span };