From 2cc71ba8813e3a651048c18d14411467517c7ddb Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 16 Aug 2023 14:28:00 -0700
Subject: [PATCH] Normalize return type of
 deduce_future_output_from_obligations

---
 compiler/rustc_hir_typeck/src/closure.rs      | 13 ++++++-------
 ...normalize-output-in-signature-deduction.rs | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 7 deletions(-)
 create mode 100644 tests/ui/async-await/normalize-output-in-signature-deduction.rs

diff --git a/compiler/rustc_hir_typeck/src/closure.rs b/compiler/rustc_hir_typeck/src/closure.rs
index 3e67afb1c3d9b..b19fb6da6defd 100644
--- a/compiler/rustc_hir_typeck/src/closure.rs
+++ b/compiler/rustc_hir_typeck/src/closure.rs
@@ -711,6 +711,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         };
 
+        let span = self.tcx.def_span(expr_def_id);
+
         let output_ty = match *ret_ty.kind() {
             ty::Infer(ty::TyVar(ret_vid)) => {
                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
@@ -724,20 +726,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .find_map(|(p, s)| get_future_output(p.as_predicate(), s))?,
             ty::Error(_) => return None,
             _ => span_bug!(
-                self.tcx.def_span(expr_def_id),
+                span,
                 "async fn generator return type not an inference variable: {ret_ty}"
             ),
         };
 
+        let output_ty = self.normalize(span, output_ty);
+
         // async fn that have opaque types in their return type need to redo the conversion to inference variables
         // as they fetch the still opaque version from the signature.
         let InferOk { value: output_ty, obligations } = self
-            .replace_opaque_types_with_inference_vars(
-                output_ty,
-                body_def_id,
-                self.tcx.def_span(expr_def_id),
-                self.param_env,
-            );
+            .replace_opaque_types_with_inference_vars(output_ty, body_def_id, span, self.param_env);
         self.register_predicates(obligations);
 
         Some(output_ty)
diff --git a/tests/ui/async-await/normalize-output-in-signature-deduction.rs b/tests/ui/async-await/normalize-output-in-signature-deduction.rs
new file mode 100644
index 0000000000000..960065a83a45d
--- /dev/null
+++ b/tests/ui/async-await/normalize-output-in-signature-deduction.rs
@@ -0,0 +1,19 @@
+// edition:2021
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+struct Foo;
+
+impl Trait for Foo {}
+pub trait Trait {}
+
+pub type TAIT<T> = impl Trait;
+
+async fn foo<T>() -> TAIT<T> {
+    Foo
+}
+
+fn main() {}