Structurally normalize async fn return type in deduce_future_output_from_obligations

Author: compiler-errors

Diff for: compiler/rustc_hir_typeck/src/closure.rs

@@ -688,8 +688,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
         });
 
+        let closure_span = self.tcx.def_span(expr_def_id);
         let ret_ty = ret_coercion.borrow().expected_ty();
-        let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
+        let ret_ty = self.try_structurally_resolve_type(closure_span, ret_ty);
 
         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
             // Search for a pending obligation like
@@ -711,8 +712,6 @@ 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| {
@@ -726,17 +725,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .find_map(|(p, s)| get_future_output(p.as_predicate(), s))?,
             ty::Error(_) => return None,
             _ => span_bug!(
-                span,
+                closure_span,
                 "async fn generator return type not an inference variable: {ret_ty}"
             ),
         };
 
-        let output_ty = self.normalize(span, output_ty);
+        let output_ty = self.normalize(closure_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, span, self.param_env);
+            .replace_opaque_types_with_inference_vars(
+                output_ty,
+                body_def_id,
+                closure_span,
+                self.param_env,
+            );
         self.register_predicates(obligations);
 
         Some(output_ty)

Diff for: tests/ui/traits/new-solver/normalize-async-closure-in-trait.rs

@@ -0,0 +1,11 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+// edition:2021
+
+#![feature(async_fn_in_trait)]
+
+trait Foo {
+    async fn bar() {}
+}
+
+fn main() {}