don't assume we can *always* find a return type hint in async fn

In particular, we sometimes cannot if there is an earlier error.

src/librustc_typeck/check/closure.rs

@@ -612,6 +612,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
                     debug!("supplied_sig_of_closure: closure is async fn body");
                     self.deduce_future_output_from_obligations(expr_def_id)
+                        .unwrap_or_else(|| {
+                            // AFAIK, deducing the future output
+                            // always succeeds *except* in error cases
+                            // like #65159. I'd like to return Error
+                            // here, but I can't because I can't
+                            // easily (and locally) prove that we
+                            // *have* reported an
+                            // error. --nikomatsakis
+                            astconv.ty_infer(None, decl.output.span())
+                        })
                 }
 
                 _ => astconv.ty_infer(None, decl.output.span()),
@@ -646,7 +656,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn deduce_future_output_from_obligations(
         &self,
         expr_def_id: DefId,
-    ) -> Ty<'tcx> {
+    ) -> Option<Ty<'tcx>> {
         debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
 
         let ret_coercion =
@@ -689,8 +699,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 } else {
                     None
                 }
-            })
-            .unwrap();
+            });
 
         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
         output_ty

src/test/ui/async-await/issues/issue-65159.rs

@@ -0,0 +1,10 @@
+// Regression test for #65159. We used to ICE.
+//
+// edition:2018
+
+async fn copy() -> Result<()> //~ ERROR wrong number of type arguments
+{
+    Ok(())
+}
+
+fn main() { }

src/test/ui/async-await/issues/issue-65159.stderr

@@ -0,0 +1,9 @@
+error[E0107]: wrong number of type arguments: expected 2, found 1
+  --> $DIR/issue-65159.rs:5:20
+   |
+LL | async fn copy() -> Result<()>
+   |                    ^^^^^^^^^^ expected 2 type arguments
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0107`.