Avoid a duplicated error case in fn_sig_suggestion.

Author: nnethercote

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -456,18 +456,14 @@ fn fn_sig_suggestion<'tcx>(
     let mut output = sig.output();
 
     let asyncness = if tcx.asyncness(assoc.def_id).is_async() {
-        output = if let ty::Alias(_, alias_ty) = *output.kind() {
-            tcx.explicit_item_super_predicates(alias_ty.def_id)
+        output = if let ty::Alias(_, alias_ty) = *output.kind()
+            && let Some(output) = tcx
+                .explicit_item_super_predicates(alias_ty.def_id)
                 .iter_instantiated_copied(tcx, alias_ty.args)
                 .find_map(|(bound, _)| {
                     bound.as_projection_clause()?.no_bound_vars()?.term.as_type()
-                })
-                .unwrap_or_else(|| {
-                    span_bug!(
-                        ident.span,
-                        "expected async fn to have `impl Future` output, but it returns {output}"
-                    )
-                })
+                }) {
+            output
         } else {
             span_bug!(
                 ident.span,