fix closure type tracking for a procedure with only an unused rest arg

Closes PR 14259

pkgs/racket-pkgs/racket-test/tests/racket/optimize.rktl

@@ -3165,6 +3165,23 @@
   (void (dynamic-require ''uses-too-much-memory-for-shift #f)))
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Make sure that closure fields are correctly type-tagged
+;; when a function has an unused rest arg:
+
+(parameterize ([current-namespace (make-base-namespace)]
+               [read-on-demand-source #f]
+               [read-accept-compiled #t])
+  (let ([o (open-output-bytes)])
+    (write (compile '(module m racket/base
+                       (require racket/fixnum)
+                       (define (test l)
+                         (define n (fxmax (length l) 1))
+                         (lambda _ n))))
+           o)
+    ;; Should succeed, as opposed to a validation error:
+    (eval (read (open-input-bytes (get-output-bytes o))))))
+
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 
 (report-errs)

racket/src/racket/src/resolve.c

@@ -2015,8 +2015,16 @@ resolve_closure_compilation(Scheme_Object *_data, Resolve_Info *info,
     /* (lambda args E) where args is not in E => drop the argument */
     new_info = resolve_info_extend(info, 0, 1, cl->base_closure_size);
     num_params = 0;
-    if (!just_compute_lift)
+    if (!just_compute_lift) {
       data->num_params = 0;
+      if (expanded_already) {
+        /* shift type map down: */
+        for (i = 0; i < closure_size; i++) {
+          scheme_boxmap_set(closure_map, i, scheme_boxmap_get(closure_map, i + 1, closure_size), closure_size);
+        }
+        SCHEME_CLOSURE_DATA_FLAGS(data) |= CLOS_HAS_TYPED_ARGS;
+      }
+    }
   } else {
     new_info = resolve_info_extend(info, data->num_params, data->num_params,
                                    cl->base_closure_size + data->num_params);