optimizer: (cdr <list?>) is a <list?>

In case it is not an error, the result is a list.

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

@@ -2535,6 +2535,25 @@
   (test-reduce 'k:list-pair? '(cdr (list 1)) #f)
 )
 
+(test-comp '(lambda (z)
+              (when (and (list? z) (pair? z))
+                (list? (cdr z))))
+           '(lambda (z)
+              (when (and (list? z) (pair? z))
+                #t)))
+(test-comp '(lambda (z)
+              (when (list? z)
+                (list? (unsafe-cdr z))))
+           '(lambda (z)
+              (when (list? z)
+                #t)))
+(test-comp '(lambda (z)
+              (when (list? z)
+                (list? (cdr z))))
+           '(lambda (z)
+              (when (list? z)
+                (begin (cdr z) #t))))
+
 (let ([test-bin
        (lambda (bin-name)
          (test-comp `(lambda (z)

racket/src/racket/src/optimize.c

@@ -3441,7 +3441,7 @@ static Scheme_Object *do_expr_implies_predicate(Scheme_Object *expr, Optimize_In
           || SAME_OBJ(app->rator, scheme_unsafe_cdr_proc)) {
         Scheme_Object *p;
         p = do_expr_implies_predicate(app->rand, info, NULL, fuel-1, ignore_vars);
-        if (SAME_OBJ(p, scheme_list_pair_p_proc))
+        if (predicate_implies(p, scheme_list_p_proc))
           return scheme_list_p_proc;
       }