enforce order of let-optionals* val/rest binding in non-chibi impl (issue 778)

lib/chibi/optional-test.sld

@@ -63,6 +63,10 @@
       (test '(0 1 (2 3 4))
           (let-optionals '(0 1 2 3 4) ((a 10) (b 11) . c)
             (list a b c)))
+      (let ((ls '()))
+        (let-optionals* ls ((a (begin (set! ls '(a b)) 'default-a))
+                            (b 'default-b))
+          (test '(default-a default-b) (list a b))))
       (test 5 (keyword-ref '(a: b: b: 5) 'b: #f))
       (test 5 (keyword-ref* '(a: b: b: 5) 'b: #f))
       (cond-expand

lib/chibi/optional.scm

@@ -29,7 +29,7 @@
 ;;> extra values are unused.
 ;;>
 ;;> \var{ls} is evaluated only once.  It is an error if any
-;;> \var{default} mutates or set!s \var{ls}.
+;;> \var{default} mutates \var{ls}.
 ;;>
 ;;> Typically used on the dotted rest list at the start of a lambda,
 ;;> \scheme{let-optionals} is more concise and more efficient than

lib/chibi/optional.sld

@@ -32,8 +32,8 @@
            (let ((tmp (op . args)))
              (let-optionals* tmp vars . body)))
           ((let-optionals* tmp ((var default) . rest) . body)
-           (let ((var (if (pair? tmp) (car tmp) default))
-                 (tmp2 (if (pair? tmp) (cdr tmp) '())))
+           (let* ((tmp2 (if (pair? tmp) (cdr tmp) '()))
+                  (var (if (pair? tmp) (car tmp) default)))
              (let-optionals* tmp2 rest . body)))
           ((let-optionals* tmp tail . body)
            (let ((tail tmp)) . body))))