repair for for expansion

Closes racket#4124

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

@@ -1458,6 +1458,29 @@
 (test '(1)
       eval-syntax #`(for/list ([(k v) (quote #,(datum->syntax #f #hash((1 . 0))))]) k))
 
+;; ----------------------------------------
+;; regression test for a missing "outer edge" scope
+
+(let ()
+  (define-sequence-syntax in-digits
+    (lambda () #'values)
+    (lambda (stx)
+      (syntax-case stx ()
+        [[(d) (_ nat)]
+         #'[(d)
+            (:do-in
+             ([(n) nat])
+             values
+             ([i n])
+             (not (zero? i))
+             ([(j d) (quotient/remainder i 10)])
+             #t
+             #t
+             [(- i 1)])]] ; <- regression would make this `i` ambigious
+        [_ #f])))
+
+  (for ([i (in-digits 12)]) i))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/collects/racket/private/for.rkt

@@ -1775,7 +1775,15 @@
                                  orig-stx
                                  stx)])))
 
-      (define ((make for*? right?) stx)
+      (define ((make for*? right?) stx-in)
+        ;; Add a fresh scope that acts the "outer edge" scope of `let`, so
+        ;; that as we expand clause transformers, we don't end up with
+        ;; ambiguities between things that started out in the form and that
+        ;; were introduced by expansion. This is necessary due to the way
+        ;; `for` performs its own expansion of clauses.
+        (define stx (internal-definition-context-add-scopes
+                     (syntax-local-make-definition-context)
+                     stx-in))
         (syntax-case stx ()
           [(_ orig-stx bindings+options . rest)
            (let ()