Fix for/fold typechecking on null accumulator

Closes PR 13259

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-app/tc-app-lambda.rkt

@@ -85,7 +85,7 @@
     ;; special case `for/list'
     [((val acc ...)
       ((~and inner-body (if e1 e2 e3:id)))
-      (null actuals ...))
+      (~and (null actuals ...) (null-exp . _)))
      #:when (free-identifier=? #'val #'e3)
      (let ([ts (for/list ([ac (in-syntax #'(actuals ...))]
                           [f (in-syntax #'(acc ...))])
@@ -99,6 +99,9 @@
                       [(tc-result1: (and t (Listof: _))) t]
                       [_ #f])
                     (generalize -Null))])
+       ;; this check is needed because the type annotation may come
+       ;; from `for/fold` and it won't necessarily be a list type
+       (tc-expr/check #'null-exp (ret acc-ty))
        (define-values (fun-results body-results)
          (tc/rec-lambda/check args body lp (cons acc-ty ts) expected))
        (add-typeof-expr lam fun-results)

pkgs/typed-racket-pkgs/typed-racket-test/tests/typed-racket/unit-tests/typecheck-tests.rkt

@@ -3113,6 +3113,15 @@
           (define f (lambda () 'hi))
           (f))
         #:msg "cannot apply function of type Procedure"]
+
+       ;; PR 13259
+       [tc-err
+        (let ()
+          (: y String)
+          (define y (for/fold: ((x : String null)) ((v : String null)) x))
+          y)
+        #:ret (ret -String)
+        #:msg "expected: String.*given: Null"]
         )
 
   (test-suite