Add in-set to Typed Racket

Closes PR 14128

original commit: 3fa3225c544160b3d2f0448012de1d7cd66b5195

pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/base-env/base-special-env.rkt

@@ -139,6 +139,9 @@
   ;; in-bytes-lines
   [(make-template-identifier 'in-bytes-lines 'racket/private/for)
    (->opt [-Input-Port -Symbol] (-seq -Bytes))]
+  ;; in-set
+  [(make-template-identifier 'in-set 'racket/private/set)
+   (-poly (a) (-> (-set a) (-seq a)))]
   ;; check-in-bytes-lines
   [(make-template-identifier 'check-in-bytes-lines 'racket/private/for)
    (-> Univ Univ Univ)]

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

@@ -1802,6 +1802,21 @@
        ;; Unit test for PR 13298. Should raise an unbound id error
        ;; instead of just allowing `x` to be undefined
        [tc-err (let () (: x Number) 3)]
+
+       ;; Sets as sequences
+       [tc-e (in-set (set 1 2 3)) (-seq -PosByte)]
+       [tc-e
+        (let ()
+          (: lst (Listof Integer))
+          (define lst
+            (for/list: : (Listof Integer) ([i : Integer (set 1 2 3)]) i))
+          (void))
+        -Void]
+       [tc-e
+        (let ()
+          (for: ([k : Symbol (in-set (set 'x 'y 'z))]) (displayln k))
+          (void))
+        -Void]
         )
   (test-suite
    "tc-literal tests"