(module
 m typed/racket
 (provide g foo)
 (: g (U String Boolean)) ; <- annotate g to be (or string boolean)
 (define g "a")

 (define (foo)
   (string-append g))
 ;; globals.rkt:6:17: Type Checker: type mismatch
 ;;   expected: String
 ;;   given: (U Boolean String)
 ;;   in: g

 ;; ^^ no warnings without the type annotation

 ;; (define (foo)
 ;;   (if (string? g)
 ;;       (+ 1 g)
 ;;       "else"))

 ;; globals.rkt:19:12: Type Checker: type mismatch
 ;;  expected: Number
 ;;  given: String
 ;;  in: g

 ;; ^^ this happens even without the type annotation

 )