Producing explicit type errors for uncovered cases in Typed Racket

explicit-failure.rkt

#lang typed/racket

(define-syntax (cond* stx)
  (syntax-case stx ()
    [(_ x clause ...)
     #`(cond clause ... [else (typecheck-fail #,stx "incomplete coverage" #:covered-id x)])]))

(: f : (U String Integer) -> Boolean)
(define (f x)
  (cond* x
         [(string? x) #t]
         [(exact-nonnegative-integer? x) #f]))

;; Produces the error:
;;  Type Checker: incomplete coverage; missing coverage of Negative-Integer in: 
;;  (cond* x ((string? x) #t) ((exact-nonnegative-integer? x) #f))