List filters override list element filters

(I'm pretty sure this is a bug)

Applying a filter to a list seems to remove any filters we previously applied to the list's elements.

#lang typed/racket/base

(: test (-> (Listof (U (List Natural) Boolean)) Boolean))
(define (test expr)
  (if (list? (car expr))
    (if (not (null? expr))
      (= 1 (car (car expr)))
      #f)
    #f))

;; test.rkt:7:11: Type Checker: Polymorphic function `car' could not be applied to arguments:
;; Domains: (Listof a)
;;          (Pairof a b)
;; Arguments: (U Boolean (List Nonnegative-Integer))

Changing the order of the if tests makes the problem go away.

(Credit to the original SO post: http://stackoverflow.com/questions/33475581/how-to-fix-the-return-value-in-this-typed-racket-code/33489459#33489459)

Start with the Typed Racket example in the issue and reproduce the error from test.rkt, comparing the two orders of the if tests. Trace how the type checker combines filters from list? and the element access; done when both equivalent orders correctly preserve the element filter and the regression is covered by a test.