[racket] typed racket and unions of vector types

I ran into this when trying to do vector-ref on a value of type In-Indexes from math/array.

If I do something like this:

#lang typed/racket

(: v : (U (Vectorof Index)
          (Vectorof Integer)))
(define v #(0))

(ann (vector-ref v 0) Integer)

Then it doesn't type check.  I understand that (Vectorof Index) isn’t a subtype of (Vectorof Integer), but still, why doesn’t this work?

Is there any reason why the type checker doesn’t do something where it checks both components of a union type and if both work then the union type works?  


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