[racket] DRRacket Check Syntax Loops Endlessly (Sample Provided)

[Ray Racine]

Fresh build from Racket git MASTER, the following causes DRRacket's Check
Syntax to loop without termination.

#lang typed/racket/base

(provide f)

(struct: (T) S1 ([x : (Listof String)]))

(struct: (T) S2 ([x : S0]
                 [g : (T -> Boolean)]))

(define-type (S0 T) (U (S1 T) (S2 T)))

(: f (String -> (S0 String)))
(define (f x)
  (S1 '()))

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