Impossible type error involving Sexp

What version of Racket are you using?

v6.12

What program did you run?

#lang typed/racket

(struct x ())
(struct y ([ x : (U #f x)]))

(: shrink-x (-> x Sexp))
(define (shrink-x a) 'x)

(: compile-y (-> y Sexp))
(define (compile-y y)
  `(x ,(shrink-x (y-x y))))

What should have happened?

In the above program, the typechecker correctly catches the erroneous application of shrink-x to a value of type (U #f x).

However, it also produces a useless "impossible" type error. Since a (List 'x Sexp) is also an Sexp, its diagnosis expected: Sexp, given: (List 'x Sexp) must be incorrect.

It seems that only the first error should be reported.

If you got an error message, please include it here.

a.rkt:11:17: Type Checker: type mismatch
  expected: x
  given: (U False x)
  in: (y-x y)
a.rkt:11:2: Type Checker: type mismatch
  expected: Sexp
  given: (List 'x Sexp)
  in: (quasiquote (x (unquote (shrink-x (y-x y)))))

Reproduce the supplied #lang typed/racket program on v6.12 and inspect how the type checker handles the quasiquoted (x ...) expression. Done means the invalid (y-x y) application still reports its error without also reporting the spurious expected Sexp/given (List 'x Sexp) mismatch.